Strategies for enhancing irrigation efficiency on turfgrass areas
Abstract
Turfgrass systems are a vital component of urban green infrastructure, providing functional, ecological, and aesthetic services in recreational, sporting, and public landscapes. However, maintaining high-quality turfgrass requires substantial water, which has become increasingly problematic in the context of global water scarcity, climate change, and environmental regulations. Efficient water use has become a priority in sustainable turfgrass management. Unlike traditional agricultural systems, turfgrass areas prioritize usability, plant growth, and aesthetics. Standards for sports turf construction focus on soil properties, drainage, and moisture retention, but increasing drought and water scarcity demand innovative irrigation strategies to maintain functionality. Traditional irrigation systems, particularly sprinkler-based methods, often result in inefficient water use due to wind drift, evaporation loss, and technically induced distribution inaccuracies. Conversely, subsurface drip irrigation offers targeted and potentially more efficient water delivery, though it poses challenges in adequately wetting the turfgrass rootzone. Irrigation delivery systems interact with soil physical properties and rootzone construction methods, thereby affecting water distribution, retention, and turfgrass quality. Efficient irrigation strategies require a nuanced understanding of soil–water dynamics, including the impact of rootzone construction methods and irrigation delivery systems on water retention characteristics and soil moisture distribution. This cumulative dissertation addresses the pressing need to enhance irrigation efficiency in turfgrass areas by systematically investigating the impact of rootzone construction methods on irrigation water distribution, retention, and turfgrass performance. This investigation focuses on using traditional and commonly used sprinkler irrigation (SPR) or subsurface drip irrigation (SDI). Moreover, the study aims to assess the application of numerical simulation models, specifically HYDRUS-2D, to predict and optimize irrigation efficiency under various rootzone–irrigation configurations, with the overarching objective of evaluating strategies to enhance irrigation efficiency in turfgrass areas. To achieve this objective, trials were conducted over several years, encompassing three complementary studies: empirical (field and controlled environment) and modeling-based. The research systematically explores how rootzone construction influences irrigation water distribution, retention, and turfgrass quality, particularly when paired with either SPR-irrigation or SDI. Furthermore, the study evaluates the application of numerical simulation models, specifically HYDRUS-2D, to predict and optimize irrigation efficiency under varying soil–irrigation configurations. The experiments were conducted under both controlled greenhouse and open field conditions. The first study was conducted under greenhouse conditions using controlled irrigation cycles and bare soil profiles (without grass cover) to isolate and analyze water distribution in response to irrigation delivery system type and rootzone construction methods. Three rootzone constructions, two two-layered (analogous to the national standard) and one three-layered, were evaluated under both SPR-irrigation and SDI. The volumetric water content (VWC) was monitored at multiple depths and time intervals following irrigation events. The second study investigated turfgrass quality and water storage in a two-year field trial (2023 and 2024) under deficit irrigation conditions, specifically 60% reference evapotranspiration (ETO). Turfgrass plots featuring perennial ryegrass (Lolium perenne L.) and employing the same three rootzone construction methods used in the greenhouse condition were irrigated using either SPR or SDI. Key performance indicators included turfgrass quality (TQ), rootzone water storage (RWS), and soil water tension (SWT). In the third study, the HYDRUS-2D finite element model was employed to simulate water dynamics within the rootzones and associated irrigation delivery systems (SPR and SDI) based on the observed greenhouse data. The model incorporated soil hydraulic parameters, determined through laboratory analysis, for each material employed in this study. The calibration and validation of the model were conducted with a focus on optimizing model quality and minimizing the discrepancies between observed and simulated outputs, particularly in relation to volumetric water content. The results of the investigations showed that the greenhouse trials revealed distinct patterns of soil moisture distribution across irrigation systems and rootzone designs. SPR-irrigated plots exhibited rapid volumetric water content (VWC) increases at shallow depths (3 cm) followed by substantial decreases within 72 hours, particularly under two-layered designs. Three-layered SDI plots showed a pronounced capillary rise in irrigation water combined with sustained irrigation water retention in the soil matrix, highlighting the system’s efficiency. The field trials demonstrated significant differences in RWS and SWT across rootzone constructions. Two-layered designs under SDI experienced high SWT values (>120 kPa), indicating insufficient moisture retention. In contrast, three-layered designs maintained lower SWT (<15 kPa) and higher RWS after 35 days of 60% deficit irrigation. TQ declined rapidly in two-layered SDI variants, with an unacceptable TQ (<6) observed 14 days after study initiation. In contrast, three-layered SDI variants achieved the highest TQ throughout the study, demonstrating their resilience under water-limited conditions. SPR-irrigated variants initially exhibited acceptable TQ but failed to maintain it beyond 28 days due to poor water retention in sandy rootzones and high evaporation loss during irrigation. An exception was the three-layered SPR variants, which maintained an acceptable TQ throughout the testing period. The HYDRUS-2D model effectively simulated water distribution under both SDI and SPR systems, with calibrated parameters yielding improved model quality values. Sensitivity analyses identified shape factors α and n determining the soil hydraulic functions as critical parameters influencing model quality, particularly under SDI conditions with high spatial variability. Incorporating hysteresis effects in the model´s soil water retention function improved the model's accuracy. The collective results underscore the complex but manageable interplay between irrigation delivery system and rootzone construction. While effective at initial near-surface wetting, sprinkler systems fail to provide lasting soil moisture retention. Conversely, SDI systems supported by a three-layered design that enhances capillary rise are highly efficient. From a sustainability standpoint, the three-layered SDI variants consistently demonstrated superior water retention under controlled conditions, as well as enhanced RWS and TQ under field conditions. These configurations enabled homogenous moisture distribution, maximized water use efficiency, and maintained high TQ—key goals in sustainable urban green and turfgrass management. The calibrated HYDRUS-2D model demonstrated its utility as a cost-effective tool for predicting water dynamics and developing efficient irrigation strategies. Future research should explore hybrid irrigation approaches combining SPR and SDI systems to assess water distribution and irrigation efficiency under open field conditions. Additionally, model-based approaches should incorporate root water uptake models to provide a more comprehensive understanding of soil–plant–water dynamics. The integrated findings of this cumulative dissertation contribute significantly to understanding how irrigation systems and rootzone construction influence soil moisture dynamics and turfgrass quality. The evidence underscores the benefits of harmonizing the rootzone construction method with the corresponding irrigation delivery system to achieve uniform moisture distribution and enhanced water use efficiency, particularly in the context of SDI. Moreover, simulation tools like HYDRUS-2D effectively support turfgrass irrigation management and water conservation efforts when appropriately calibrated. The synergy between empirical findings and modeling results supports a paradigm shift toward precise, site-specific irrigation strategies that enhance irrigation efficiency in turfgrass areas. Nonetheless, further trials in urban and sports turf environments are essential to facilitate the broader implementation of these strategies.
Full text
Jan Cordel STRATEGIES FOR ENHANCING IRRIGATION EFFICIENCY ON TURFGRASS AREAS Dissertation Universität Osnabrück Hochschule Osnabrück
II Strategies for enhancing irrigation efficiency on turfgrass areas Dissertation zur Erlangung des Doktorgrades Doktor der Naturwissenschaften (Dr. rer. nat) des Fachbereiches Kulturund Sozialwissenschaften der Universität Osnabrück in Kooperation mit der Hochschule Osnabrück Fakultät Agrarwissenschaften und Landschaftsarchitektur vorgelegt von Jan Cordel geboren in Bonn Osnabrück den 05.06.2025 Gutachter*in: Prof. Dr. Gabriele Broll Institut für Geographie Universität Osnabrück Prof. Dr. Rüdiger Anlauf Fakultät Agrarwissenschaften und Landschaftsarchitektur Hochschule Osnabrück Prof. Dr. Bernd Leinauer Extension Plant Sciences New Mexico State University
III Index Chapter 1 General Introduction ........................................................................................ 1 1.1 Background and objectives ................................................................................. 2 1.2 Turfgrass irrigation management ........................................................................ 5 1.3 Research objectives and hypotheses ................................................................ 11 Chapter 2 Scientific publication within the context of this work.................................... 14 2.1 Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions ...................................................... 15 2.2 Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance ..................................................................................................... 38 2.3 Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D ....................................................................... 53 Chapter 3 General Discussion ......................................................................................... 83 3.1 Enhancement of turfgrass rootzone water retention characteristics and water use efficiency .................................................................................................... 84 3.2 Harmonization of rootzone construction method and irrigation delivery system under open field conditions .............................................................................. 87 3.3 Identification of a model-based approach for optimizing irrigation scheduling in turfgrass management ...................................................................................... 89 Chapter 4 Conclusion ...................................................................................................... 92 Summary ......................................................................................................................... 95 Zusammenfassung .......................................................................................................... 98 References ..................................................................................................................... 102 Acknowledgements ....................................................................................................... 118 Annex ........................................................................................................................ 119 Conference contributions as first author ................................................................. 120 Erklärung an Eides über die Eigenständigkeit der erbrachten wissenschaftlichen Leistung .................................................................................................................... 121
IV Abbreviations and acronyms 2A, 2LSTD two-layered rootzone construction method 2B, 2LUHFC two-layered rootzone construction method 3, 3LUHFC three-layered rootzone construction method α soil hydraulic property shape factor for the water retention curve αw soil hydraulic property shape factor for the wetting water retention curve θ(ψ) water content at matric potential ψ ϴr residual water content ϴs water content at saturation ϴsw water content at saturation for the wetting water retention curve CM rootzone construction method CSIL, URTS C coarse sand intermediate layer DFLU drainage flux DG, DS drainage gravel layer DIN deutsches Institut für Normung ET evapotranspiration ETO reference evapotranspiration FLL Forschungsgesellschaft Landschaftsentwicklung und Landschaftsbau FSIL, URTS F fine sand intermediate layer H2D HYDRUS-2D HSRM, STD high silt rootzone mix IS irrigation system 𝐾ψ hydraulic conductivity at matric potential ψ LSRM, UHFC low silt rootzone mix m soil hydraulic property shape factor for the water retention curve MAE mean absolute error NDVI normalized difference vegetation indices n soil hydraulic property shape factor for the water retention curve NSE Nash-Sutcliffe efficiency q subsurface drip irrigation water flux R2 correlation coefficient RMSE root mean square error ROS reactive oxygen species RWS rootzone water storage SDI subsurface drip irrigation Se effective water content SOIL_S soil water storage SPR sprinkler irrigation SWT soil water tension TC turfgrass coverage TQ turfgrass quality USGA United States Golf Association VWC volumetric water content
1 Chapter 1 General Introduction
General Introduction 2 1.1 Background and objectives Turfgrass areas constitute integral components of urban ecosystems, offering a diverse array of ecosystem services, social benefits, and economic advantages, particularly within urban environments (Beard, 1973; Beard and Green, 1994). These include promoting recreational activities, enhancing the aesthetic value of urban landscapes, and supporting carbon sequestration, which is crucial for mitigating climate change (Braun and Bremer, 2019; Qian and Follett, 2012; Selhorst and Lal, 2011). Generally, ecosystem services are divided into four components: provisioning, regulating, cultural, and supporting services (Thompson and Kao-Kniffin, 2017). Provisioning services include products derived from grasses, such as seed and livestock feed (Larson et al., 2016). Established grass swards contribute to ecosystem regulation by reducing urban heat, filtering water, and storing carbon (Monteiro, 2017). In addition, grass-covered areas offer aesthetic and cultural benefits, serving as visually appealing spaces and providing opportunities for recreation (Monteiro, 2017). Finally, grasses support ecosystem functions by enhancing soil formation and nutrient cycling (Kopp and Guillard, 2002). Managing irrigation and fertilization regimes can influence the carbon sequestration rates in turfgrass, highlighting the importance of implementing sustainable practices to maximize these benefits (Braun and Bremer, 2019; Selhorst and Lal, 2011). Carbon sequestration in turfgrass areas typically occurs at rates ranging from 0 to 5.0 Mg C ha-1 year-1, contingent upon management practices, turfgrass species, and environmental conditions (Braun et al., 2023; Braun et al., 2024). For instance, golf courses exhibit carbon sequestration rates between 0.4 and 1.58 Mg C ha-1 year-1, which vary according to their age and the intensity of maintenance (Coyne, 2023). In addition to carbon sequestration, turfgrass provides excellent erosion and runoff control through its dense canopy, root structure, and soilstabilizing properties. It is particularly effective in urban and suburban landscapes, roadsides, and buffer zones near water bodies (Stier et al., 2013). Further, turfgrass plays a crucial role in improving soil quality by enhancing organic matter, stabilizing soil, boosting microbial activity, and reducing erosion and nutrient losses (Braun et al., 2024). In addition to ecological functions, in urban landscapes, the presence of turfgrass enhances their aesthetic value and supports recreational activities, thereby promoting social well-being (Barnes and Watkins, 2023; Philocles et al., 2023). It functions as a crucial surface for sports such as soccer, golf, baseball, and football, offering a non-heatable, injury-protective, and impact-absorbing playing surface that no other vegetation or synthetic turf can fully replicate (Monteiro, 2017). Unlike agricultural systems, where yield is the primary objective, turfgrass management prioritizes year-round usability, vegetation-supportive properties, and aesthetic appeal. These requirements can be contradictory, as optimizing functionality often compromises sustainability (Hejduk et al., 2012). The challenge lies in enhancing functionality while ensuring long-term sustainability, a difficult balance to achieve given that improvements in one aspect may negatively affect the other (Philocles et al., 2023). As urbanization expands and managed turfgrass areas increase, the environmental concerns associated with these systems are expected to intensify. In particular, the emphasis on sustainable and ecologically responsible management practices has grown due to rising concerns about climate change and the increasing threat of water scarcity (Braun et al., 2022; Felipe et al., 2014; Hejduk et al., 2012;
General Introduction 3 Morris and Shearman, 1998). Turfgrass typically requires 25–38 l m-2 of water per week to support optimal growth (Christians et al., 2016). Sprinkler irrigation systems are the established standard for turf irrigation. Still, climatic factors, such as wind drift and technical distribution inaccuracies, lead to low irrigation efficiency and nutrient leaching in the soil matrix, which is usually weak in sorption and water retention (Chartzoulakis and Bertaki, 2015; Fidanza, 2023). In contrast, subsurface drip irrigation (SDI) systems are characterized by direct water delivery into the plant rootzone, which can increase the irrigation system´s efficiency (Leinauer and Makk, 2007) and is already standard in modern land use systems (e.g., intensive vegetable production or urban greens). Nevertheless, these systems experience limited adoption in turfgrass areas due to multiple factors, primarily associated with the lack of understanding of soil–physical relationships and the increased technical complexity of subsurface irrigation compared to sprinkler irrigation. Consequently, the potential for significant water conservation in turfgrass areas remains largely unrealized (Leinauer, 2020). Moreover, the rootzone construction method (multi-layered systems), in conjunction with the associated irrigation system (sprinkler or SDI), constitutes a highly complex design in terms of irrigation water distribution and presents significant challenges for irrigation scheduling regarding the targeted and efficient utilization of water (Stier et al., 2013). Consequently, irrigation remains a critical issue, especially in regions with insufficient precipitation to sustain healthy and visually appealing turfgrass (James, 2011). In this context, the primary objective of this thesis was to evaluate strategies for enhancing irrigation efficiency in turfgrass areas by examining the effects of rootzone construction methods and associated irrigation systems on irrigation efficiency and utilizing predictive numerical models (HYDRUS-2D) to optimize irrigation scheduling in turfgrass management to ensure the targeted and efficient use of water resources. To achieve this, a study was conducted from 2021 to 2024 under both greenhouse and field conditions. The greenhouse study, performed without grass cover, focused on the fundamental physical principles of water movement, whereas the field study employed an identical experimental setup, incorporating turfgrass to assess real-world applicability. Structure of the thesis This dissertation is structured as cumulative work. Individual components have been published in peer-reviewed journals and reproduced as discrete chapters (2.1 – 2.3) in this thesis. Preceding each chapter, supplementary information is provided, including bibliographic data and delineation of individual author contributions. Chapter 1 presents a general introduction to the research topic. The challenges arising from diverse and often conflicting functional requirements of turfgrass areas, as well as the increasing demand for sustainability in these areas, are elucidated. In addition, the underlying factors that contribute to or exacerbate this issue are discussed. Furthermore, this chapter delineates the research questions and study objectives of this thesis, along with the corresponding hypotheses which are formulated at the end of this chapter.
General Introduction 4 In Chapter 2, three peer-reviewed publications are included in subchapters: - Chapter 2.1 deals with the impact of various rootzone construction methods and irrigation systems on soil moisture distribution and retention under greenhouse conditions (without grass cover). Identifying how these factors can be harmonized to enhance irrigation efficiency and develop sustainable, environmentally resilient turfgrass areas was important. The experiment involved twoand three-layered rootzones, irrigated using both sprinkler and subsurface drip irrigation (SDI) systems. - Chapter 2.2 presents the results of a two-year field study conducted with an experimental setup identical to the greenhouse study (Chapter 2.1) but with grass cover. This study evaluated the effects of rootzone construction methods and irrigation delivery systems on water retention characteristics and perennial ryegrass performance under 60% reference evapotranspiration (ETO) deficit irrigation. Various rootzone constructions were investigated using sprinkler and subsurface drip irrigation (SDI) systems. Furthermore, it was essential to evaluate the interactions between rootzone construction and irrigation systems regarding irrigation efficiency; this provided valuable insight that enhancing irrigation efficiency while maintaining functionality necessitates harmonizing rootzone construction methods and irrigation delivery systems. - Chapter 2.3 aims to compare measured field data with predicted data on irrigation water distribution in turfgrass rootzones to verify and enhance the accuracy of the HYDRUS-2D simulation model. For this purpose, data were collected under controlled greenhouse conditions across unvegetated plots with twoand three-layered rootzone construction methods, each receiving water via subsurface drip irrigation (SDI) or sprinkler (SPR). Water content was monitored at various depths and time intervals. The hydraulic soil parameters required for the simulation model were determined through laboratory analysis. To improve model performance, sensitivity analysis and model calibrations were conducted with the overall goal of providing an efficient and reliable tool for optimizing irrigation scheduling in turfgrass management. Chapter 3 presents a general discussion in a broader context of the results obtained from all experimental investigations conducted within the scope of this thesis. Furthermore, the remaining research gap is described in more detail. Finally, Chapter 4 concludes the potential and challenges of strategies to enhance irrigation efficiency in turfgrass areas.
General Introduction 5 1.2 Turfgrass irrigation management The proper irrigation of turfgrass stands as a critical factor for maintaining optimal turfgrass health while achieving aesthetic appeal and functional maintenance across different landscapes, including sports fields, golf courses, and urban green spaces that serve as recreational areas for city residents. Without proper irrigation practices, turfgrass develops drought stress rapidly, producing various severe negative impacts that damage its growth and quality through multiple vital survival and vitality processes. The absence of proper irrigation causes multiple negative effects, including damage to cell structures and physiological processes, inhibited growth leading to developmental stunting, physical changes in grass appearance, and substantial landscape visual deterioration (Serba et al., 2022). Challenges in turfgrass irrigation management Multiple difficulties exist during the irrigation management of turfgrass due to environmental, technological, and operational elements. Turfgrass irrigation control presents multiple difficulties, especially in water-limited areas or those under extreme climatic conditions. The current water scarcity situation, along with climate change effects and other water competing demands, requires irrigation methods for turfgrass to be reassessed. The large water usage allocated to urban turfgrass irrigation demands the implementation of water conservation methods that preserve turfgrass quality (Litvak et al., 2017; Sevostianova et al., 2025). The limited availability of water causes severe difficulties for turfgrass irrigation management because conventional watering methods lead to excessive usage of available water resources. Turfgrass uses a significant amount of urban water distribution, making it an essential target for water preservation projects (Litvak et al., 2017; Sevostianova et al., 2025). The identified problems call for different approaches (Hejl et al., 2023; Sandor et al., 2021; Serba et al., 2024) that focus on reducing water usage but simultaneously maintain professional and recreational turfgrass quality (Serena et al., 2022; Serena et al., 2020a; Serena et al., 2020b). The research demonstrates that water quality plays an essential role in affecting turfgrass development and nitrogen absorption along with its final performance level. The research conducted by Fan et al. (2014) confirmed that using reclaimed water for irrigation resulted in changes to turfgrass growth rates and nitrogen uptake efficiency, yet revealed that low-quality water usage alters the turf system's nutrient patterns. Research conducted over several years proves that infrequent irrigation leads to decreased turfgrass quality which demonstrates the need for proper water quality and quantity (Chang et al., 2013; GómezArmayones et al., 2018). Turfgrass represents one of the most water-consuming agricultural crops since it utilizes major quantities of urban water resources (Haghverdi et al., 2021). Freshwater irrigation requirements create additional water scarcity problems which generate conflicts between agricultural operations, industrial facilities, and municipal water services. In arid regions the heavy application of landscape irrigation products, including turfgrass and ornamental grasses, exacerbates the existing water scarcity problems in these areas (Litvak et al., 2014). The growing shortage of water in urban areas requires turfgrass management to adopt efficient irrigation approaches. These strategies enable the maintenance of healthy turf alongside reduced water consumption and limited water resources relief (Serba et al., 2022).
General Introduction 12 irrigation efficiencies on turfgrass quality and water retention characteristics. The specific research questions, objectives, and hypotheses guiding this study are detailed below. Topic 1: Enhancement of Turfgrass Rootzone Water Retention Characteristics and Water Use Efficiency Research Question: How can the risk of inefficient water usage during turfgrass irrigation be mitigated? Hypotheses: • In general, rootzone construction methods and irrigation delivery systems significantly influence water use efficiency. • Sprinkler irrigation leads to pronounced downward movement of irrigation water followed by rapid drying of near-surface areas and low water use efficiency, particularly in highly permeable rootzones. • The construction method for a well-performing subsurface irrigation system (SDI) should be optimized to provide sufficient capillary action for the homogenous wetting of the rootzone, thereby enabling efficient and targeted use of irrigation water. The hypotheses are examined in detail in Paper 1 (Section 2.1) and Paper 2 (Section 2.2). Topic 2: Harmonization of rootzone construction method and irrigation delivery system under open field conditions Research Question: Does the harmonization of the rootzone construction method and the irrigation delivery system increase irrigation efficiency without adversely affecting turfgrass quality parameters? Hypotheses: • Sprinkler-irrigated variants are hypothesized to exhibit a rapid decline in turfgrass quality under deficit irrigation conditions. • SDI-irrigated variants are anticipated to yield the lowest turfgrass quality when soil physical properties are not considered in the rootzone construction method relative to the applied irrigation technology. Furthermore, it is hypothesized that SDI-irrigated variants employing construction methods designed to facilitate an optimal operational environment for this irrigation technique are expected to exhibit the highest turfgrass quality parameters. • Matching the rootzone construction method and irrigation delivery system is necessary to increase irrigation efficiency in turfgrass areas. The hypotheses are examined in detail in Paper 2 (Section 2.2).
General Introduction 13 Topic 3: Identification of a model-based approach for optimizing irrigation scheduling in turfgrass management Research Question: Are simulations with a numerical model (HYDRUS-2D) an effective tool to optimize irrigation scheduling in turfgrass management and contribute to model-based improvements in irrigation efficiency? Hypotheses: • The complexity of the overall design resulting from the rootzone construction method and associated irrigation delivery system will influence the model’s applicability and necessitate model calibration for achieving high model quality. • In the context of subsurface irrigation, due to the increased complexity of soil physical processes and interactions, a precise parameterization of soil hydraulic properties, coupled with a more sophisticated model calibration procedure, is expected to be necessary to achieve acceptable model qualities. • A model-based approach based on high-accuracy simulation models will facilitate precise analysis within complex designs of rootzone construction methods and the associated irrigation delivery systems and, consequently, enhance irrigation efficiency. The hypotheses are examined in detail in Paper 3 (Section 2.3).
14 Chapter 2 Scientific publication within the context of this work
15 2.1 Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions J. Cordel1, W. Prämaßing1 and R. Anlauf1 1 Faculty of Agriculture Sciences and Landscape Architecture, Osnabrück University of Applied Sciences, Am Krümpel 31, 49090 Osnabrück, Germany Citation: Cordel, J., Prämaßing, W., Anlauf, R. (2024): Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions. Eur.J.Hortic.Sci. 89 (2), 1-14. https://doi.org/10.17660/eJHS.2024/007 Significance of this study: What is already known on this subject? Highly functional materials and construction methods are used for turfgrass areas at the expense of sorption and water retention. Dry periods necessitate artificial irrigation to maintain function and plant growth. Sprinkler irrigation systems are the standard, but harm irrigation water efficiency. Subsurface irrigation systems offer targeted and precise water distribution in the rootzone, but are not widely used in turfgrass areas. What are the new findings? This study shows that a targeted and homogeneous water distribution with a subsurface irrigation system is possible, but soil physical processes and corresponding rootzone construction methods must be considered to achieve the desired effect. What is the expected impact on horticulture? The harmonization of the rootzone construction method and irrigation system improves the irrigation water's efficiency. This study provides corresponding information on which designs the irrigation water efficiency can be increased. Keywords: hybrid irrigation, irrigation water efficiency, soil moisture retention, spatiotemporal soil moisture distribution, sprinkler irrigation, subsurface irrigation, turfgrass areas Author contributions: Jan Cordel: Conceptualization, Investigation, Methodology, Formal analysis, Validation, Visualization, Writing – original draft. Rüdiger Anlauf: supervision, writing – review and editing. Wolfgang Prämaßing: writing – review and editing. Acknowledgements: The authors would like to thank Herbert Pralle and Anne Friederike Borchert for their assistance with statistical data analysis.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 16 Abstract This study investigated the effects of various rootzone construction methods and irrigation systems on soil moisture distribution and retention in sports field soils under greenhouse conditions. The goal was to identify how these factors can be harmonized to enhance irrigation efficiency and develop sustainable, environmentally resilient turfgrass areas. The experiment involved 2and 3-layered rootzones, irrigated using both sprinkler and subsurface drip irrigation (SDI) systems. Eighteen plots were set up in a greenhouse, each receiving 10 L m-2 h-1 of water for 1 or 2 hours via SDI or sprinkler. Soil moisture was monitored at different depths and times to assess the distribution and retention patterns. Results showed that both the rootzone construction and irrigation system significantly influenced soil moisture behavior. Sprinkler irrigation led to a quick moisture increase followed by a sharp decline due to high percolation and low retention. In contrast, SDI caused minimal changes, especially in near-surface areas, due to insufficient upward water movement. The 2-layered SDI setup, even with doubled irrigation, did not enhance capillary rise or humidification in the main rooting zone up to 12 cm depth. However, the 3-layered SDI variant demonstrated a more effective capillary rise, resulting in even moisture distribution and better water retention. At 12 cm depth, the 3-layered SDI setup maintained higher residual moisture than other variants. This suggests that the benefits of SDI are most pronounced in 3-layer constructions or other setups that adequately account for capillary rise in the root zone. The findings indicate that a hybrid approach combining SDI and sprinkler systems could effectively meet the dual requirements of sports fields: sufficient near-surface humidification for turfgrass establishment and efficient water resource utilization. Graphical Abstract
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 17 Introduction Sports turf areas are unique parts of urban ecosystems with strongly divergent requirement profiles. The focus here is not on yield, as in agriculture, but on providing functional cross-season sports, vegetation-favorable properties, and aesthetically pleasing turf. The users and operators’ focus are on the season-spanning usability of the sports turf areas with correspondingly high functionality and aesthetics. The resulting requirements are sometimes contradictory, and factors optimized for functionality are at the expense of these systems’ sustainability (Hejduk et al., 2012). National and international technical standards define sports turf surfaces corresponding to soil-mechanical and physical properties, e.g., by German Institute for Standardization (DIN18035-4, 2018), Landscape Development and Landscaping Research Society (FLL, 2008), United States Golf Association (USGA, 2018). The increase in dry summers due to climate change and scarcity of water resources has led to a substantial increase in the focus on the sustainability of sports turf. The current situation makes the conversion or adaptation of these systems inevitable. Future-oriented sports turf areas should be configured with a holistic view of the relevant legal, economic, and ecological requirements, adapted and implemented according to the climatic location and the user’s requirements profile. Climatic conditions and the frequentation of sports turf areas cause a strain on the plant population and a change in soil structure. Therefore, maintenance management adapted to the external conditions and influences is crucial for the functional preservation of turf sports fields. The use of potable water for turfgrass irrigation, essential for basic maintenance, has invited critical discussions. Optimal growth of established turf typically necessitates an average water intake of 25 to 38 mm per week, primarily due to evapotranspiration (ET) (Christians et al., 2016). Balancing any discrepancies between precipitation and ET is crucial to maintaining vital growth. The focus here is on the interaction between the irrigation system and rootzone construction method, significantly influencing soil moisture distribution and retention. Factors such as the water application method (e.g., surface or subsurface) and distribution uniformity of the irrigation system, along with soil physical properties such as the content of plant available water in the rootzone, markedly affect these dynamics. Understanding these complex behavioral patterns within irrigation management is crucial to prevent overwatering and to ensure efficient water resource utilization. The worsening water scarcity has spawned innovative approaches, especially in recent years, aimed at sustaining turf sports fields into the future. Sprinkler irrigation systems are the standard for turf irrigation. However, climatic factors like wind drifts and technically induced distribution inaccuracies may cause low irrigation efficiency and intensive percolation and nutrient leaching in the soil matrix, which is weak in sorption and water retention (Chartzoulakis and Bertaki, 2015; Fidanza, 2023). There are several strategies to reduce/eliminate potable water use for turf irrigation. One starting point is subsurface water delivery in the rootzone of turfgrasses, which can cause increased irrigation system efficiency (Serena et al., 2020b). In modern land use systems (e.g., intensive vegetable production or urban greens), irrigation systems with water delivery in the immediate rootzone (subsurface irrigation) are already a standard (Lamm et al., 2012). However, these systems experience only shallow acceptance on sports turf fields due to numerous factors, related to the ignorance of soil-physical relationships and the higher
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 18 technical complexity of subsurface irrigation, vis-à-vis sprinkler irrigation. Many elementary and complex soil physical relationships can be identified in the soil matrix in multi-layered rootzone construction methods. Developing strategies for increasing irrigation efficiency requires a holistic analysis of the soil construction and the irrigation technology used. Mainly related to subsurface irrigation, the potential of significant water savings on sports turf fields remains mostly untapped (Leinauer, 2020). The actual efficiency of an irrigation technology is determined by numerous climatic, technical, vegetation-related, and soil physical factors. The soil matrix, with the central function of water retention (making it available to plants) and water transport (draining off excess water to maintain durability and playability), is an elementary factor here. Synthetic turf pitches requiring less maintenance and care have long been preferred in urban areas and are widely used (Felipe et al., 2014; Fleming et al., 2023). Due to better playability and economic importance, however, the current focus is increasingly on grass or hybrid turf pitches (McCarty et al., 2016). Therefore, sustainability and ecological maintenance management have acquired urgency (Itten et al., 2020; James, 2011). There is now intense pressure to address climatic changes, particularly to prepare for impending water shortages (Christians et al., 2016; Johnson et al., 2013; Turgeon and Fidanza, 2017). Diverse aspects of playability, aesthetic requirements, efficiency, and sustainability should be integrated into an all-round compatible compromise (Steinke and Ervin, 2013; Straw et al., 2020). Turf construction methods, irrigation systems, and management are, therefore, crucial in terms of sustainability (Follis et al., 2009; A Kowalewski et al., 2015; Alec Kowalewski et al., 2015; McCoy et al., 2007). Irrigation systems with high distribution accuracy and efficient water application can provide the basis for vital turf growth and the sustainable use of water resources (Stier et al., 2013). An efficient irrigation system should minimize losses from wind drift, surface runoff, percolation, and evaporation (Carrow et al., 2002). Sprinkler irrigation systems using rotary, multi-jet, and spray sprinklers are the standard on turf sports fields. In contrast, subsurface drip irrigation (SDI) systems are characterized by direct water delivery into the plant’s rootzone. The typical rooting depth of cool-season turfgrasses is subject to seasonal variations (Turgeon, 1991) and ranges between 5 and 15 cm (Landschoot, 2018). The advantages of subsurface irrigation are the unrestricted usability of the area during the irrigation process, reduced disease pressure due to less moisture penetration of the turf, and efficient near-root water delivery uninfluenced by evapotranspiration and wind drift (Beard, 1973; Carrow et al., 2008; Leinauer, 1998). The disadvantages of SDI are higher installation costs, difficulty in germinating turfgrass seedings (Leinauer and Makk, 2007), limited soil aeration work and incompatibility with surface applied products, e.g., granular fertilizer (Suarez-Rey et al., 2000). Higher discharge rates of irrigation systems (sprinkler or SDI) tend to increase vertical spreading more than horizontal spreading because the ratio of gravitational forces to capillary forces increases with higher amounts of water. Horizontal and vertical soil wetting from the drip application of a given volume of water will be determined by soil hydraulic properties depending on the texture of the soil and will
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 19 not be significantly impacted by drip management parameters such as discharge rate and pulsed water application (Skaggs et al., 2010). Under drip irrigation, the vertical wetting front advances faster in sandy soil than in clayey soil, but the horizontal wetting front in clay soil advances more vis-à-vis sandy soil due to higher capillary forces in the smaller pores in clay (Bajpai and Kaushal, 2020). Sanddominated rootzones are widely used for high-quality sports facilities because of their good drainage properties, greater air-filled pore space after compaction, and more consistent playing characteristics (Baker, 2006). The particle size distribution also has a major influence on the physical properties of a rootzone mix, and a compromise is required between the advantages of coarser sand for hydraulic conductivity and air-filled pore space and finer sands for moisture retention and stability. Uniform grain-size sand is associated with better drainage and greater total pore space but at the expense of stability (Baker, 2003). To increase waterholding capacity, rootzones are amended with peat (Leinauer and Makk, 2007), and soil surfactants enhance moisture retention in turfgrass rootzones (Leinauer and Makk, 2007; Leinauer et al., 2001). Water retention and water movement issues primarily influence decisions about sports turf construction methods (Bigelow and Soldat, 2013). Construction methods for sports turf are of three types: a) sport turf pitches based on natural soil with no technical drainage or just essential pipe drainage, b) sand carpets–typically 100–150 mm of sand, or a sand-dominated rootzone over the native soil, c) suspended water tables, i.e., 300 mm of rootzone material over a gravel drainage layer of 100–150 mm, and often an optional coarse sand intermediate layer (choker layer) of 100 mm between the rootzone material and the gravel (Baker, 2006). The most widely used method, including by the USGA, is the suspended water table. The gravel drainage layer supports rapid water movement to the drainage lines, while the choker layer prevents particle migration of fine sand into the gravel layer (Stewart, 2004). The soil texture difference between the gravel and the rootzone mix creates a capillary break due to the abrupt pore size difference between the gravel and the rootzone layer to enhance soil moisture retention in the rootzone (McCarty et al., 2016). Water retention in the rootzone layer is determined by the physical properties of the rootzone material used, its depth, the presence of an intermediate layer, and the coarseness of the underlying gravel-drainage layer (Baker and Binns, 2001). Brown and Thomas (1980) reported that the total available water in a 300 mm rootzone was lower when an intermediate layer was included, and the highest when the rootzone was directly installed over a graveldrainage layer. Baker and Binns (2001) found that the amount of fine material in the intermediate layer does not affect the rootzone water retention ability but significantly influences the intermediate layer’s water retention, which increased when sand (grid 0.5–1.0 mm) was added to a 1–4 mm grid. Taylor et al. (1993) calculated the water retention in different root zone mixes packed over four different sub-layer designs and found that in root zone mixes where the sand particle size is predominantly 0.25–1.0 mm in diameter, different sub-layers increased the water retention at the bottom affected the root zone mix (250–300 mm depth), while no significant difference in water retention could be detected in the center (125–175 mm depth) and near the surface (0–50 mm depth).
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 20 Moreover, a rootzone’s highest water retention ability was detected in rootzone materials where sand (grid 0.1–0.5 mm) was fine and medium-sized. The commonly used highly functional construction methods of a sports turf area have adverse effects on the efficiency of an irrigation system. Depending on the structure and physical properties of the construction materials, different soil moisture distributions can be observed, irrespective of the irrigation system (Leinauer and Makk, 2007; Stier et al., 2013). Till now, how different rootzone construction methods and their associated irrigation systems, and occurring interactions, affect soil moisture distribution and how these may affect irrigation water efficiency, has not been investigated in detail. To address this knowledge gap, this study evaluated different rootzone construction methods of sports turf areas and associated irrigation systems concerning soil moisture distribution and retention. The goal was to investigate the basic physical principles of water movement under greenhouse conditions in unvegetated experimental plots. Interactions between the experimental factors were analyzed to provide conclusions for the effective and targeted use of water resources for turfgrass irrigation. The construction method and irrigation system are hypothesized to impact soil moisture distribu-tion and retention strongly. Particularly in the area of subsurface irrigation, it is expected that the construction method for a well-performing SDIsystem should be optimized to provide sufficient capillary action for homogenous humidification of the rootzone and thereby enable efficient and targeted use of irrigation water. Materials and methods Experimental setup This study was conducted under greenhouse conditions using bare soil profiles (plots with no grass cover) to investigate how different rootzone construction types and associated irrigation systems impact soil moisture distribution and retention. The research area (11.94 m x 4.71 m) was designed as a completely randomized 2-factorial split plot with three replications for each treatment and comprised six main plots. Each main plot (4.11 m x 1.70 m) was repeated thrice employing three construction types (two 2-layered, one 3-layered design) and two different irrigation systems (sprinkler and SDI). Each of the six main plots was subdivided into three plots (1.70 m x 1.37 m) for the three replications. Two different irrigation cycles (discharge amount cycle 1 = 10 L m-2, and 2 = 20 L m-2) were applied. After irrigation cycle 1, all plots underwent a drying phase (about four weeks with an average evaporation rate of 3.22 mm day-1) to achieve a uniform initial soil volumetric water content. The initial volumetric water content was between 9 and 10 Vol.-% without any significant differences between the plots. A total of 5 materials–STD, UHFC (rootzone layer), URTS F (intermediate layer), URTS C, and DS (gravel drainage layers) were used for the three construction types. Table 1 lists the physical properties of each material.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 21 STD is generally used for sports turf areas with a common requirement for functionality and sorption capability, consisting of sand, peat, and humus-rich topsoil. The material UHFC (ultrahigh-function), also consisting of sand and peat but without topsoil, has high functionality at the expense of sorption capability with a lower silt content compared to STD, and a higher hydraulic conductivity but water holding capacity (field capacity) similar to STD. URTSF is an intermediate layer material consisting of medium-sized sand (grain size up to 0.5 mm) without silt, with a much higher hydraulic conductivity and lower water holding capacity (fc) compared to STD and UHFC. DS is a gravel drainage layer material with a particle size distribution of 0–8 mm, higher hydraulic conductivity, and lower silt content than the rootzone materials (STD and UHFC). URTS C is a much finer drainage material consisting of very coarse sand with a grain size of up to 2 mm, much higher hydraulic conductivity, and lower bulk density than DS. Table 1 Overview of materials used and their associated physical properties Material Physical Properties Texture *Grid Bulk density **Ksat ***Pore Space ****Field Capacity Gravel Sand Silt Clay (-) (%) (mm) (g cm-3) (mm h-1) (%vol) (%vol) STD (-) 89.58 10.42 (-) 0–2 1.55 220 41.5 15.9 UHFC (-) 98.27 1.73 (-) 0–2 1.46 535 44.9 13.6 URTS C (-) 99.78 0.22 (-) 0.2–2 1.60 6,081 39.6 4.6 URTS F (-) 99.58 0.42 (-) 0.1–0.5 1.41 1,465 46.6 6.4 DS 31.54 66.10 2.36 (-) 0–8 1.80 916 32.1 6.5 Notes. * particle size distribution was determined according to (DIN EN ISO, 2017) ** hydraulic conductivity Ksat was determined in the laboratory using an Eijkelkamp-Permeameter (Eijkelkamp, 2017) *** pore space was determined from particle density using gaspycnometry (DIN, 2019) **** field capacity was determined by hanging water column in a sand bed according to (DIN EN ISO, 2020) The construction layers had a total thickness of 24 cm, consisting of rootzone layers (0–12 cm), gravel drainage layer (12–24 cm) for the 2-layered constructions, intermediate layer (12–18 cm) and drainage layer (18-24 cm) for the 3-layerd constructions, and plus a 26 cm sand carpet base with a grain size up to 2 mm for surface drainage. Three rootzone construction methods were applied. Two construction types (2LSTD and 2LUHFC) according to the German Standards for Sports Grounds (DIN, 2018) consisted of a 2layered design, including a 12 cm peat-amended sandy rootzone (STD or UHFC) over a 12 cm gravel drainage layer (DS) with a total thickness of 24 cm. One construction method comprised a 3-layer design based on the guidelines of the United States Golf Association (USGA, 2018) (3LUHFC) which included a 12 cm rootzone (UHFC) over a 6 cm intermediate sand layer (URTS F) and an underlying 6 cm drainage layer (URTS C). Figure 1 provides an overview of the construction types, associated materials, and irrigation systems. In each case, all materials
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 28 Within the 2-layer variants, doubling the irrigation quantity did not cause a more pronounced capillary rise of irrigation water or higher humidification of the observation area. Contrary behavior was exhibited by the 3-layer variant 3LUHFC, with a more pronounced capillary rise of irrigation water. Further, the graphically processed maps indicate homogeneous moisture penetration at 12 hours in combination with sustained irrigation water retention in the soil matrix until the end of the test. Irrespective of the irrigation amount, the graphically prepared maps indicate an insufficient capillary rise of irrigation water for humidification of near-surface areas in the SDI-irrigated plots.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 29 Figure 3 Spatiotemporal soil moisture distribution of sprinklerand SDI-irrigated plots within irrigation cycle 1. The filled circle shows the position of the SDI-System.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 30 Figure 4 Spatiotemporal soil moisture distribution of sprinklerand SDI-irrigated plots within irrigation cycle 2. The filled circle shows the position of the SDI-System.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 31 Water balance The net water values stored in the top 15 cm of the soil profile for each irrigation cycle are presented in Table 3 as a function of the difference between the initial amount (Observation time 0) and the amount at each observation time. Table 3 Calculated net water values (mm) stored in the top 15 cm of the soil profile based on the averaged VWC (%vol) at observation depths of 3, 6, and 12 cm per irrigation cycle and observation time. Means followed by a common letter (separately for each observation time, discharge rate and irrigation system) are not significantly different based on Tukey´s HSD test (p < 0.05) *IC ***ISYS **CM OBSERVATION TIME (hrs) 0 4 12 24 48 72 (L m-2) (-) (-) (hrs) 10 SPR 2LSTD 0 12.29 b 5.66 b 5.83 b 3.73 b 2.53 b 2LUHFC 0 10.75 a 4.94 a 3.30 a 2.14 a 1.16 a 3LUHFC 0 10.49 a 7.28 b 6.31 b 3.96 b 3.00 b SDI 2LSTD 0 3.72 a 1.84 a 1.07 a -0.05 a 0.48 a 2LUHFC 0 5.87 b 2.91 b 1.70 a 0.55 a 0.82 a 3LUHFC 0 7.33 c 7.55 c 6.15 b 4.94 b 3.90 b 20 SPR 2LSTD 0 14.73 b 13.73 b 13.47 b 9.08 c 7.19 a 2LUHFC 0 12.54 a 12.92 a 11.45 a 6.04 a 6.67 a 3LUHFC 0 14.18 b 13.01 a 12.27 ab 7.66 b 6.76 a SDI 2LSTD 0 5.98 a 2.99 a 3.19 a 1.84 a 1.01 a 2LUHFC 0 5.74 a 4.42 b 3.59 a 3.27 b 3.55 b 3LUHFC 0 10.18 b 10.46 c 9.67 b 8.68 c 6.82 c Notes. *IC Irrigation Cylec **CM Construction Method ***ISYS Irrigation Sytem Water balance results vary greatly depending on soil construction, particularly the irrigation system used. Irrigation cycle 1 (10 L m-2) increases the amount of water stored in the upper 15 cm soil profile of 10.5–12.3 mm after 4 hours for the sprinkler irrigation variants. In contrast, the SDI-irrigated variants show an increase between 3.7 and 7.3 mm. Irrigation cycle 2 (20 L m-2) increased the values to 12,5–14.7 mm (sprinklers) and 5.7–10.2 mm (SDI). In particular, the results of the 2-layer SDI irrigation variants indicate that when the irrigation amount doubles (10 to 20 L m-2), the total amount of water stored in the top 15 cm increases only slightly. Also of note is the slight increase (0–4 hours) and decrease (4–72 hours) in the SDIirrigated variants compared to the sprinkler-irrigated variants. From an observation time of 12
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 32 hours, the 3-layer SDI-irrigated variant 3LUHFC_SDI shows significantly higher (p < .01) values within both irrigation cycles than the 2-layer SDI-irrigated variant. Looking at the results at the end of the experiment within cycle 1, variant 3LUHFC_SPR shows significantly higher (p < .05) values than variant 2LUHFC_SPR and not significantly higher values than 2LSTD_SPR. In contrast, within cycle 2 there are no significant differences. The results of the 2-layer variants also show that higher water volumes can be determined within both irrigation cycles when using sprinklers instead of SDI (Figure 5). In addition, with subsurface irrigation, the highest values can be achieved with a 3-layer root zone construction (3L_SDI). The results also indicate the beneficial effect of the 3-layer sprinkler irrigation variant (3L-SPR vs. 2L_SPR), particularly within irrigation cycle 1. The differences between the net water and the irrigated amounts are due to percolation and evaporation losses (Figure 5). Figure 5 Summarized net water values (mm) stored in the top 15 cm for all observation depths (3 to 12 cm) and observation times (4 to 72 hours) of the 2-and 3-layer sprinkler-irrigated (2L_SPR, 3L_SPR) and SDI-irrigated variants (2L_SDI, 3L_SDI), separately for irrigation cycle 1 (10 L m-2) and irrigation cycle 2 (20 L m-2)
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 33 Discussion Rapid drainage is essential for all sports fields. It quickly removes large amounts of water and prevents the cancellation of games due to waterlogging and unstable playing surfaces. However, the rootzone construction method and the associated irrigation system should support a homogeneous soil moisture distribution and sufficient soil moisture retention for supplying grass plants. The results indicate that the investigated variants have significantly different characteristics concerning soil moisture distribution and retention. The material composition, the rootzone construction method, and the irrigation system used are the determining factors. The previously described and analyzed results indicate that the rootzone construction method particularly significantly influences soil moisture distribution and retention. Temporal change of soil moisture and soil moisture retention The present results generally show high percolation rates within the 2-layered sprinklerirrigated plots, along with low soil moisture retention in the SDI-irrigated plots. The 2-layer variants exhibit excellent drainage properties of elementary importance for maintaining the function and playability of turfgrass areas, especially during heavy rain, but have only low retention properties. This indicates a high percolation rate due to the materials' physical properties (coarse soil texture and high saturated hydraulic conductivity), and the findings agree with others' Fidanza (2023) and Chartzoulakis and Bertaki (2015). Conversely, despite the difference in particle size distribution between the layers 1 and 2 (STD and DS or UHFC and DS), it was impossible to achieve irrigation water retention in the observed area. Taylor et al. (1993) reported that the highest retention was obtained when the rootzone was placed directly on the gravel layer due to a capillary break on account of the abrupt pore size difference between the gravel and the rootzone layer. Our results do not align with this, which is potentially due to the different texture and composition of the gravel layer DS which, unlike the material used by Taylor et al. (1993), does not consist of 100 % gravel (grid > 2 mm) but has 31.5 % gravel, 66 % sand, and 2.5 % silt content (grid 0–8 mm). In terms of the construction method, it can be observed that either a high sand content or insufficient gravel content in the DS Layer leads to increased pore continuity with the overlaying rootzone layer (STD, UHFC). This, in turn, enables higher rates of percolation and infiltration. Contrastingly, the 3-layered variants show the favorable effects of an intermediate layer on soil moisture retention, especially in SDI-irrigated plots. The results of the 3-layer SDI-irrigated variant indicate high water retention of the intermediate layer URTS F as well as a homogeneous distribution of the SDI-irrigation water within the layer. Due to the texture and the dominating high proportion of fine and medium sand (grain size 0.1–0.5 mm), these material properties have a favorable effect on soil moisture retention, as well as on the horizontal water movement which concurs with the findings of Bajpai and Kaushal (2020) and Baker and Binns (2001), who could achieve improved retention properties of the intermediate layer by adding 0.1–0.5 mm fine sand. Further, the significantly higher soil moisture retention in the intermediate layer indicates a capillary break between the upper URTS F and the lower URTS C layers. The remaining soil moisture at the end of the trial within both irrigation cycles indicates the highest soil moisture retention, compared to the other variants. The lack of fine
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 34 sand in the URTS C layer (grain size 0.2–2 mm) is potentially responsible for this observation, as it leads to a limited pore continuity within both layers (URTS F-URTS C) and a correspondingly higher retention behavior in the intermediate upper layer. Similarly, the high proportion of fine and medium sand in the rootzone UHFC causes high capillary activity, which, on the one hand, allows a corresponding increase of irrigation water in the upper areas and, on the other, also causes more pronounced retention properties than in the comparison 2layer variants. This aligns with the results of Taylor et al. (1993), demonstrating the highest retention properties for rootzone mixes with a dominant 0.1–0.5 mm proportion, but cannot be confirmed for the 2-layer rootzone construction method 2LUHFC. Presumably, the improved retention properties of the upper rootzone can have an effect only if there is a corresponding reduced percolation or infiltration behavior caused by capillary break against gravity pull between adjacent layers. However, from the present study, it is clear that although high capillary activities occur in the 3-layer SDI-irrigated variant, a force imbalance in favor of the gravitational force occurs above an observation depth of 3 cm, preventing a further capillary rise of the irrigation water. The resulting lack of moisture penetration in near-surface areas in SDI will lead to difficulties in germinating turfgrass seeding Leinauer and Makk (2007) and will strongly impair the flushing in of surface-applied products (e.g., granular fertilizer and herbicides). Spatiotemporal soil moisture distribution and water balance SDI systems use irrigation water more targeted and efficiently for turfgrass irrigation than the established sprinkler irrigation (Fidanza, 2023). However, the water balance results indicate that the amount of stored water in the upper 15 cm within the 2-layer construction tends to be higher in the investigated area with sprinkler-irrigation than SDI-irrigation (Figure 5). The results are confirmed by the spatiotemporal soil moisture distribution presented in Figures 3 and 4, which indicate that the installation of the SDI system in a material (DS) that does not have a corresponding capillary action leading to a rapid movement of the irrigation water to deeper soil zones where it is not plant-available. Further, the results reveal that higher irrigation amounts (10 to 20 L m-2) cannot offset this fact and confirm the results of Skaggs et al. (2010) that an increasing irrigation quantity leads only to more pronounced downward irrigation water spreading. This fact indicates that a general statement regarding an increase in efficiency using an SDI system vis-à-vis a sprinkler system does not apply to every rootzone construction method. Effective management of capillary tension in the soil matrix surrounding the SDI system (URTS F) was found to be critical. Ensuring high capillary tension together with an optimized soil texture characterized by a high proportion of small pores in the upper root zone (UHFC) is essential. These factors are crucial in enhancing soil's capillary forces against the force of gravity and allowing pronounced horizontal and vertical movement in the upper rootzone of irrigation water. The spatiotemporal results visualized the previously described facts and confirmed the interpretation in the results section. Particularly, the homogeneous water distribution and pronounced retention of the 3-layer subsurface-irrigated variant 3LUHFC compared to the other variants regarding the efficient use of the resource water for turfgrass irrigation comes into focus. Further, the homogeneous soil moisture distribution of the 3-layer subsurface-irrigated variant provides the basic requirements for a uniform turfgrass quality aspect.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 35 Generally, the results indicate good drainage properties of the investigated 2-layer and 3-layer rootzone construction methods due to the rapid drying near the surface and low moisture accumulation in this zone. However, this can cause rapid drought stress to the plants, especially during dry weather, and irrigation seems unavoidable for maintaining the functionality and aesthetics of the turfgrass. Limitations of the study and future research As these results were determined under controlled greenhouse conditions without turfgrass cover, the effect of turfgrass on soil moisture distribution and retention requires further field investigation. An interpretation of the results from a functional perspective of the turfgrass area indicates that the results of this study cannot be used to decide whether the required higher capillary rise of the finer materials can lead to waterlogging problems during heavy rainfall, but should be a further concern for research. Additionally, in the sense of an efficient, targeted use of the resource water for turfgrass irrigation, on the one hand, a harmonization of the rootzone construction method and the associated irrigation system is necessary and a hybrid irrigation approach (SDI and sprinklers) seems advantageous for fulfilling the entire requirement profile for turfgrass irrigation. Conclusion Durable and sustainable turfgrass areas are essential for maintaining the playing activity of numerous amateur and professional sport clubs. However, like all other ecosystems, they are under increased pressure to deal more intensively and specifically with water shortages caused by climate change. Here, maximizing irrigation water efficiency is the most significant future challenge and should ideally occur before being mandated by law. However, an increase in efficiency should consider all the requirements of users and managers to ensure the future acceptance of a turfgrass area. The present results indicate that increasing irrigation water efficiency while maintaining functionality seems possible but requires a harmonization of the rootzone construction method and the associated irrigation system. Especially in the case of SDI, complex soil physical processes and the rootzone construction methods derived therefrom must be considered to ensure efficient use of the resource water. Further, it was shown that the rootzone construction methods adapted to subsurface irrigation allow a more targeted use of water, vis-à-vis the established standard of sprinkler irrigation. However, deficits must be expected here too, particularly due to insufficient capillary moisture rise to near-surface areas. The results indicate that a hybrid irrigation approach (SDI and sprinkler) could be an adequate solution to meet a sports turf area's entire requirement profile and increase irrigation water efficiency.
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 36 SUPPLEMENTAL INFORMATION-Table S1 RESULTS of ANOVA testing the effects of different construction methods, irrigation systems, discharge rates, observation depths, observation times, and their interactions on the volumetric soil moisture content Main Effects Discharge rate OBSERVATION DEPTH (cm) 3 6 12 OBSERVATION TIME (hrs) 4 12 24 48 72 4 12 24 48 72 4 12 24 48 72 (-) (L m -2) * ** *** ** NS * *** *** *** *** *** *** *** *** *** Construction Method 10 * * * * NS *** ** ** ** *** *** *** *** *** *** 20 * * * * NS *** ** ** ** *** *** *** *** *** *** Irrigation System 10 * NS * NS * * NS NS NS NS NS NS NS NS NS 20 * * * * * * * * * NS NS * * NS NS Construction Method x Irrigation System 10 * NS *** ** * * ** *** ** ** *** *** *** *** *** 20 ** * * ** ** ** *** ** *** *** *** *** *** *** *** Note. NS not signifikant * Significant at the .05 probability level ** Significant at the .01 probability level *** Significant at the .001 probability level
Impact of rootzone construction and irrigation methods on soil moisture in sports fields under greenhouse conditions 37 SUPPLEMENTAL INFORMATION-Table S2 Average initial VWC at 3, 6, and 12 cm observation depth IRRIGATION CYCLE CONSTRUCTION METHOD IRRIGATION SYSTEM MEAN SD (-) (-) (-) (%) (%) 1 2LSTD SPR 9.18 0.74 SDI 9.24 0.45 2LUHFC SPR 9.44 0.93 SDI 9.46 0.75 3LUHFC SPR 9.62 0.81 SDI 10.01 1.04 2 2LSTD SPR 9.18 0.30 SDI 8.60 1.05 2LUHFC SPR 8.82 0.41 SDI 8.07 0.82 3LUHFC SPR 9.29 0.33 SDI 8.55 1.16
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 44 accumulated evapotranspiration. These periods were selected based on the typical climatic conditions at the experimental site, which are characterized by low rainfall and high evapotranspiration. Irrigation was provided on Monday, Wednesday, and Friday at 8:00 a.m. using either a sprinkler (sprinkler main plot) or a subsurface drip irrigation system (subsurface drip main plot). Irrigation amounts were determined based on the previous days reference evapotranspiration ETO (FAO Penman-Monteith method), as estimated by an on-site weather station (iMetos 3.3, Pessl Instruments, Weiz, Austria). Precipitation of 12.80 mm in 2023 and 16.30 mm in 2024, respectively, were subtracted from the irrigation quantity applied in each case (Table 2). Table 2. Irrigation scheduling, cumulative reference evapotranspiration (ETO), precipitation, and amount of irrigation water applied during 60% ETO deficit irrigation treatments, across the study periods 2023 and 2024 Period Date IRRIGATION SCHEDULING CUMULATIVE VALUES Quantity Frequency Duration ETO Precipitation Irrigation (-) (-) (% ETO) (-) (weeks) –––––––––––– mm ––––––––––––– 2023 06/26/23 – 07/31/23 60 Mon. – Wed. – Fri. 5 110.6 12.8 53.6 2024 06/24/24 – 07/29/24 60 Mon. – Wed. – Fri. 5 114.9 16.3 52.6 Turfgrass performance evaluations During the experiment, several turfgrass performance parameters were evaluated at the beginning of each period in 2023 and 2024, followed by weekly intervals. Turfgrass quality and living ground cover were assessed using a visual rating scale recommended by the National Turfgrass Evaluation Program (Krans & Morris, 2007). Visual turfgrass quality was evaluated on a scale of 1 to 9 (quality and color), where 1 = completely brown, 6 = acceptable, and 9 = optimum quality, color, density, and uniformity. Turfgrass coverage was assessed by visual estimation on a 0 – 100% scale based on the percentage of surface area covered with originally planted living species, according to the European Standard (EN12231, 2003). Normalized difference vegetation indices (NDVI) were collected on the same schedule to quantify stress (Park et al., 2004) using a GreenSeeker NDVI handheld crop sensor (Trimble, Sunnyvale, CA, USA). Additionally, drone image data were acquired at the end of each period using a DJI Mavic 3 Multispectral (DJI, Shenzhen, China) and analyzed using PIX4D fields version 2.8.4. Analysis of soil water tension and irrigation water storage Soil water tension, represented as an absolute value of the matric potential in non-saline soils (Shock & Wang, 2011), was used to analyze irrigation water movement. The corresponding matric potential was permanently measured using soil-water tension sensors. For this purpose, 72 Watermark WM-S-100 matrix potential sensors (Irrometer Company Inc., Riverside, CA, USA) were installed (nine per main plot) at a depth of 8 cm to describe the water tension at the main rooting depth. The calculation of irrigation water storage in the upper 12 cm rootzone layer was based on the soil volumetric water content (VWC) observed in the field
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 45 plots at 0–4, 4–8, and 8–12 cm depths. A soil sampler was used through lateral openings with a 50 mm diameter metal tube driven into the soil and withdrawn to obtain soil samples. Soil gravimetric water content was measured by oven-drying the soil samples at 105 °C, and VWC was calculated by multiplying it with the bulk density. All data were collected on the same schedule as the turfgrass performance evaluation (i.e., once per week). Data analysis Statistical analyses were performed based on a 2-factorial split-plot design using RStudio version 1.2.5042. Differences among treatments for each parameter were determined via twoway ANOVA (F1 = rootzone construction method, F2 = irrigation system, and years as a fixed factor) with the function “lmer” using package “car”. All data fulfilled the criteria of a normal distribution, homogeneity of variance, and sphericity. Tukey’s contrasts were calculated for pairwise comparison at the 5% level using the function “cld/emmeans” in package “multcomp” (Piepho, 2004). Results Under 60% ETo deficit irrigation, ANOVA revealed significantly different behaviors and interactions across the study periods in 2023 and 2024. The construction method, irrigation system, and construction method × irrigation system interaction were all statistically significant for soil water tension, rootzone water storage, turfgrass quality, turfgrass coverage, and NDVI (Tables 3, 4, and 5). Development of soil water tension In a strictly physical sense, the matrix potential is negative. This study used absolute values for the matrix potential, expressed as soil water tension (SWT). Therefore, a high SWT indicates dry soil, whereas a low SWT indicates wet soil. Table 3. Development of soil water tension (8 cm depth) and analysis of variance under 60% ETO-deficit irrigation across the two five-week study periods. MAIN EFFECTS Days after study initiation 0 7 14 21 28 35 –––––––––––––––––––––––––––––––––––– kPa ––––––––––––––––––––––––––––––––––––––– CM IS 2A SPRINKLER 6.1 a 7.2 b 8.6 b 11.4 b 17.9 b 25.1 b 2B SPRINKLER 6.1 a 6.9 b 8.3 b 11.0 b 18.7 b 25.5 b 3 SPRINKLER 6.0 a 7.5 b 8.5 b 8.5 d 15.5 bd 23.3 b 2A SDI 6.2 a 18.1 a 40.8 a 67.2 a 141.8 a 161.9 a 2B SDI 6.0 a 19.6 a 33.8 c 56.3c 87.7 c 119.8 c 3 SDI 6.1 a 6.4 b 6.9 b 8.3 d 11.2 d 12.5 d ANOVA ––––––––––––––––––––––––––––––––––– p > F –––––––––––––––––––––––––––––––––––––– CONSTRUCTION METHOD (CM) NS *** *** *** *** *** IRRIGATION SYSTEM (IS) NS ** *** *** *** *** YEAR NS NS NS ** NS NS CM x IS NS *** *** *** *** *** CM x YEAR NS NS * *** NS NS IS x YEAR NS NS NS ** NS NS Means in the same column followed by a common letter are not significantly different based on Tukey´s honestly significant difference test (p < 0.05) Note. NS not significant * Significant at the .05 probability level ** Significant at the .01 probability level *** Significant at the .001 probability level
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 46 Across the five-week study periods in 2023 and 2024, the average soil water tension within all plots at 8 cm depth initially measured 6 kPa and subsequently increased to an average of about 25 kPa after 35 days of 60% ETO deficit irrigation across all sprinkler irrigated variants. In contrast, the 2-layered subsurface drip irrigated variants 2A-SDI and 2B-SDI became much dryer, and the tension reached 162 kPa and 120 kPa, respectively. The 3-layered SDI irrigated variant 3-SDI showed a relatively low increase, with soil water tension reaching only 13 kPa. A marked increase in soil moisture tension was observed starting at 21 days post-study initiation across the sprinkler-irrigated variants 2A and 2B, whereas the SDI-irrigated variants 2A and 2B exhibited a similar increase starting at 14 days. The 3-layered SDI variant showed a very small increase, only starting after 28 days (Table 3). Rootzone water storage Generally, significantly different patterns of rootzone water storage (RWS) could be observed during the study periods in 2023 and 2024. In the upper 12 cm, the initial RWS averaged 19.64 mm. A 35-day lasting 60% ETO deficit irrigation treatment caused a substantial decrease in RWS across all variants, with the significantly highest decrease observed within the sprinklerand 2-layered subsurface drip irrigation-irrigated variants and the significantly lowest decrease within the 3-layered SDI irrigated variant 3. Table 4. Development of rootzone water storage in the upper 12 cm and analysis of variance under 60% ETO-deficit irrigation across all five-week study periods. MAIN EFFECTS Days after study initiation 0 7 14 21 28 35 ––––––––––––––––––––––––––––––––––––– mm –––––––––––––––––––––––––––––––––––––– CM IS 2A SPRINKLER 20.3 a 19.2 b 18.8 b 16.2 b 12.5 b 4.9 a 2B SPRINKLER 19.6 a 16.6 c 13.8 c 11.9 c 9.1 c 3.6 b 3 SPRINKLER 19.7 a 18.9 bc 17.8 b 18.7 d 11.5 b 5.3 a 2A SDI 19.3 a 11.1 a 4.9 a 4.6 a 4.4 a 4.3 ab 2B SDI 19.1 a 12.2 a 5.4 a 5.2 a 4.9 a 4.5 ab 3 SDI 19.7 a 19.1 b 17.9 b 16.7 b 14.8 d 13.5 c ANOVA ––––––––––––––––––––––––––––––––––– p > F –––––––––––––––––––––––––––––––––––––– CONSTRUCTION METHOD (CM) NS *** *** *** *** *** IRRIGATION SYSTEM (IS) NS ** *** *** * ** YEAR NS NS NS NS NS NS CM x IS NS *** *** *** *** *** CM x YEAR NS NS NS * NS NS IS x YEAR NS NS NS NS NS NS Means in the same column followed by a common letter are not significantly different based on Tukey´s honestly significant difference test (p < 0.05) Note. NS not significant * Significant at the .05 probability level ** Significant at the .01 probability level *** Significant at the .001 probability level At the end of the periods, the RWS of the sprinkler-irrigated and the 2-layered SDIirrigated variants finally averaged 4.5 mm (3.6-5.3 mm). In contrast, the 3-layered SDI-irrigated variant revealed the significantly highest RWS (13.5 mm) across all variants after 35 days of ETO 60% deficit irrigation treatment. With an average ETO of 3.2 mm/day, this corresponds to a water storage of approximately 4.2 days for the 3-layered SDI variant compared to slightly more than one day for all other variants (Table 4).
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 47 Turfgrass performance The construction method (CM), irrigation system (IS), and CM × IS significantly affected turfgrass quality (TQ), turfgrass coverage (TC), and NDVI under a 60% ETO deficit irrigation treatment (Table 5). Comparison of 2023 and 2024 (Figure 2) revealed analog patterns across the 2-layered sprinklerand SDI-irrigated variants. Contrastingly, the 3-layered sprinklerand SDI-irrigated variants exhibited slightly higher turfgrass performance parameters in 2024. Across both periods, a rapid drop after study initiation in TQ, TC, and NDVI among the 2-layered SDI irrigated variants was observed (Figure 2). Furthermore, these variants exhibited an unacceptable TQ (< 6) 14 days after study initiation, in combination with a significant decrease in turfgrass coverage (24.25%). The 2-layered SPR-irrigated variants (2A-SPR and 2B-SPR) revealed no significant differences across all treatments. After 35 days, TQ dropped below 6, averaging 5.4 (2A-SPR) and 5.9 (2B-SPR), with TC at 78% (2A-SPR) and 81% (2B-SPR). The 3-layered variants (3-SPR and 3-SDI) showed a similar trend, but with significantly higher turfgrass quality, averaging 7.7 (3-SDI) and 7.1 (3-SPR). In summary, only the 3-layered variants with an intermediate layer achieved acceptable TQ (≥ 6). The NDVI patterns mirrored turfgrass quality and coverage, showing consistency with the lowest values in the 2-layered SDI irrigated variants. No significant differences were observed among the other variants 35 days after study initiation, although the 3-layered variants showed slightly higher NDVI values (Figures 2 and 3). Table 5. Analysis of variance for the entire study period on turfgrass quality parameters as affected by 60% ETO-deficit irrigation. MAIN EFFECTS Days after study initiation 0 7 14 21 28 35 ANOVA ––––––––––––––––––––––––––––––––––– p > F –––––––––––––––––––––––––––––––––––––– TURFGRASS QUALITY CONSTRUCTION METHOD (CM) NS *** *** *** *** *** IRRIGATION SYSTEM (IS) NS ** *** *** *** ** YEAR NS NS * NS NS NS CM x IS NS *** *** *** *** *** TURFGRASS COVERAGE CONSTRUCTION METHOD (CM) ** *** *** *** *** *** IRRIGATION SYSTEM (IS) NS ** *** *** *** *** YEAR * NS * NS * NS CM x IS ** *** *** *** *** *** NDVI CONSTRUCTION METHOD (CM) NS * *** ** *** *** IRRIGATION SYSTEM (IS) NS * ** * ** ** YEAR NS NS NS NS NS NS CM x IS NS * *** *** *** *** Note. NS, not significant NDVI, normalized difference vegetation index * Significant at the .05 probability level ** Significant at the .01 probability level *** Significant at the .001 probability level
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 48 Figure 2. Turfgrass quality (line denotes an acceptable quality level of 6), visual turfgrass coverage (%), and NDVI across the study periods 2023 and 2024, as affected by a 60% ETO deficit irrigation treatment. Means followed by a common letter separately for each assessment day are not significantly different based on Tukey´s honestly significant difference test (p < 0.05).
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 49 Figure 3. Overview of the experimental site with sprinkler (SPR) and sub-surface drip irrigation (SDI) and analysis of the normalized difference vegetation index (NDVI) values in 2023 utilizing multispectral drone imagery, following 35 days of 60% ETO deficit irrigation.
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 50 Discussion Proper drainage prevents water logging and stabilizes sports field surfaces. Effective rootzone construction and irrigation systems optimize water use, maintain uniform soil moisture, and enhance turfgrass quality. Supplemental irrigation sustains turf health, appearance, density, and resilience to traffic, pests, and environmental stress. Therefore, sustainable management practices and water conservation strategies are vital. Research has shown that some cool-season species sustain acceptable quality with <60% ET replacement (Braun et al., 2022). This study examined 2and 3-layered rootzone methods with sprinkler and SDI systems under 60% ETO deficit irrigation in perennial ryegrass, analyzing interactions between rootzones, irrigation systems, and turfgrass performance. The results revealed significant differences and interactions in irrigation water usage efficiency and turfgrass performance parameters within the evaluated variants. The determining factors were material composition, rootzone construction method, and irrigation delivery system. Irrigation water movement and rootzone water storage The results demonstrated that a 60% ETO replacement under sprinkler irrigation resulted in substantial decreases in rootzone water storage and an increase in soil water tension, especially in the sprinkler variants, within 28 days. This indicates that under 60 ETo replacement, insufficient rootzone water retention properties caused by the physical properties of the material (coarse soil texture and high saturated hydraulic conductivity), irrigation delivery systems (sprinkler) susceptibility to evaporation loss, and induced distribution inaccuracies which are consistent with Fidanza (2023), Chartzoulakis and Bertaki (2015), and Carrow et al. (2002). Further, Taylor et al. (1993) reported that the highest retention and RWS were achieved when the rootzone was placed directly on the gravel layer because it created a capillary break owing to the abrupt pore size difference between the gravel and rootzone layers. However, the current results across variant 2A-SPR are different, likely because of the distinct texture and composition of the gravel drainage layer DG, which consists of 31.5% gravel, 66% sand, and 2.5% silt (grid 0–8 mm), unlike the material used by Taylor et al. (1993), which is made entirely of gravel (grid > 2 mm). Subsurface drip irrigation (SDI) systems enhance irrigation efficiency by delivering water directly and precisely to the rootzone (Fidanza, 2023). However, rootzone water storage within the 2-layered variants tends to be significantly higher under sprinkler irrigation than under SDI irrigation. The results indicate that the installation of the SDI System in a layer (DG or CSIL) characterized by low retention properties and lacking corresponding capillary action leads to an insufficient capillary rise of irrigation water. This agrees with Cote et al. (2003), who demonstrated that a greater quantity of water is distributed beneath the emitter plane in a highly permeable rootzone material that is drip irrigated and increasing the irrigation amounts only resulted in more pronounced downward water spreading (Skaggs et al., 2010). In contrast, the 3-layered variant exhibited beneficial effects of a fine-textured intermediate layer on soil moisture retention and rootzone water storage, especially under SDI irrigation. The remaining quantity of stored water in the rootzone across all assessment days
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 51 indicated the highest soil moisture retention compared to the other variants. The lack of fine sand in the 3. layer (CSIL, grain size 0.2–2 mm) was potentially responsible for this observation, as it resulted in limited pore continuity between layer 2 and layer 3 (FSIL and CSIL) and correspondingly higher retention behavior in the intermediate layer 2 (FSIL). Similarly, the high proportion of fine and medium sand in the rootzone LSRM (layer 1) resulted in high capillary activity, which allowed a corresponding capillary rise of irrigation water in the upper areas and, in contrast, resulted in more pronounced water retention than in the comparison 2-layer variants. This aligns with the results of Taylor et al. (1993), who demonstrated the highest retention properties for rootzone mixes with a dominant 0.1–0.5 mm proportion. However, this cannot be confirmed for the 2-layer rootzone construction method 2B with an identical rootzone material (layer 1: LSRM) as variant 3. Presumably, the improved retention properties of the upper rootzone can have an effect only if there is a corresponding reduced percolation or infiltration behavior caused by a capillary break against the gravity pull between adjacent layers. Turfgrass quality parameters under deficit irrigation treatment Evaluating turfgrass quality parameters under 60% ETo deficit irrigation among different rootzone construction methods and associated irrigation delivery systems resembled the trends observed in irrigation water dynamics. The 2-layered SDI irrigated variants revealed unacceptable turfgrass quality (<6) 14 days after study initiation due to insufficient rootzone water storage (upper 12 cm) as well as high soil water tension (at a depth of 8 cm). Replacements of 60% ETO increased soil water tension within 2A_SDI and 2B_SDI to a range between 162 and 120 kPa, consistent with Aronson et al. (1987), who reported unacceptable turfgrass quality when soil water tension rose to more than 80 kPa. Braun et al. (2022) reported that a minimum deficit irrigation replacement of 59% of evapotranspiration is necessary to maintain a minimum acceptable turfgrass quality of ≥ 6 for perennial ryegrass. The results partially align with these observations; 2-layered sprinkler irrigated variants achieved a turfgrass quality > 6 with a 60% ETO replacement only until 28 days after study initiation. In the remaining time period, turfgrass quality was insufficient. Notably, the 3-layered sprinkler and SDI variants maintained TQ > 6 throughout the study periods. This demonstrates the efficacy of these configurations in sustaining turfgrass quality under reduced water input. Under drought stress, plant leaves show a noticeable reduction in near-infrared (NIR) reflection and redder irradiance, decreasing the normalized difference vegetation index (NDVI). In this context, our results align with other studies (Bell et al., 2002; Trenholm et al., 1999) that have demonstrated that NDVI values correlated with visual turfgrass quality and color, as they exhibited significantly lower values across the 2-layered SDI irrigated variants as well as significantly highest values across the 3-layered variants at 60% ETO replacement. Efficiency of irrigation water usage The study emphasizes the significant impact of material composition, rootzone construction methods, and associated irrigation delivery systems on irrigation water-usage efficiency and turfgrass performance. These findings underscore the challenge of sandy rootzone construction materials, which have difficulty retaining irrigation water for plant use
Turfgrass irrigation: Analyzing the effects of rootzone construction and irrigation delivery system on water retention characteristics and perennial ryegrass performance 52 because of their low water retention and high proportion of air-filled porosity at field capacity, along with the need to optimize the irrigation delivery systems to their working environment. The results revealed that harmonizing the rootzone construction method and associated irrigation delivery systems is crucial for improving turfgrass irrigation efficiency and maintaining acceptable turfgrass performance parameters, particularly for subsurface drip irrigation systems. Properly matching the rootzone medium characteristics with the specific irrigation method is essential for maximizing water-usage efficiency and sustaining high-quality turfgrass under deficit irrigation conditions. Conclusion Maximizing irrigation water-use efficiency is a significant challenge that should be met before legal mandates. Our results showed that enhancing irrigation efficiency while maintaining functionality requires harmonizing rootzone construction methods and irrigation delivery systems. Specifically, SDI systems must address complex soil physical processes and rootzone construction to optimize water use. The 3-layered construction method with an intermediate finer-textured layer atop a coarser drainage layer yielded the most favorable results. Harmonized rootzone construction methods, especially subsurface drip irrigation, allow more targeted water use than standard sprinkler systems, although they may lead to insufficient capillary moisture in near-surface areas.
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 53 2.3 Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D J. Cordel1, R. Anlauf1, W. Prämaßing1 and G. Broll2 1 Faculty of Agriculture Sciences and Landscape Architecture, Osnabrück University of Applied Sciences, Am Krümpel 31, 49090 Osnabrück, Germany 2 Institute of Geography, University of Osnabrück, Seminarstr. 19, 49074 Osnabrück, Germany Citation: Cordel, J., Anlauf, R., Prämaßing, W., Broll, G. (2025): Predicting water distribution and optimizing irrigation management in turfgrass rootzones using HYDRUS-2D. Hydrology, 12(3), 53. https://doi.org/10.3390/hydrology12030053 Keywords: turfgrass management; turfgrass irrigation; water use efficiency; predictive models for irrigation Author contributions: Jan Cordel: Conceptualization, Investigation, Methodology, Formal analysis, Validation, Visualization, Writing – original draft. Rüdiger Anlauf: supervision, writing – review and editing. Wolfgang Prämaßing: writing – review and editing. Gabriele Broll: writing – review and editing.
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 60 Figure 1. Overview of construction types of the 2-layer (2A, 2B) and 3-layer (3) systems consisting of 5 rootzone components: high-silt rootzone mixture (HSRM), low-silt rootzone mixture (LSRM), coarse sand intermediate layer (CSIL), fine sand intermediate layer (FSIL) and drainage gravel (DG) with the associated irrigation systems: sprinkler (SPR) and subsurface drip irrigation (SDI). The circles indicate the position of the SDI system, with a spacing of 33 cm and an installation depth of 16.5 cm. In each case, all materials were installed with a bulk density of 95% of the standard Proctor density (DIN18035-4, 2018) (HSRM = 1.55, LSRM = 1.46, FSIL = 1.41, DG = 1.80, CSIL = 1.60 g cm−3) and two systems used for irrigation. Due to the plot size and to maximize the water distribution uniformity, SPR-irrigated plots were hand-watered with a discharge rate of 10 mm h−1. The SDI system used was a line-source drip system consisting of porous pipes (radius r = 0.9 cm) with a discharge rate of 3 L h−1 m−1 (9.09 L h−1 m−2) resulting from a line spacing of 33 cm. The SDI installation depth was 16.5 cm, and the operating pressure was 0.2 bar. Experimental Setup and Measurements Experimentally measured data were collected under greenhouse conditions using bare soil profiles (plots with no grass cover). The research area (11.94 m × 4.71 m including nonconsidered edge areas of the actual test plots) was designed as a completely randomized twofactorial split plot with three replications for each treatment. The total area comprised six main plots (each 4.11 m × 1.70 m; three for SPR and three for SDI irrigation). Each of the six main plots was divided into three plots (1.70 m × 1.37 m) for the three construction types (two 2layered and one 3-layered design). Two different irrigation cycles with discharge amounts of cycle 1 = 10 mm (the usual irrigation amount for turfgrass and used for model calibration) and 2 = 20 mm (used for model validation) were applied, both with an intensity of 10 mm h−1. After irrigation cycle 1, all plots underwent a drying phase (approximately four weeks with an average evaporation rate of 3.22 mm day−1) to achieve a uniform initial soil volumetric water content. The initial volumetric water content was between 9 and 10 vol.% without significant
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 61 differences between the plots. The collected dataset for each evaluated SPR or SDI irrigated variant (2A, 2B, and 3) comprised 30 measurements (n=30), which were derived from a) three distinct observation depths (3, 6, and 12 cm) and b) ten specific observation times (0.00, 0.17, 0.33, 0.50, 2.00, 4.00, 8.00, 12.00, 24.00, 48.00 hours after irrigation initiation). Data were collected in two separate operations during both cycles and to ensure an identical initial volumetric water content (VWC), irrigation cycle 2 commenced following a drying phase of three weeks. Soil samples were taken with a soil sampler through lateral openings at −3, −6, and −12 cm using a 50 mm diameter metal tube, which was driven into the soil and withdrawn. The soil gravimetric water content was measured by oven-drying the soil samples at 105 °C, and the VWC was calculated by multiplying it with bulk density. For each replication and observation time, a total of 15 soil samples were collected from five measurement points (center of the plot above the SDI line and at 8.25 and 16.5 cm right and left of the center; at similar positions for the SPR variants) and at depths of 3, 6, and 12 cm. The five water content values for one depth for each sampling were averaged. After sampling, the voids were refilled with the same soil material. Simulation Model, Initial and Boundary Conditions HYDRUS-2D (H2D) finite element model version 5.04 (Šimunek et al., 2012), a wellknown parametric model that connects volumetric water content to matric potential, as proposed by van Genuchten (Van Genuchten, 1980), was employed to simulate the distribution of irrigation water in bare soil profiles. The parametric model incorporates parameters such as ϴr, ϴs, α, n, and m and is integrated into the HYDRUS-2D model as follows: θ(ψ)=ϴ𝑟+ θs−θr (1+|𝛼∙ψ|𝑛)𝑚 (1) where ϴψ is the water content at matric potential ψ; ϴr is the residual water content (cm3 cm−3); ϴs is the saturated soil water content (cm3 cm−3); α, n, and m describe the shape of the function without physical meaning; and m is usually fixed as 1 − 1/n (Anlauf et al., 2012). The previously discussed formulation can be integrated with Mualem’s equation (Mualem, 1976) to elucidate the unsaturated hydraulic conductivity function, and it is also incorporated in the HYDRUS-2D model: with 𝑆𝑒=ϴ−ϴ𝑟 ϴ𝑠−ϴ𝑟 (3) where Kψ is the hydraulic conductivity at matric potential ψ; Ks is the saturated hydraulic conductivity; Se is the effective water content; L is a parameter describing the pore structure of the soil, usually set to 0.5; and m is fixed as m = 1 − 1/n (Anlauf et al., 2016; Raviv et al., 2019; Šimůnek et al., 2008). 𝐾ψ=𝐾s∙𝑆𝑒 𝐿∙[1−(1−𝑆𝑒 1 𝑚)𝑚]2 (2)
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 62 The simulation was initiated by creating a model setup with the correct material layers, initial conditions, and boundary conditions within H2D that corresponded to the experimental setup. The observed volumetric soil water content in the soil profile was taken as the initial water content for all simulated scenarios. The model´s soil surface was subjected to atmosphere boundary conditions for soil water evaporation (EV) corresponding to greenhouse conditions with 0.013 cm h−1. A free drainage boundary condition was imposed at the bottom of the soil profile. On the rightand left side of the soil profile, a no-flux boundary was used (Figure 2). Figure 2. Triangular grid used for HYDRUS-2D simulations for SDI (left) and SPR (right) variants and related boundary conditions. SPR irrigation was scheduled in accordance with the experimental setup with an intensity of 1 cm h−1. For SDI, a variable flux boundary was used around the SDI emitter (Figure 2). During irrigation, the drip pipe boundary had a constant water flux, which was obtained by dividing the emitter discharge flow rate of 3 L (h × m)−1 by the surface of the drip pipe as: 𝑞=Emitter discharge flow rate pipe surface area =( 3000 cm3 (h)−1 2 ∙ 0.9 cm∙ π ∙ 100 cm) =5.306 cm h−1 (4) During the no-irrigation period, the flux was kept at zero. The simulation model domain was 50 cm deep. The high differences between the soil hydraulic parameters of the different materials, as well as the significant temporal-spatial variability under SDI, necessitated a node spacing of 0.50 mm, with progressively closer spacing down to 0.25 mm around the SDI pipe and each layer interface.
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 63 Model Quality Evaluation Criteria The evaluation of the HYDRUS-2D model encompassed the assessment of model quality, sensitivity analysis, and model calibration based on the predicted and measured volumetric soil water content (VWC) dataset (n=30) of each variant within the upper 12 cm. The correlation coefficient (R2), root mean square error (RMSE), mean absolute error (MAE), and Nash-Sutcliffe efficiency (NSE) were calculated using the following equation to assess the model quality parameters: R2=(𝑆𝑋𝑌 𝑆𝑋𝑆𝑌)2 (5) 𝑅𝑀𝑆𝐸=√1𝑁∑(𝑋𝑖 − 𝑌𝑖)2 𝑁 𝑖=1 (6) 𝑀𝐴𝐸= 1𝑁∑|(𝑋𝑖 − 𝑌𝑖)| 𝑁 𝑖=1 (7) 𝑁𝑆𝐸=1−∑(𝑋𝑖 − 𝑌𝑖)2 𝑁 𝑖=1 ∑(𝑋𝑖 − 𝑋𝑎𝑣)2 𝑁 𝑖=1 (8) where SXY is the covariance between the variables X (measured data) and Y (predicted data). SX and SY are the standard deviations of measured and predicted data, respectively. Xi is the measured data, Yi = the predicted data, Xav = the average of the measured data, and N = the number of observations. The correlation coefficient should be close to 1. The root mean square error (RMSE) is a commonly utilized metric for evaluating the agreement between measured and simulated values and should be close to zero. A widely accepted standard that does not consider overor under-forecasting is the mean absolute error (MAE). In an ideal scenario, the MAE should be nearly zero. Both the root mean square error (RMSE) and mean absolute error (MAE) share the same units as the measured and predicted values (Wallach et al., 2006). Lastly, the NSE (Nash and Sutcliffe, 1970) is a normalized statistic commonly used to evaluate hydraulic models and compares residual and measured variance (Moriasi et al., 2007). The parameter equals zero when the square of the differences between the measured and predicted values equals the variability in the measured data. If the NSE value is negative, the measured mean is a more accurate predictor than the model (Ahnert et al., 2007). If the model gives perfect results, the NSE = 1. An acceptable-quality model should have an NSE > 0.5 (Anlauf et al., 2012; Wallach et al., 2006). Input Parameter, Parameter Sensitivity, Model Calibration and Validation The water retention drying curves were determined using Eijkelkamp standard sandbox apparatus (DINEN13041, 2012) to assess the water content at pF 1, 1.8, and 2.5. The water content at pF 4.2 was determined using a pressure plate apparatus, as per the DIN EN ISO 11274 (2019). The soil water retention curves were then parameterized according to the vanGenuchten equation (Hartmann et al., 2018) by adjusting the θr (residual water content), α,
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 64 and n (shape factors) values using the EXCEL solver function to match the measured water content and water suction drying curve values. The saturation water content θs was fixed at the total porosity (TP). The material´s hysteretic behavior was evaluated based on the capillary rise of water in the materials employed in this study within experimental containers composed of 10 rigid plastic rings, each measuring 2 cm in height (Anlauf et al., 2012). The rings were filled with the material of interest (95% bulk density of Proctor density DPR), and a flooding depth of 1 cm was maintained for 48 h before the water content was determined gravimetrically. It was assumed that the water tension in the rings is defined by the distance to the water table, and the water content at equilibrium is analogous to the water retention curve (Raviv et al., 2019). The wetting water retention curve was initially parameterized by adjusting and establishing the parameter α to αw (Šimůnek et al., 2008). The analytical determined soil parameters used in HYDRUS-2D are presented in Table 2. Table 2. Analytically determined soil parameters used in the HYDRUS-2D program. Sensitivity analyses and model calibration focused on the materials used in the top two layers, including HSRM, LSRM (rootzone layers), FSIL (intermediate layer), and DG (drainage gravel layer) within each construction method (2A, 2B, and 3) and irrigation system (SDI and SPR). The HYDRUS-2D model was used to analyze the sensitivity of the soil hydraulic parameters θr, α, and n (representing the soil water retention drying curve parameterization) while excluding the exact analytically determinable parameter θs (fixed at total porosity), as well as the parameters αw and θsw (representing the opportunities in the HYDRUS code for describing material´s hysteresis behavior). The study evaluated the impact of a 20 % incremental increase in these parameters on simulation output (VWC) after 10 mm irrigation (cycle 1) regarding the deviation in model efficiency (NSE). For model calibration, the Levenberg-Marquardt optimization algorithm (Marquardt, 1963), in conjunction with the HYDRUS-2D code (Šimůnek et al., 2016), was employed to inversely estimate the desired soil hydraulic parameters. These parameters were determined through the systematic minimization of differences between observed and simulated state variables (i.e., VWC). The total differences are expressed by an objective function, ϕ, which may be defined as (Nakhaei and Šimůnek, 2014): 𝜙(𝛽,𝛾)= ∑𝑣𝑗 𝑗=𝑚𝑦 𝑗=1 ∑𝑤𝑖,𝑗[𝑦𝑖∗(𝑧,𝑡𝑖)−𝑦𝑖(𝑧,𝑡𝑖,𝛽)] 𝑖=𝑛𝑗 𝑖=1 ² where the right side represents the residuals between the measured (yi*) and corresponding model-predicted (yi) space-time variables using the soil hydraulic parameters of the optimized parameter vector, β. The initial summation aggregates the residuals for all measurement types (my) (i.e., VWC), whereas the variable nj in the subsequent summation Parameters HSRM LSRM FSIL CSIL DG ϴs (cm3 cm−3) 0.415 0.430 0.466 0.394 0.321 ϴr (cm3 cm−3) 0.060 0.076 0.011 0.014 0.014 Ks (cm h−1) 22.019 64.854 146.474 608.12 91.612 α 0.089 0.061 0.055 0.228 0.085 αw 0.118 0.119 0.077 0.300 0.156 n 1.728 2.090 2.719 1.929 2.063 l 0.5 0.5 0.5 0.5 0.5 (9)
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 65 denotes the number of measurements for a specific measurement type j. Assuming that measurement errors within a given measurement type are independent and uncorrelated, the weighting factor values for vj can be chosen to ensure either equal weighting of data types through a normalization procedure or weighting proportional to the reciprocal of the measurement variance for type j (Clausnitzer and Hopmans, 1995). The model calibration process involved (i) calibrating separately the four soil hydraulic parameters identified as most sensitive through sensitivity analysis (scenario F1–F4), and (ii) calibrating simultaneously, based on previous research (Huang et al., 2005; Kool and Parker, 1987; McCoy and McCoy, 2009; Šimůnek et al., 2008), the shape parameters αw and n (scenario F5) as well as αw and ϴsw (scenario F6). In total, six distinct and independent scenarios (F1–F6) were utilized for the calibration procedures, each without internal weighting of inversion data. Following calibration, the most suitable model for simulation was identified and subsequently validated using an independent dataset (irrigation cycle 2). This validation step was conducted to evaluate the model's reliability. Irrigation Management Evaluation The HYDRUS-2D code was also used in hypothetical instances to examine the effect of irrigation treatments, which are given in Table 3. The parameters for the boundary and flow domain remained unchanged from previous descriptions. Based on previous research (Cordel, 2025; Cordel et al., 2024) that demonstrated the favorable water retention characteristics of the three-layered construction method 3, this methodology was employed to evaluate the efficacy of various irrigation approaches regarding water usage. The analysis focused on two key factors influencing irrigation efficiency, i.e. drainage flux and soil water storage within the simulation domain (upper 50 cm). Four different irrigation approaches were assessed. Each approach involved applying a total irrigation amount of 10 mm through either SPR (SPR 1–4), SDI (SDI 1–4), or HYBRID (HYBRID 1–4) in up to five irrigation events within a 12 h period. Table 3. Irrigation management parameters within the four irrigation approaches (1-4) under SPR, SDI, and HYBRID (SPR+SDI) irrigation across construction method 3. Irrigation approach Irr. events within 12 h Water applied per charge (mm) Proportion 0 3 6 9 12 SPR SDI (-) (-) hours (%) (%) SPR-1 1 10.00 0 0 0 0 100 0 SPR-2 2 7.50 0 2.50 0 0 100 0 SPR-3 3 5.00 0 2.50 0 2.50 100 0 SPR-4 4 5.00 1.25 1.25 1.25 1.25 100 0 SDI-1 1 10.00 0 0 0 0 0 100 SDI-2 2 7.50 0 2.50 0 0 0 100 SDI-3 3 5.00 2.50 2.50 0 100 SDI-4 4 5.00 1.25 1.25 1.25 1.25 0 100 HYBRID-1 1 5.00 (SPR), 5.00 (SDI) 0 0 0 0 50 50 HYBRID-2 2 7.50 (SPR) 0 2.50 (SDI) 0 0 75 25 HYBRID-3 3 5.00 (SPR) 0 2.50 (SDI) 0 2.50 (SDI) 50 50 HYBRID-4 4 5.00 (SPR) 1.25 (SDI) 1.25 (SDI) 1.25 (SDI) 1.25 (SDI) 50 50
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 66 Results Model Quality Evaluation The uncalibrated model's performance was evaluated using the model efficiency parameter (NSE). Further the results were graphically visualized by comparing the observed values against the simulation output differences, presented through spatial maps (ordinary Kriging method). Figure 3 shows the observed values (left) against the difference values (right) of construction methods 2A, 2B, and 3 under SPR irrigation, each averaged across the entire observation time 0-48 hours following 10 mm irrigation. Figure 4 is analogous to Figure 3, but in each case under SDI. The NSEs ranged from 0.27 (2A_SDI) to 0.72 (3_SPR), with the SPR variants generally achieving NSEs above 0.5. Further model quality parameters of the uncalibrated models are depicted in Figure 7. Graphical analysis showed that, for SPR variants 2A and 2B, the model tended to calculate higher predicted values than observed values. At an observation depth of 6 cm, the average overestimation was 0.85 vol. % for 2A and 1.22 vol. % for 2B. At a depth of 12 cm, the average overestimation was 0.94 vol. % for 2A and 1.55 vol. % for 2B. In the case of SDI, an average overestimation within variant 2A of 1.27 vol. % (6 cm depth) and 3.11 vol. % (12 cm depth) was observed. In contrast, variant 3_SDI exhibited a different pattern, with the model generating lower predicted values than the observed ones. The average underestimation was 1.54 vol. % at a depth of 6 cm and 2.27 vol. % at a depth of 12 cm. Similar to the model’s performance, variant 3 showed the most minor differences among SPR variants, while 2B displayed the least variation among SDI variants.
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 67 Figure 3. Volumetric water content within SPR variants 2A_SPR, 2B_SPR, and 3_SPR at observation depths of 3, 6, and 11 cm (averaged values across the entire observation time 048 hours) as observed values (left) and differences between the observed and predicted values (right).
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 68 Figure 4. Volumetric water content within SDI variants 2A_SDI, 2B_SDI, and 3_SDI at observation depths of 3, 6, and 11 cm (averaged values across entire observation time 0-48 hours) as observed values (left) and differences between the observed and predicted values (right).
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 69 Sensitivity Analysis The sensitivity analyses of the 5 parameters ϴr, α, n (normal drying WRC), ϴsw , and αw (wetting WRC) presented in Figure 5 illustrate the respective deviations in model efficiency (NSE) under a soil hydraulic parameter perturbation of +20 % within each variant's upper two layers (Layer 1 and Layer 2). Figure 5. Influence of a 20 % perturbation of soil hydraulic parameters ϴr, n, ϴsw, αw, and α on model efficiency deviation (NSE) across Layer 1 and Layer 2 of the variants (a) 2A_SPR) to (f) 3_SDI during irrigation cycle 1 (10 mm).
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 76 Up to 12 hours (8 hours for IA1), the water storage was very similar within the irrigation approaches with a tendency of generally minimal higher water storage in the SDI variants. Generally, the water storage was higher in IA1 and IA2 compared to IA3 and IA4. Strong differences could be identified between the water storage in the different irrigation systems after 24 hours: the SPR variant always showed the lowest water storage followed by the SDI irrigation. The hybrid irrigation system always had the highest water storage. The hybridapproach results at 48 h were 3.33 mm (IA4), 3.61 mm (IA3), 2.69 mm (IA2), and 2.39 mm (IA1) (Figure 4). Since the cumulative drainage is naturally determined by the water storage, these values were always inversely related to the storage (Figure 4). In summary, focusing on the maximum residual soil water storage and minimum drainage flux of each irrigation technique (SPR, SDI, HYBRID) and approach (IA1—IA4) at an observation time of 48 h, the following order was observed for soil water storage: HYBRID-IA3 (3.61 mm) > SDI-IA4 (2.53 mm) > SPR-IA3 (0.38 mm), and for drainage flux: HYBRID-IA3 (2.57 mm) < SDI-IA4 (3.59 mm) < SPR-IA3 (5.46 mm). The difference of water storage plus drainage flux to the 10 mm irrigation is due to the actual evaporation during the 24 hours. Discussion Turfgrass areas, including golf courses and sports fields, provide essential ecosystem services such as carbon sequestration, oxygen production, water purification, and heat dissipation, contributing to climate change mitigation and urban environmental quality (Beard, 1973; Beard and Green, 1994; Braun et al., 2023). Irrigation management must support efficient irrigation water usage, uniform soil moisture distribution, and adequate soil moisture retention to maintain acceptable turfgrass quality. This study evaluated the HYDRUS-2D model for simulating irrigation water distribution, demonstrating its effectiveness while highlighting the need for calibration in multilayered rootzone construction methods, particularly under SDI. Theoretical Aspects The model performance varied depending on the construction method and the irrigation system used. The initially uncalibrated model utilizing analytical determined soil hydraulic parameters demonstrated already acceptable performance for SPR variants (NSE 0.58–0.72) but suboptimal performance for SDI (NSE 0.27–0.52). The model tended to overestimate the two-layered SPR and SDI variants, consistent with Ghazouani et al. (2019), who reported overestimation of modeled water content values compared to measured values, with 2A_SDI displaying the lowest model accuracy. In contrast, the three-layered SDI variant notably underestimated soil water content, particularly in the lower parts of the rootzone (observation depths of 6 and 12 cm). Sensitivity analysis of the parameters indicated that the shape factor n exhibited the highest sensitivity within both layers and across all variants, whereas shape factor α (drying
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 77 WC) demonstrated the lowest sensitivity, which is consistent with the results of Inoue et al. (1998). Soil hydraulic parameter ϴsw shows the second highest sensitivity, aligning with the findings of Abbasi et al. (2003). The sensitivity analysis further demonstrated the need for the precise parameterization of soil hydraulic parameters, as construction method 2B/Layer 2 exhibited a decrease in the NSE value of −0.23 when the shape factor n was increased by 20 % under SPR irrigation. In contrast, this modification increased the NSE value of +0.19 under SDI (Figure 5). The soil hydraulic parameters for each material were independently calibrated using the HYDRUS inverse solution method across the calibration scenario F1-F6. However, the results demonstrated that the model structure significantly influenced the calibrated material's soil hydraulic properties. Generally, the calibrated αw (wetting WRC) values resulted in αw/α ratios averaging 1.90 across all variants (calibration scenario F1), indicating substantially greater αw values than the corresponding drying curve values. This finding aligns with the study of McCoy and McCoy (2009) and approximates the value of 2 (αw/α ratio) suggested by Kool and Parker (1987) for estimating wetting curve parameter αw. This supports using this approximation when measured wetting curve data are unavailable. Nevertheless, the observations indicated that, particularly under a two-parameter simultaneous calibration (scenario F5 and F6), the model's inverse solution estimated parameters exhibited high variability, notably in layer 2 and material DG. The values of the factor ϴsw showed values 0.121 and 0.321 cm3 cm-3 within scenario F6 and material DG. Shape factor n showed changeable behavior with a partially low to strongly pronounced increase and decrease depending on the construction method and associated irrigation system with values ranging between 1.764 and 3.005 (scenario F5, material DG); these findings do not agree with the results of McCoy and McCoy (2009), which showed generally lower n values when considering the typical characteristics of the hysteresis wetting curve response. Even under single-parameter calibration (scenario F2, material DG) parameters αw values ranged from 0.152 to 0.388 (Table 4). These adjustments likely represent the model´s attempt to compensate for errors (differences between observed and predicted VWC) in layer 1. Given this perspective, prior to utilizing calibrated values that yield the highest improvement in model efficiency, it is imperative to ascertain whether these calibrated parameters fall within a physically plausible range, especially in the context of directly measurable parameters (i.e., ϴr and ϴs). Furthermore, our findings suggest that, specifically under SDI conditions, the shape parameter nw (wetting curve) should be additionally considered within the HYDRUS code to more accurately characterize the hysteresis behavior of the material and facilitate a shape factorbased model calibration approach. Furthermore, the models revealed pronounced gradients at the interface between neighboring layers (Layers 1 and 2) due to markedly different soil physical properties of the materials employed. For instance, in construction method 3, Layer 1 (HSRM) demonstrated a saturated hydraulic conductivity (Ks) of 649 mm h−1, while Layer 2 (FSIL) showed a value of 1465 mm h−1. Similarly, substantial water content and water suction gradients are observed at
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 78 the interface of Layer 1 and 2 in construction method 2A, where Layer 1 exhibited a Ks of 220 mm h−1 and Layer 2 had a Ks of 916 mm h−1. These findings indicate that complex model structures involving multilayered rootzone construction methods comprising adjacent layers with significantly different soil physical properties and highlighting the necessity for model calibration, particularly under SDI conditions. Notwithstanding the observed local variations, the NSE values consistently remained positive, indicating substantial agreement between the measured soil water content and the model’s simulated values (Nash and Sutcliffe, 1970) despite the complexity of the conditions to which the model was subjected, that is, multi-layered rootzone construction methods and irrigation systems with high spatial variability. Following calibration, the model strongly agreed with the measured data, achieving NSEs up to 0.81 for SDI and 0.75 for SPR variants. Calibration demonstrated that SPR variants exhibited NSE values > 0.5 even under uncalibrated conditions. Nevertheless, if calibration for further improvement is to be conducted, the results indicated that consideration of layer 1 across SPR variants is sufficient (Figure 6). In contrast, under SDI, a significant improvement in model quality could be observed by considering both layers (Layers 1 and 2). A plausible explanation is that during SDI, the model primarily utilized the wetting retention curve in Layer 2. However, during drainage and redistribution, both layers and their corresponding wetting and drying curves and the connections between them necessitated consideration within the model calibration procedure. Practical Significance Despite discrepancies between the calculated and measured values of soil water content, these variations typically remained below 4 vol. % and are consistent with standard field measurement uncertainties. These uncertainties are significantly influenced by the specific measurement technology employed (e.g., measurement with a frequency domain reflectometry (FDR) sensor), the measurement point's location, and the measurement's timing. Regarding practical significance, it is essential to note that the model demonstrates efficacy in multilayer construction methods with highly complex soil physical relationships and under irrigation systems with considerable spatial variability (i.e., SDI), significantly influencing irrigation water dynamics (Dabach et al., 2015). Consequently, this tool facilitates the adaptation of irrigation management to the specific onsite structural conditions of sports turf areas and enables the implementation of efficient irrigation practices. In the context of irrigation management evaluation, the model data of the three-layered variant (construction method 3) indicated that, under SPR irrigation, dividing the total irrigation water quantity into multiple smaller applications (approaches 3 and 4) generally resulted in higher soil water storage and lower drainage flux compared to single large applications. Furthermore, the model data under the hybrid irrigation approach indicated the most efficient use of resource water, characterized by the highest soil water storage and lowest drainage flux compared to the other irrigation approaches (Figure 8). The practical applicability or the requirement profile of an irrigation technique should also be emphasized, as maintenance practices typically conducted on turfgrass areas necessitate surface irrigation (i.e., SPR) for establishing seeding or flushing in granulated fertilizers. In the context of
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 79 turfgrass irrigation, the primary focus remains on the targeted application of irrigation water into the root zone, which should ideally be applied with minimized evaporation loss, surface runoff, and moisture penetration of the turf to reduce disease pressure. The results indicate that under a hybrid irrigation approach, these requirements are attainable, further demonstrating the potential of HYDRUS-2D as a cost-effective and valuable tool for evaluating and optimizing turfgrass irrigation management. Limitations of the study and further research While the calibrated models exhibited strong overall performance, some discrepancies between measured and predicted values remained, particularly for the three-layered construction under SDI. This suggests that further refinement of the HYDRUS code, such as implementing nw as an additional shape factor, may be necessary to account for the material's hysteresis behavior more precisely. Moreover, to evaluate the fundamental model performance across various rootzone construction methods and irrigation systems, the study was conducted on bare soil profiles, and the incorporation of turfgrass and its associated root water uptake patterns would undoubtedly influence water distribution and irrigation efficiency. Future research should incorporate root water uptake models to provide a more comprehensive understanding of soil-plant-water dynamics and relationships in multi-layered construction methods for turfgrass areas, wherein an increasing complexity for model calibration could be anticipated. Conclusions This study demonstrated the efficacy of the HYDRUS-2D model in simulating irrigation water distribution across various turfgrass rootzone construction methods. The key findings include the following: (i) The initial model performance varied depending on the construction method and irrigation system, with acceptable to high performance for SPR variants (NSE 0.58–0.72) but suboptimal performance for SDI variants (NSE 0.27–0.52). (ii) Sensitivity analysis revealed that the shape factor n exhibited the highest sensitivity, whereas the shape factor α showed the lowest sensitivity across all variants. Model calibration significantly improved the performance, achieving NSEs up to 0.81 for SDI and 0.75 for SPR variants, respectively. This calibration appears necessary, particularly under SDI, to enhance model quality among multilayered rootzone construction methods. The calibrated αw/α ratios averaged 1.90 across all variants, aligning with previous research and supporting this approximation when the measured wetting curve data are unavailable. (iii) The evaluation of irrigation management revealed that dividing the total irrigation water quantity into multiple smaller applications generally resulted in higher soil water storage and lower drainage flux than single large applications. (iv) A hybrid irrigation approach combining SPR and SDI systems showed the most efficient use of water resources. While the calibrated models demonstrated good overall performance, some discrepancies between the measured and predicted values persisted, particularly for the three-layered construction under the SDI. This finding suggests that further refinement of the
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 80 model is necessary. These findings indicate that HYDRUS-2D has the potential to be an efficient tool for evaluating irrigation strategies in turfgrass areas. However, future research should incorporate root water uptake models to provide a more comprehensive understanding of soilplant-water dynamics in multilayered construction methods for turfgrass areas. Overall, this modeling approach has the potential to optimize irrigation management in turfgrass rootzones, thereby enhancing water-use efficiency and minimizing resource wastage. Consequently, this optimization improves the sustainability, capacity for carbon sequestration, and ecosystem services of turfgrass, which are particularly crucial in urban settings.
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 81 Table S1. Influence of 20 % perturbation of soil hydraulic parameters Θr, n, ϴsw, αw, and α on the absolute model efficiency deviation (ABS NSEdeviation) across Layer 1 and Layer 2 of the variants 2A_SPR and 3_SDI, irrigation cycle 1 (10 mm) including the absolute an total absolute mean per soil hydraulic parameter and layer respectively. Soil hydraulic parameter 2A_SPR 2B_SPR 3_SPR 2A_SDI 2B_SDI 3_SDI 2A_SPR 2B_SPR 3_SPR 2A_SDI 2B_SDI 3_SDI Absolut Mean Layer 1 Layer 2 Layer 1 Layer 2 ϴr 0.12 0.14 0.06 0.20 0.07 0.09 0.02 0.00 0.00 0.02 0.03 0.03 0.11 0.02 n 0.01 0.28 0.62 0.22 0.39 0.55 0.10 0.23 0.09 0.02 0.19 0.03 0.35 0.11 ϴsw 0.19 0.15 0.04 0.33 0.02 0.14 0.01 0.01 0.01 0.09 0.11 0.09 0.14 0.05 αw 0.02 0.03 0.06 0.09 0.10 0.18 0.01 0.11 0.06 0.05 0.02 0.00 0.08 0.04 α -0.02 0.00 -0.03 0.01 -0.01 -0.04 0.01 -0.01 0.00 0.04 -0.01 -0.03 0.02 0.01 Total Absolut Mean 0.14 0.05 Table S2. Model efficiency values (NSEs) across model calibration scenarios (F1 – F6) used in isolated implementation (Layer 1 and Layer 2), and combined implementation (Layer 1+2) across variants 2A_SPR and 3_SDI, irrigation cycle 1 (10 mm). Construction method Irrigation F1 F2 F3 F4 F5 F6 F1 F2 F3 F4 F5 F6 F1 F2 F3 F4 F5 F6 system Layer 1 Layer 2 Layer 1+2 2A SPR 0.71 0.74 0.70 0.72 0.75 0.75 0.65 0.69 0.68 0.71 0.53 0.67 0.71 0.74 0.70 0.72 0.72 0.75 2B 0.60 0.61 0.60 0.63 0.62 0.60 0.38 0.60 0.59 0.64 0.20 0.57 0.60 0.59 0.60 0.63 0.61 0.61 3 0.72 0.72 0.72 0.72 0.72 0.72 0.70 0.72 0.71 0.71 0.71 0.70 0.72 0.71 0.72 0.72 0.71 0.72 2A SDI 0.43 0.72 0.70 0.75 0.78 0.68 0.15 0.09 0.11 0.12 0.54 -0.03 0.49 0.72 0.76 0.71 0.79 0.81 2B 0.45 0.37 0.35 0.60 0.54 0.31 0.64 0.64 0.65 0.66 0.73 0.72 0.45 0.34 0.35 0.60 0.81 0.33 3 0.49 0.64 0.49 0.54 0.73 0.54 0.61 0.48 0.58 0.59 0.61 0.61 0.48 0.64 0.49 0.54 0.76 0.57 Mean (SPR) 0.67 0.68 0.67 0.69 0.70 0.69 0.58 0.67 0.66 0.69 0.48 0.65 0.67 0.68 0.67 0.69 0.68 0.69 Mean (SDI) 0.46 0.58 0.51 0.63 0.68 0.51 0.47 0.41 0.44 0.46 0.63 0.43 0.48 0.57 0.53 0.62 0.79 0.57
Predicting water distribution and optimizing irrigation scheduling in turfgrass management using HYDRUS-2D 82 Table S3. Development of soil water storage (upper 50 cm), cumulative drainage flux and corresponding cumulative evaporation of 3-layered construction method 3 and irrigation approaches 1-4 under SPR and SDI irrigation (Cycle 1 = 10 mm); observation time: 4, 8, 12, 24, and 48 hours after irrigation initiation. Amount of irrigation Irrigation approach Irrigation events within 12 hours Observation time (h) 4 8 12 24 48 EVAP SOIL_S DFLU EVAP SOIL_S DFLU EVAP SOIL_S DFLU EVAP SOIL_S DFLU EVAP SOIL_S DFLU (mm) (-) (-) (mm) 10 SPR-1 1 0.54 9.61 0.05 1.07 9.22 1.26 1.61 7.86 0.94 3.22 3.04 4.77 6.44 0.00 6.60 SPR-2 2 0.54 7.13 0.05 1.07 9.25 0.10 1.61 8.80 0.24 3.22 4.90 3.11 6.44 0.00 6.30 SPR-3 3 0.54 4.63 0.05 1.07 6.75 0.10 1.61 6.36 0.15 3.22 6.88 1.15 6.44 0.38 5.46 SPR-4 4 0.54 5.87 0.05 1.07 6.74 0.10 1.61 7.61 0.16 3.22 6.01 1.95 6.44 0.00 5.87 10 SDI-1 1 0.54 9.79 0.05 1.07 9.31 0.24 1.61 8.17 0.55 3.22 5.24 2.84 6.44 1.98 4.64 SDI-2 2 0.54 7.33 0.05 1.07 9.41 0.11 1.61 8.83 0.48 3.22 5.82 2.45 6.44 1.57 4.84 SDI-3 3 0.54 4.87 0.05 1.07 7.00 0.10 1.61 6.61 0.17 3.22 7.08 1.28 6.44 2.01 4.23 SDI-4 4 0.54 6.12 0.05 1.07 7.03 0.10 1.61 7.88 0.18 3.22 6.81 1.59 6.44 2.53 3.59 10 HYBRID-1 1 0.54 9.45 0.05 1.07 9.08 0.11 1.61 8.62 0.32 3.22 6.19 1.79 6.44 2.39 3.26 HYBRID-2 2 0.54 6.98 0.05 1.07 9.29 0.10 1.61 8.92 0.16 3.22 6.79 1.23 6.44 2.69 3.09 HYBRID-3 3 0.54 4.47 0.05 1.07 6.78 0.10 1.61 6.40 0.15 3.22 7.70 0.68 6.44 3.61 2.57 HYBRID-4 4 0.54 5.81 0.05 1.07 6.78 0.10 1.61 7.74 0.15 3.22 7.51 0.63 6.44 3.33 2.64
83 Chapter 3 General Discussion
General Discussion 84 The present research aimed to evaluate strategies for enhancing irrigation efficiency in turfgrass areas, focusing on two key factors: the rootzone construction method and the irrigation delivery system. To this end, the study employed a combination of diverse field and greenhouse experiments conducted over several years, as well as numerical model-based simulations utilizing the HYDRUS-2D software. In the following chapter, the results of these various experiments are discussed collectively in an overarching perspective. In alignment with the overarching aim, this discussion is structured around the three central research questions formulated in Section 1.3 (Topics 1–3, pp. 12–13), along with their corresponding hypotheses. Each research question was addressed through specific empirical studies, which are presented in the three papers that constitute this cumulative dissertation. Section 3.1 discussed the findings presented in Paper 1 (Section 2.1) and Paper 2 (Section 2.2) in relation to the hypothesis associated with Topic 1. Section 3.2 focuses on the results of Paper 2 (Section 2.2) in the context of the hypothesis of Topic 2. Finally, section 3.3 discussed the outcomes of Paper 3 (Section 2.3) in relation to the hypothesis of Topic 3. 3.1 Enhancement of turfgrass rootzone water retention characteristics and water use efficiency The prevalent methods employed in constructing highly functional rootzones for turfgrass areas negatively impact the water use efficiency of irrigation systems. This effect is observed regardless of the irrigation system used, as the structure and physical properties of the construction materials influence variations in soil moisture distribution and retention (Clothier and Green, 1994; Leinauer and Makk, 2007; Stier et al., 2013; Yin et al., 2012). In all trials, under greenhouse and field conditions, it was shown that the rootzone construction method and the associated irrigation delivery system significantly influenced water use efficiency and different water usage characteristics during turfgrass irrigation. Thus, it was possible to confirm this hypothesis. The evaluated results in both trials (greenhouse and field) demonstrated that after sprinkler irrigation, an increase in rootzone water storage followed by a rapid decrease could be observed (Section 2.1, Table 3, p.31, Section 2.2, Table 4, p.46). This indicates that insufficient rootzone water retention properties caused by the physical properties of the material (coarse soil texture and high saturated hydraulic conductivity), the irrigation delivery systems (sprinkler), susceptibility to evaporation loss, and induced distribution inaccuracies are consistent with Fidanza (2023), Chartzoulakis and Bertaki (2015), Cordeiro et al. (2010), and Carrow et al. (2002). Taylor et al. (1993) reported that the highest retention and rootzone water storage were achieved when the rootzone was placed directly on the gravel layer because it created a capillary break owing to the abrupt pore size difference between the gravel and rootzone layers. However, the current results across the two-layered sprinkler-irrigated variants are different, likely because of the distinct texture and composition of the used gravel drainage layer. SDI systems enhance irrigation efficiency by delivering water directly and precisely to the rootzone (Fidanza, 2023). However, water storage in the upper 15 cm (Section 2.1, Figure 5, p.32) and in the upper 12 cm (Section 2.2, Table 4, p.46) of the two-layered variants tends to be significantly higher under sprinkler irrigation than under SDI irrigation. The results indicate that installing the SDI system in a layer characterized
General Discussion 85 by low retention properties and lacking corresponding capillary action leads to an insufficient capillary rise regarding irrigation water, which agrees with Grabow et al. (2005) and Cote et al. (2003). Furthermore, increasing the volume of irrigation water from 10 mm to 20 mm does not mitigate this issue (Section 2.1, Table 3, p.31 and Figure 5, p.32), as supported by the findings of Skaggs et al. (2010). It ultimately results in a further significant decline in water use efficiency. The three-layered variant exhibited the beneficial effects of a fine-textured intermediate layer on rootzone water storage in the upper 15 cm (Section 2.1, Figure 5, p.32) and 12 cm (Section 2.2, Table 4, p.46), respectively, especially under SDI irrigation. These findings indicate that effective management of capillary tension in the soil matrix surrounding the SDI system, as well as capillary connection to the rootzone, was found to be critical; they are crucial in enhancing soil´s capillary forces against the force of gravity and allowing pronounced horizontal and vertical movement in the rootzone of irrigation water, with the overall result of a significant increase in water use efficiency. Under sprinkler irrigation, significant downward movement of irrigation water was observed, followed by rapid drying of near-surface areas. Thus, it was also possible to confirm this hypothesis. The patterns of irrigation water movement under sprinkler irrigation are confirmed by the results obtained from the greenhouse experiment, as evidenced by the temporal changes in soil moisture within the upper 12 cm of the rootzone (Section 2.1, Figure 2, p.25), as well as by the spatiotemporal distribution of soil moisture (Section 2.1, Figures 3 and 4, pp.29–30). Throughout all trials, the two-layered sprinkler-irrigated variants consistently demonstrated high percolation rates. These variants, which are consistent with the national standard construction method (according to DIN 18035-4), generally exhibited superior drainage properties, which are fundamentally crucial for maintaining the functionality and playability of turfgrass areas, particularly during periods of heavy rainfall. However, the highly permeable sandy rootzone materials and the associated construction methods exhibit low water retention capabilities, thereby facilitating the gravitational movement of irrigation water into plant-unavailable regions. Regarding the construction method, it is evident that enhancing pore continuity in adjacent layers promotes increased rates of percolation and infiltration, consequently reducing water use efficiency. This observation is supported by the model-based analysis utilizing HYDRUS-2D, which identified the highest drainage flux and the lowest soil storage of irrigation water within the upper 50 cm of the soil matrix under sprinkler irrigation (Section 2.3, Figure 8, p.75). Moreover, the materials employed exhibit a low nutrient adsorption capacity due to their high sand and low clay content. Coupled with the pronounced downward movement of water under sprinkler irrigation, as depicted by the spatiotemporal soil moisture distribution maps (Section 2.1, Figures 3 and 4, pp.29–30), significant leaching of applied nutrients is anticipated (Dekker and Bouma, 1984). The present findings highlight the challenges associated with sandy rootzone construction materials, which exhibit limited capacity to retain irrigation water for plant utilization due to their low water retention and high air-filled porosity at field capacity. This underscores the necessity of optimizing irrigation management practices, and our findings regarding irrigation water movement and retention characteristics in sandy rootzones align with the observations of Alhammadi and Al-Shrouf (2013), Song et al. (2008), and Carrow et al. (2002). Under greenhouse conditions, the results focused on the principles of water movement without considering vegetation. Consequently,
92 Chapter 4 Conclusions
Conclusions 93 The increasing strain on freshwater resources, driven by climate change, urban expansion, and rising demand for recreational turfgrass areas, has intensified the need for efficient and sustainable irrigation strategies. This cumulative dissertation addresses this challenge through three peer-reviewed studies, each aligned with the research questions and corresponding hypotheses outlined in Section 1.3 (Topics 1–3, pp. 12–13). Topic 1, focused on improving rootzone water retention and water use efficiency, was examined in Papers 1 and 2 (section 2.1 and 2.2). Topic 2, addressing the harmonization of rootzone construction methods and irrigation delivery systems, was explored in Paper 2. Topic 3, investigating model-based optimization of irrigation scheduling in turfgrass management was addressed in Paper 3 (section 2.3). Across the greenhouse and field studies (Papers 1 and 2), a consistent pattern emerged: the effectiveness of any irrigation system heavily depends on the harmonization of the irrigation delivery system and the associated rootzone construction method. The studies confirmed that, although widely used, traditional sprinkler (SPR) irrigation exhibit notable inefficiencies when applied to high-permeability, sand-based rootzones commonly used for high-quality sports facilities due to their consistent playing characteristics. Specifically, twolayered SPR-irrigated variants, where gravitational forces outweighed capillary action, experienced rapid percolation losses and subsequent surface drying, resulting in reduced rootzone water storage and lower turfgrass quality (TQ). In contrast, three-layered SPRirrigated variants consistently maintained acceptable TQ, particularly under deficit irrigation conditions, specifically 60% reference evapotranspiration (ETO), due to the intermediate layer´s water retention properties. The implementation of a subsurface drip irrigation system (SDI) alongside capillarityenhancing three-layer rootzone designs significantly improved water distribution, retention, and utilization. These setups, which incorporate adequately performing environments for the SDI system, effectively facilitated upward and lateral irrigation water movement, resulting in more homogeneous rootzone wetting and improved water availability throughout the turfgrass rooting depth. Notably, SDI variants installed in harmonized rootzone construction methods achieved the highest TQ during deficit irrigation periods. While SPR systems performed adequately in the short term, SDI systems ensured longer-term TQ resilience when optimally executed. Variants with mismatched SDI-rootzone combinations, particularly those lacking suitable capillary activity, exhibited steep declines in TQ, underscoring the risks of employing SDI without prior assessment of physical soil properties. The findings indicate that discrepancies between SDI systems and inappropriate rootzone construction methods can reduce both irrigation efficiency and TQ compared to conventional SPR systems. These results largely support the hypotheses proposed in Topics 1 and 2, except for the three-layered sprinkler models. Contrary to the initial prediction of a rapid decline in TQ under SPR-irrigation, these variants maintained acceptable TQ outcomes. In future research, follow-up experiments should employ hybrid irrigation approaches combining SPR and SDI systems to assess water distribution and irrigation efficiency over extended testing periods in diverse climatic conditions. In addition to empirical investigations, the role of numerical simulation models, particularly HYDRUS-2D, in evaluating and optimizing irrigation strategies in multi-layered
Conclusions 94 turfgrass rootzones was examined (Paper 3). Model calibration showed that accurate predictions of water dynamics under SDI and SPR systems require the careful parameterization of soil hydraulic properties. Nevertheless, the calibrated models exhibited a high degree of correlation with field data, demonstrating their potential as reliable tools for guiding irrigation management, evaluating system designs, and informing best turfgrass irrigation practices. The findings emphasize the importance of a model-based approach and model-based decision support in turfgrass irrigation. As demonstrated, predictive models can simulate complex interactions between rootzone structure, irrigation dynamics, and environmental conditions, insights that are challenging to obtain through field experimentation alone. Furthermore, calibrated models enabled the evaluation of hypothetical scenarios, such as the division of irrigation events or hybrid SDI-SPR systems, allowing for more strategic and efficient irrigation planning. These findings fully confirm the hypothesis proposed under Topic 3. In future studies, root water uptake models should be integrated to better understand soil–plant–water dynamics in multi-layered turfgrass rootzones. Overall, the results of this cumulative dissertation indicate that integrating rootzone engineering, irrigation delivery system selection, and advanced modeling represents a viable path toward more efficient and sustainable turfgrass irrigation strategies. The resulting synergies not only enhance irrigation efficiency but also contribute to broader goals such as resource conservation, climate resilience in urban green infrastructure, and the development of practical frameworks for addressing emerging challenges in turfgrass irrigation under changing environmental conditions.
95 Summary Turfgrass systems are a vital component of urban green infrastructure, providing functional, ecological, and aesthetic services in recreational, sporting, and public landscapes. However, maintaining high-quality turfgrass requires substantial water, which has become increasingly problematic in the context of global water scarcity, climate change, and environmental regulations. Efficient water use has become a priority in sustainable turfgrass management. Unlike traditional agricultural systems, turfgrass areas prioritize usability, plant growth, and aesthetics. Standards for sports turf construction focus on soil properties, drainage, and moisture retention, but increasing drought and water scarcity demand innovative irrigation strategies to maintain functionality. Traditional irrigation systems, particularly sprinkler-based methods, often result in inefficient water use due to wind drift, evaporation loss, and technically induced distribution inaccuracies. Conversely, subsurface drip irrigation offers targeted and potentially more efficient water delivery, though it poses challenges in adequately wetting the turfgrass rootzone. Irrigation delivery systems interact with soil physical properties and rootzone construction methods, thereby affecting water distribution, retention, and turfgrass quality. Efficient irrigation strategies require a nuanced understanding of soil–water dynamics, including the impact of rootzone construction methods and irrigation delivery systems on water retention characteristics and soil moisture distribution. This cumulative dissertation addresses the pressing need to enhance irrigation efficiency in turfgrass areas by systematically investigating the impact of rootzone construction methods on irrigation water distribution, retention, and turfgrass performance. This investigation focuses on using traditional and commonly used sprinkler irrigation (SPR) or subsurface drip irrigation (SDI). Moreover, the study aims to assess the application of numerical simulation models, specifically HYDRUS-2D, to predict and optimize irrigation efficiency under various rootzone–irrigation configurations, with the overarching objective of evaluating strategies to enhance irrigation efficiency in turfgrass areas. To achieve this objective, trials were conducted over several years, encompassing three complementary studies: empirical (field and controlled environment, Papers 1 and 2, section 2.1 and 2.2) and modeling-based (Paper 3, section 2.3). The research systematically explores how rootzone construction influences irrigation water distribution, retention, and turfgrass quality, particularly when paired with either SPR-irrigation or SDI. Furthermore, the study evaluates the application of numerical simulation models, specifically HYDRUS-2D, to predict and optimize irrigation efficiency under varying soil–irrigation configurations. The experiments were conducted under both controlled greenhouse and open field conditions. The first study (Paper 1) was conducted under greenhouse conditions using controlled irrigation cycles and bare soil profiles (without grass cover) to isolate and analyze water distribution in response to irrigation delivery system type and rootzone construction methods. Three rootzone constructions, two two-layered (analogous to the national standard) and one three-layered, were evaluated under both SPR-irrigation and SDI. The volumetric water content (VWC) was monitored at multiple depths and time intervals following irrigation events. The second study (Paper 2) investigated turfgrass quality and water storage in a two-year field trial (2023 and
Summary 96 2024) under deficit irrigation conditions, specifically 60% reference evapotranspiration (ETO). Turfgrass plots featuring perennial ryegrass (Lolium perenne L.) and employing the same three rootzone construction methods used in the greenhouse condition were irrigated using either SPR or SDI. Key performance indicators included turfgrass quality (TQ), rootzone water storage (RWS), and soil water tension (SWT). In the third study (Paper 3), the HYDRUS-2D finite element model was employed to simulate water dynamics within the rootzones and associated irrigation delivery systems (SPR and SDI) based on the observed greenhouse data. The model incorporated soil hydraulic parameters, determined through laboratory analysis, for each material employed in this study. The calibration and validation of the model were conducted with a focus on optimizing model quality and minimizing the discrepancies between observed and simulated outputs, particularly in relation to volumetric water content. The results of the investigations showed that the greenhouse trials revealed distinct patterns of soil moisture distribution across irrigation systems and rootzone designs. SPRirrigated plots exhibited rapid volumetric water content (VWC) increases at shallow depths (3 cm) followed by substantial decreases within 72 hours, particularly under two-layered designs. Three-layered SDI plots showed a pronounced capillary rise in irrigation water combined with sustained irrigation water retention in the soil matrix, highlighting the system’s efficiency. The field trials demonstrated significant differences in RWS and SWT across rootzone constructions. Two-layered designs under SDI experienced high SWT values (>120 kPa), indicating insufficient moisture retention. In contrast, three-layered designs maintained lower SWT (<15 kPa) and higher RWS after 35 days of 60% deficit irrigation. TQ declined rapidly in two-layered SDI variants, with an unacceptable TQ (<6) observed 14 days after study initiation. In contrast, three-layered SDI variants achieved the highest TQ throughout the study, demonstrating their resilience under water-limited conditions. SPR-irrigated variants initially exhibited acceptable TQ but failed to maintain it beyond 28 days due to poor water retention in sandy rootzones and high evaporation loss during irrigation. An exception was the threelayered SPR variants, which maintained an acceptable TQ throughout the testing period. The HYDRUS-2D model effectively simulated water distribution under both SDI and SPR systems, with calibrated parameters yielding improved model quality values. Sensitivity analyses identified shape factors α and n determining the soil hydraulic functions as critical parameters influencing model quality, particularly under SDI conditions with high spatial variability. Incorporating hysteresis effects in the model´s soil water retention function improved the model's accuracy. The collective results underscore the complex but manageable interplay between irrigation delivery system and rootzone construction. While effective at initial near-surface wetting, sprinkler systems fail to provide lasting soil moisture retention. Conversely, SDI systems supported by a three-layered design that enhances capillary rise are highly efficient. From a sustainability standpoint, the three-layered SDI variants consistently demonstrated superior water retention under controlled conditions, as well as enhanced RWS and TQ under field conditions. These configurations enabled homogenous moisture distribution, maximized water use efficiency, and maintained high TQ—key goals in sustainable urban green and turfgrass management. The calibrated HYDRUS-2D model demonstrated its utility as a costeffective tool for predicting water dynamics and developing efficient irrigation strategies.
Summary 97 Future research should explore hybrid irrigation approaches combining SPR and SDI systems to assess water distribution and irrigation efficiency under open field conditions. Additionally, model-based approaches should incorporate root water uptake models to provide a more comprehensive understanding of soil–plant–water dynamics. The integrated findings of this cumulative dissertation contribute significantly to understanding how irrigation systems and rootzone construction influence soil moisture dynamics and turfgrass quality. The evidence underscores the benefits of harmonizing the rootzone construction method with the corresponding irrigation delivery system to achieve uniform moisture distribution and enhanced water use efficiency, particularly in the context of SDI. Moreover, simulation tools like HYDRUS-2D effectively support turfgrass irrigation management and water conservation efforts when appropriately calibrated. The synergy between empirical findings and modeling results supports a paradigm shift toward precise, site-specific irrigation strategies that enhance irrigation efficiency in turfgrass areas. Nonetheless, further trials in urban and sports turf environments are essential to facilitate the broader implementation of these strategies.
98 Zusammenfassung Rasenflächen stellen einen essenziellen Bestandteil der städtischen grünen Infrastruktur dar und bieten funktionale, ökologische und ästhetische Leistungen in Erholungs-, Sportund öffentlichen Grünanlagen. Die Pflege qualitativ hochwertiger Rasenflächen ist jedoch mit einem erheblichen Wasserverbrauch verbunden. Angesichts global zunehmender Wasserknappheit, des Klimawandels sowie strenger werdender Umweltvorgaben stellt dies eine wachsende Herausforderung dar. Eine effiziente Wassernutzung hat sich daher zu einem vorrangigen Handlungsfeld im nachhaltigen Rasenmanagement entwickelt. Rasenflächen unterscheiden sich in ihren Zielen von traditionellen landwirtschaftlichen Systemen, wobei die Hauptziele die Strapazierfähigkeit, das Pflanzenwachstum und die Ästhetik sind. Nationale und internationale Standards geben vor, wie Rasenflächen zu bauen sind. Der Fokus liegt dabei auf den bodenphysikalischen Eigenschaften der Baustoffe, insbesondere auf deren Wasserinfiltrationsleistung. Angesichts zunehmend trockener Sommermonate und begrenzter Wasserressourcen sind jedoch innovative Beregnungsstrategien erforderlich, die einen effizienten Einsatz der Ressource Wasser ermöglichen und zugleich die Funktionalität und Belastbarkeit der Rasenflächen erhalten. Traditionelle Beregnungsmethoden, insbesondere überflurbasierende Sprinklerbewässerung (SPR), führen aufgrund von Winddrift, Verdunstungsverlusten und technisch bedingten Verteilungsungenauigkeiten oft zu einer ineffizienten Wassernutzung. Die unterirdische Tropfbewässerung (SDI) stellt demgegenüber eine zielgenauere und potenziell effizientere Wasserzufuhr dar, ist jedoch mit der Herausforderung verbunden, die Befeuchtung der Rasentragschicht sowie der Hauptwurzelzone der Rasengräser ausreichend sicherzustellen. Die Interaktion der Bewässerungssysteme mit den bodenphysikalischen Eigenschaften und der Bodenbauweise für Rasenflächen beeinflusst die Wasserverteilung, -retention und Qualität der Rasendecke. Effiziente Beregnungsstrategien erfordern ein differenziertes Verständnis der Boden-WasserDynamik, einschließlich der Auswirkungen der Bodenbauweise und des Bewässerungssystems auf die Wasserretentionseigenschaften und die Bodenfeuchtigkeitsverteilung. Die vorliegende kumulative Dissertation widmet sich der dringenden Notwendigkeit, die Effizienz der Beregnung auf Rasenflächen zu optimieren. Zu diesem Zweck werden die Auswirkungen der Bauweisen für Rasenflächen auf die Verteilung, Speicherung und Effizienz des applizierten Bewässerungswassers systematisch untersucht. Der Fokus liegt auf der Anwendung traditioneller und häufig eingesetzter Sprinklerbewässerung (SPR) sowie unterirdischer Tropfbewässerung (SDI). Ein weiterer Fokus liegt auf der Bewertung der Anwendung numerischer Simulationsmodelle, insbesondere HYDRUS-2D, zur Vorhersage und Optimierung der Beregnungseffizienz unter verschiedenen Bauweisen-BewässerungssystemsKonfigurationen. Das übergeordnete Ziel dieser Untersuchung besteht in der Evaluierung von Strategien zur Steigerung der Beregnungseffizienz auf Rasenflächen. Zur Erreichung dieses Ziels wurden über mehrere Jahre Versuche durchgeführt, die drei sich ergänzende Studien umfassten: empirische (Feldund kontrollierte Umgebung, Paper 1 und Paper 2, Abschnitt 2.1 und 2.2) und modellbasierte Studien (Paper 3, Abschnitt 2.3). Untersuchungsgegenstand war der Einfluss unterschiedlicher Bauweisen von Rasenflächen auf die Wasserverteilung, -speicherung und die Qualität der Rasendecke, insbesondere in
Zusammenfassung 99 Kombination mit Sprinklerbewässerung (SPR) bzw. unterirdischer Tropfbewässerung (SDI). Darüber hinaus bewertet die Studie den Einsatz numerischer Simulationsmodelle, insbesondere HYDRUS-2D, zur Vorhersage und Optimierung der Beregnungseffizienz unter variierenden Bodenbauweisen und Bewässerungssystemen. Die Experimente wurden sowohl unter kontrollierten Gewächshausbedingungen als auch unter Freilandbedingungen durchgeführt. Die erste Studie (Paper 1) fand im Gewächshaus statt und nutzte kontrollierte Bewässerungszyklen sowie vegetationsfreie Bodenprofile (ohne Rasendecke), um die Wasserverteilung isoliert in Abhängigkeit von Bewässerungssystem und Bauweise der Rasenfläche zu analysieren. Hierzu wurden drei Bauweisen untersucht: zwei zweischichtige Varianten, analog zum nationalen Standard, sowie eine dreischichtige Bauweise. Die Bewertung erfolgte unter Einsatz sowohl von Sprinklerbewässerung (SPR) als auch von unterirdischer Tropfbewässerung (SDI). Der volumetrische Wassergehalt (VWC) wurde nach den jeweiligen Bewässerungsvorgängen in mehreren Bodentiefen und Zeitintervallen erfasst. Die zweite Studie (Paper 2) untersuchte die Qualität der Rasendecke sowie die Wasserspeicherung innerhalb der Rasentragschicht im Rahmen eines zweijährigen Feldversuchs (2023 und 2024) unter Bedingungen der Defizitbewässerung, konkret bei 60 % der Referenz-Evapotranspiration (ETO). Die mit Deutschem Weidelgras (Lolium perenne L.) bestockten Versuchsflächen wurden, analog zu den im Gewächshaus verwendeten drei Bauweisen, entweder mittels Sprinklerbewässerung (SPR) oder unterirdischer Tropfbewässerung (SDI) bewässert. Die wichtigsten Leistungsindikatoren umfassten die Rasenqualität (TQ), die Wasserspeicherung (RWS) sowie die Bodensaugspannung (SWT) in der Rasentragschicht. In der dritten Studie (Paper 3) wurde das Finite-Elemente-Modell HYDRUS2D eingesetzt, um die Wasserdynamik innerhalb der Rasentragschicht sowie der zugehörigen Bewässerungssysteme (SPR und SDI) auf Grundlage der im Gewächshaus erhobenen Beobachtungsdaten zu simulieren. Die Modellierung der Wasserverteilung basierte auf bodenhydraulischen Parametern, die durch Laboranalysen für die in dieser Studie verwendeten Materialien ermittelt wurden. Die Kalibrierung und Validierung des Modells zielten darauf ab, die Modellqualität zu optimieren und die Abweichungen zwischen gemessenen und simulierten volumetrischen Wassergehalten zu minimieren. Die Ergebnisse der Untersuchungen zeigen, dass die Gewächshausversuche deutliche Unterschiede in den Mustern der Bodenfeuchtigkeitsverteilung in Abhängigkeit von Bewässerungssystem und Bauweise erkennen lassen. In den mit Sprinklern bewässerten Parzellen wurde ein rascher Anstieg des volumetrischen Wassergehalts (VWC) in oberflächennaher Tiefe (3 cm) beobachtet, dem innerhalb von 72 Stunden ein deutlicher Rückgang folgte, insbesondere bei zweischichtigen Konstruktionen. Demgegenüber zeigten die dreischichtigen Bauweisen mit unterirdischer Tropfbewässerung (SDI) einen ausgeprägten kapillaren Aufstieg des applizierten Wassers sowie eine hohe Retention in der Bodenmatrix, was die Effizienz dieser Systemkonfiguration unterstreicht. Die Feldversuche zeigten signifikante Unterschiede in der Wasserspeicherung (RWS) und Bodenwasserspannung (SWT) in Abhängigkeit von der Bauweise der Rasenfläche. Bei zweischichtigen Konstruktionen unter SDI wurden hohe SWT-Werte (>120 kPa) gemessen, was auf ein unzureichendes Retentionsverhalten hinweist. Im Gegensatz dazu zeigten dreischichtige Aufbauten nach 35 Tagen unter 60 % Defizitbewässerung deutlich niedrigere SWT-Werte (<15 kPa) sowie eine erhöhte Wasserspeicherung (RWS). Bei den zweischichtigen, mit SDI bewässerten Varianten
Zusammenfassung 100 kam es zu einem raschen Rückgang der Rasenqualität (TQ), wobei bereits 14 Tage nach Studienbeginn inakzeptable TQ-Werte (<6) festgestellt wurden. Im Gegensatz dazu zeigten die dreischichtigen SDI-Varianten während der gesamten Versuchsdauer höchste TQ-Werte. Die mit Sprinklern bewässerten Varianten erreichten zu Beginn ebenfalls akzeptable TQ-Werte, konnten diese jedoch aufgrund unzureichender Wasserretention in den sandbasierten Rasentragschichten und hoher Verdunstungsverluste während der Bewässerung nicht länger als 28 Tage aufrechterhalten. Hiervon abweichend zeigten die dreilagigen SPR-bewässerten Varianten über den gesamten Versuchszeitraum hinweg akzeptable TQ-Werte (>6). Das HYDRUS-2D-Modell simulierte die Wasserverteilung sowohl unter SDIals auch unter Sprinklerbewässerungssystemen zuverlässig, wobei die Kalibrierung der Modellparameter zu einer deutlichen Verbesserung der Modellqualität führte. Im Rahmen der Sensitivitätsanalysen erwiesen sich die Formparameter α und n der Wasserretentionsfunktion als kritisch für die Modellqualität, insbesondere unter Bedingungen der unterirdischen Tropfbewässerung (SDI). Die Modellqualität konnte durch die Integration von Hystereseeffekten in den Simulationsprozess deutlich gesteigert werden. Die Gesamtergebnisse verdeutlichen die komplexen, jedoch systematisch erfassbaren Interaktionen zwischen der Bewässerungsmethode und der Bauweise von Rasenflächen. Die Sprinklerbewässerung erweist sich zwar als wirksam bei der initialen, oberflächennahen Durchfeuchtung, ist jedoch nicht in der Lage, eine nachhaltige, pflanzenverfügbare Wasserspeicherung im Wurzelraum der Rasengräser sicherzustellen. Abweichend hierzu zeigen SDI-Systeme, die im Rahmen einer dreischichtigen Bauweise den kapillaren Aufstieg des applizierten Beregnungswassers fördern, eine deutlich höhere Effizienz hinsichtlich der Wasserverfügbarkeit im Wurzelraum. Aus der Analyse geht hervor, dass die dreischichtigen SDI-Systeme sowohl unter kontrollierten als auch unter Feldbedingungen durchweg höhere Wassereffizienzen und Rasenqualitäten als die Vergleichsvarianten aufwiesen. Diese Konfigurationen ermöglichen eine homogene Wasserverteilung, maximieren die Wassernutzungseffizienz und gewähren selbst bei einer Defizitbewässerung von 60% eine hohe Qualität der Rasendecke. Das kalibrierte HYDRUS-2D-Modell hat sich als kostengünstiges Instrument zur Vorhersage der Wasserdynamik und zur Information über effiziente Beregnungsstrategien bewährt. Zukünftige Forschungsarbeiten sollten hybride Bewässerungskonzepte untersuchen, die SPRund SDI-Systeme kombinieren, um die Wasserverteilung und Beregnungseffizienz weitergehend zu analysieren. Darüber hinaus sollten modellbasierte Ansätze Modelle zur Wasseraufnahme der Rasenwurzeln einbeziehen, um ein umfassenderes Verständnis der Boden-Pflanze-Wasser-Dynamik zu ermöglichen. Die integrierten Ergebnisse dieser drei Studien tragen wesentlich zum Verständnis bei, wie Bewässerungssysteme und die Bauweise von Rasenflächen die Bodenwasserdynamik sowie die Qualität der Rasendecke beeinflussen. Die Ergebnisse betonen die Vorzüge einer Harmonisierung der Bauweise der Rasenfläche mit dem entsprechenden Bewässerungssystem, um eine gleichmäßige Wasserverteilung und eine optimierte Wassernutzungseffizienz zu erzielen, insbesondere im Kontext der unterirdischen Tropfbewässerung. Darüber hinaus unterstützen Simulationswerkzeuge wie HYDRUS-2D bei entsprechender Kalibrierung effektiv das Beregnungsmanagement und die Bemühungen zur Wassereinsparung. Die Synergie zwischen empirischen und modellierten Ergebnissen
Zusammenfassung 101 begünstigt einen Wandel hin zu präzisen, standortspezifischen Beregnungsstrategien, die eine Steigerung der Beregnungseffizienz auf Rasenflächen ermöglichen. Dennoch sind zusätzliche Versuche in öffentlichen Grünund Sportanlagen für die effektive Umsetzung dieser Strategien erforderlich.
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