Comments on Uncertainty in Groundwater Governance in the Volcanic Canary Islands, Spain

Water 2015, 7, 2952-2970; doi:10.3390/w7062952 water ISSN 2073-4441 www.mdpi.com/journal/water Article Comments on Uncertainty in Groundwater Governance in the Volcanic Canary Islands, Spain Emilio Custodio 1, María del Carmen Cabrera 2,*, Roberto Poncela 3, Tatiana Cruz-Fuentes 2, Gema Naranjo 2 and Luis Olavo Puga de Miguel 4,† 1 Department of Geo-Engineering, Technical University of Catalonia (UPC), Jordi Girona, 1-3, Barcelona ES-08034, Spain; E-Mail: em[email protected] 2 Departamento de Física (GEOVOL), Universidad de las Palmas de Gran Canaria, Campus Universitario de Tarifa, Las Palmas de Gran Canaria ES-35017, Spain; E-Mails: [email protected] (T.C.-F.); [email protected] (G.N.) 3 Hydrogelogist-Consultant, Santa Cruz de Tenerife 38111, Spain; E-Mail: [email protected] 4 La Laguna University, La Laguna (Tenerife) 38001, Spain; E-Mail: [email protected] † Retired. * Author to whom correspondence should be addressed; E-Mail: mcarme[email protected]; Tel.: +34-928-454-478; Fax: +34-928-452-922. Academic Editor: Athanasios Loukas Received: 30 March 2015 / Accepted: 8 June 2015 / Published: 17 June 2015 Abstract: The uncertainty associated with natural magnitudes and processes is conspicuous in water resources and groundwater evaluation. This uncertainty has an essential component and a part that can be reduced to some extent by increasing knowledge, improving monitoring coverage, continuous elaboration of data and accuracy and addressing the related economic and social aspects involved. Reducing uncertainty has a cost that may not be justified by the improvement that is obtainable, but that has to be known to make the right decisions. With this idea, this paper contributes general comments on the evaluation of groundwater resources in the semiarid Canary Islands and on some of the main sources of uncertainty, but a full treatment is not attempted, nor how to reduce it. Although the point of view is local, these comments may help to address similar situations on other islands where similar problems appear. A consequence of physical and hydrological uncertainty is that different hydrogeological and water resource studies and evaluations may yield different results. Understanding and coarsely evaluating uncertainty helps in reducing administrative OPEN ACCESS Water 2015, 7 2953 instability, poor decisions that may harm groundwater property rights, the rise of complaints and the sub-optimal use of the scarce water resources available in semiarid areas. Transparency and honesty are needed, but especially a clear understanding of what numbers mean and the uncertainty around them, to act soundly and avoid conflicting and damaging rigid attitudes. However, the different situations could condition that what may be good in a place, may not always be the case in other places. Keywords: uncertainty; groundwater; governance; Canary Islands 1. Introduction Natural processes and natural behavior are essentially uncertain due to a stochastic component, which is always present at any spatial and time scale, even when they are well known and accurately measured. This uncertainty can be called essential or non-reducible. Other uncertainty components are added, often dominant ones under common circumstances. These are due to knowledge or epistemic deficiency, measurement and monitoring deviations, bias and errors. They are also due to the application of conceptual models that are inadequate or erroneous or that are based on insufficient analysis or a partial or too simplified consideration of the involved contributing processes. These last sources of uncertainty are often the dominant ones. A further kind of uncertainty is that derived from using falsified, invented data and concealed and deformed facts, which is unfortunately something that happens from time to time, if not the common situation in some cases. These non-essential kinds of uncertainty can be decreased with improved effort and care, although with increased human effort and time devoted to checking the data and results. However, the economic expenses involved may be high, and the cost could become excessive and disproportionate in the context and in the economic and social value of what has to be managed in a particular case. This is reflected in the popular saying: the perfect is the enemy of good. All that has been said above is valid on hydrological grounds, and particularly for groundwater, despite that the data and results are often presented with many digits. Most of them are actually non-significant. Giving them communicates an illusory expectation of accuracy to non-experts and especially to those that make decisions and conform policies. Often, only two or three digits are valid and, in some cases, only one. It is important to calculate, show and internalize the real accuracy of the data and results. Quite a large uncertainty is common in hydrometeorology, surface water hydrology and groundwater hydrology, as is the normal case in the evaluation of natural magnitudes, with different nuances in each case. In the two mentioned sciences, time variability has more weight than in groundwater hydrology (hydrogeology), in which land and ground properties play a dominant role in uncertainty. Furthermore, environmental, economic and social results, which are as or more important to water users and society, are quite uncertain. Uncertainty can be measured by deviations from a centered value, such as the mean, median or mode. For data close to a normal (Gaussian) distribution, the uncertainty of a magnitude with mean value m can be measured by the standard deviation (σ) and be given as m ± σ (comprising 68.3% of the possible Water 2015, 7 2954 realizations) or as m ± 2σ (comprising 95.4% of possible realizations). It can be also given as dimensionless as m/σ, which is called the coefficient of variation (CV), as an absolute value or as a percent. For non-normal data distributions that cannot be mathematically transformed into a normal one, uncertainty can be measured as the difference between to symmetrical percentiles, such as 20% and 80%, and refers to the median as a centered value. When data do not form series with at least a few terms or concepts that cannot be quantified, uncertainty is appreciated. As much as possible, it is convenient to qualify these data according to some references or categories in order to get an approximate semi-quantified value. The uncertainty of some calculation results can be obtained from the squared root of their statistical variance. In the case of normally-distributed variables, the variance of a calculated result is the weighted sum of the variances of the different variables involved, following the error propagation rule. In the case of closed formulae, the variances and the respective weighting factors can be readily obtained by calculation, but in more complex cases, a sensitivity analysis and expert evaluation of the variable variances and the weighting factors are required. The object of this paper is neither to advance the study of uncertainty in water and groundwater resources, which is developed in a large number of recent publications, such as [1–5], and many others, nor to provide a detailed analysis of hydrological, economic and social uncertainty components, but to comment on the actual circumstances on the Canary Islands. The Canary Islands, besides the local interest, present circumstances that are also found on other high volcanic and non-volcanic islands in arid and semiarid areas. Commenting on their circumstances may help to solve or avoid some groundwater governance problems in other areas, taking care of the sometimes dominant influence of the local framework. Thus, the contents do not present a systematic analysis, nor quantitative considerations, nor the way to solve accuracy aspects, but just problems found in practice. They may be relevant in many situations in the semiarid part of Europe as a result of the application of the European Water Framework Directive by many untrained water authorities or poorly-informed politicians. Thus, this paper deals with the uncertainty problems in the particular case of the Canary Islands, due to their special conditions for water governance, which are more clear cut than in other areas of Spain. The MASE project (Minería del Agua Subterránea en España (Groundwater Mining in Spain)) [6] has focused on the Canary Islands, in addition to southeastern continental Spain, to collect and analyze information [7]. Most comments refer to Gran Canaria and Tenerife Islands, which are the main islands according to their economic output and population, with important intensive groundwater use. 2. The Canary Islands The Canary Islands (the Canaries) are a volcanic archipelago of the region known as Macaronesia, consisting of seven major islands and a few small isles and islets. They are located in the eastern Atlantic Ocean, between 27°37′ and 29°25′ N and 13°20′ and 18°10′ W, across from the Saharan coast of Africa (Figure 1). Along a length of 400 km from east to west, the islands are Lanzarote (LZ), Fuerteventura (FV), Gran Canaria (GC), Tenerife (TF), La Gomera (GO), La Palma (LP) and El Hierro (HI). There are large economic, population and physical variations from island to island. The Canary Islands are one of the Autonomous Regions of Spain. The total surface area is 7447 km2. About 2.2 million inhabitants live on them, plus a large number of tourist visitors year-round. Water 2015, 7 2955 Figure 1. Situation of the Canary Islands. The location of the example areas in Gran Canaria mentioned in the text are shown. Stressed water conditions dominate, and intensive groundwater exploitation is common, even with groundwater mining [8]. From the hydrogeological point of view, the islands consist of a low permeability volcanic core, covered and surrounded by younger, more permeable, heterogeneous volcanic materials and some derived sediments. Circumstances vary from island to island and from site to site. Groundwater flows towards the coast, and on the way down, under the original natural conditions, spring areas appeared and fed some permanent flows in some tracts of the deep gullies. They have mostly disappeared due to intensive groundwater development. Besides direct channeling of spring water, where it is still available, groundwater is intensively developed through horizontal and vertical works. Horizontal works consist of tunnels (water galleries) penetrating deep enough to intersect the water saturated formations at the moment; they are more developed in Tenerife. Vertical works are deep, large diameter wells (Canarian wells), excavated using mining technology. Both of them may contain secondary works, mainly water galleries and small diameter, long horizontal boreholes to increase the chance of intersecting permeable features. More recently, deeply-penetrating, mechanically-drilled wells have been introduced or used to deepen existing Canarian wells. Seawater desalination was introduced to Lanzarote in the early 1960s and in the early 1970s to Gran Canaria. All of this water is now commonly used for urban supply and for irrigation of cash crops. In addition, brackish groundwater is freshened by means of reverse osmosis or electrodialysis, mostly for crop irrigation in small, private facilities. Reclamation of treated urban waste water for agricultural uses started in the late 1970s, but its development is lagging behind expectations. Groundwater is the most important natural water resource in the high islands and the dominant one, greater than sea water desalination and still less expensive, in spite of the extreme conditions for obtaining it [6,7]. Groundwater development by means of galleries and wells started in the late 19th century and become intensive after 1930 and especially in the period 1950–1970. Administratively, each island is currently a separate water district (Consejo Insular de Aguas or Insular Water Council). The variable island circumstances create special conditions for water governance. Local water legislation follows, but is different from, the Spanish Water Act to consider the special circumstances in the islands. Currently it incorporates the European Water Framework Directive, mostly Water 2015, 7 2956 through the Spanish Water Act, as the Canarian Parliament has not yet enacted a review of their previous water law. Table 1 summarizes the estimated water resources in the 2000–2010 decade, after groundwater extraction started to decrease. Table 1. Average water resources in the 2000–2010 decade, when groundwater extraction started to decrease and the use of reserves was dwindling. Values in hm3/year (1 hm3 = 1 cubic hectometer = 1 million m3 = 1 × 106 m3). Data come from the 2010 Island Water Plans. Data may vary according to the report and author and are the result of applying non-uniform methodologies. They are given only to show the order of magnitude. Uncertainty has not been given, but it is high. LZ, Lanzarote; FV, Fuerteventura; GC, Gran Canaria; TF, Tenerife; GO, La Gomera; LP, La Palma; HI, El Hierro. Island LZ FV GC TF GO LP HI Total Total Water Precipitation 134 184 466 865 140 518 101 2408 Surface runoff 1.3 5 75 20 8 1.5 0.6 111 Recharge 3.3 14 87 239 63 265 27 642 Available Resources Surface water 0.1 0.0 24 0.0 1.4 0 0 25 Groundwater 0.5 2 100 * 180 * 4.5 58 2.4 347 * Springs 0 0.0 0.1 5 7 10 0 22 Seawater desalination 19 12 60 19 0 0 0.5 110 Brackish water desalination 0.5 2 18 0 0 0 0 20 Reuse 0 0 12 8 0 0 0.4 20 Note: * A significant fraction comes from the depletion of groundwater reserves. 3. Considerations on Some Uncertainty Components of Groundwater Resources Evaluation in Gran Canaria and Tenerife Many situations and facts contribute to knowledge and management uncertainty. They influence water governance. It is difficult to analyze them briefly; they are not treated systematically, but only those that are considered relevant in the local circumstances are discussed. Some aspects are highlighted as those that contribute the most under common circumstances. For the sake of brevity, only some of the components that refer to the evaluation of groundwater resource quantity will be considered, leaving aside those related to water salinity and quality, which are more complex, although not less important and a growing concern, and these also are an essential part or water governance. Groundwater resources are generally quantified by a water balance of the aquifer or aquifer system. The water balance has four main general components: recharge, discharge, storage change and water exchange with other units in the ground. The balance is carried out in a given volume, so boundary conditions are often important to define some terms. Boundary and aquifer conditions are often difficult to define and may greatly contribute to uncertainty. Furthermore, hydrological boundaries may greatly differ from administrative and political boundaries that have to be adopted in practice. The use of political boundaries, often demanded by politicians and the water administration, increases uncertainty, as conditions within these boundaries are difficult to clearly and accurately define from a Water 2015, 7 2957 hydrogeological point of view, both for direct use or to split results obtained when hydrological boundaries are used. This is often a serious cause of litigation unless rules to resolve partitions that are not clearly defined by applying hydrological reasoning. On the Canary Islands, each island as a whole has a well-defined boundary at the coast line, but when a part of it has to be considered (e.g., when the scale of study is decreased), problems in defining appropriate boundary conditions appear [9–11]. Recharge is often the most difficult to evaluate for the water balance, as the wide time and space variability of precipitation and atmospheric conditions combines with large land, terrain and vegetal cover variability. Recharge evaluation is an important scientific and technical challenge that is accompanied by a noticeable uncertainty in all of its aspects. The quantification of recharge can be undertaken through a wide set of methods, but which are not equivalent among themselves, as the results they yield depend on the spatial and time scale. Calculations involve assumptions of the processes and the estimation of local and lumped physical parameters, sometimes risky ones, whose uncertainty increases the more the terrain differs from a homogeneous granular one. To increase the accuracy, independent methods have to be applied, if possible. Diverse general publications present different recharge evaluation and calculation methods [12–17]. For a relatively large territory, the possible methods to calculate recharge from precipitation reduce to soil water balance, balance of atmospheric chloride deposition, water table fluctuations, numerical modeling and, recently, satellite-based observations. They are not always applicable and may yield different results (net and total recharge, time variable recharge or long-term average recharge). Another problematic aspect can be the calibration, which is not always possible in some of the methods. In some cases, uncertainty can be restricted by using other more detailed associated methodologies. Rainfall recharge may be made more complex for evaluation when soil and rock discontinuities favor recharge, avoiding soil storage. In some circumstances, concentrated recharge from surface water bodies and rivers may be important, especially the more arid the climate is. This recharge involves high uncertainty, especially for ephemeral flows. Recharge resulting from excess irrigation water may become important in many areas, and the uncertainty of water application is a new component to be considered. Recharge uncertainty is especially acute in the Canary Islands due to conspicuous altitudinal changes, extensive areas of dissected relief and important geological changes. To increase accuracy, much territorial detail is needed, but in reality, there is not enough meteorological data, even if the actual density of stations is in general greater than what is common in continental Spain and on most islands. Increasing the number of meteorological stations is too costly. On the Canary Islands, recharge is derived from rainfall infiltration, with a small effect of snow and surface water infiltration. However, storm runoff is sporadically produced, which may have some relevance in water resources, especially in some areas of Gran Canaria Island, as well as the recharge produced by return irrigation flows in the peripheral agricultural areas of Gran Canaria, Tenerife and La Palma Islands. Gauging the many small, ephemeral and sporadic gullies is too costly and even practically unfeasible. In the case that there are some dams to temporarily store water, there is the possibility to calculate occasional runoff, but the infiltration tract mostly remains unknown. These reservoir dams are rare, and in fact, they are only significant in Gran Canaria and La Gomera, where they are in the high parts; so, they only yield information in the headwaters in the infrequent cases in which they are monitored. This monitoring is not an easy task, because they usually also store groundwater from wells and water galleries. Water 2015, 7 2958 La Aldea aquifer, located at the west of Gran Canaria, is a good example of an aquifer that is isolated from the island’s main aquifer and that needs specific water planning conditions. A numerical model allowed reducing and estimating recharge uncertainty (Figure 2). The La Aldea aquifer is highly sensitive to the hydraulic conductivity and storage coefficient of the basalts, the alluvial units and the scree deposits, so the uncertainty of these parameters has a large influence on the results and the derived decisions. Uncertainty has been reduced to some extent by considering salinity transport [18]. Measurement of these parameters by hydraulic tests is rarely feasible and at a high cost relative to the improvement in the results. The high groundwater salinity is the result of high evapoconcentration due to the aridity and the return irrigation flows in the agricultural areas using imported water and local groundwater, with variable proportions according to surface water availability each year. This helps to reduce the uncertainty of groundwater rainfall recharge relative to that of return irrigation flows and external water irrigation loses. Figure 2. La Aldea area (NW of Gran Canaria). (A) Groundwater head contours (m asl) and flow paths corresponding to the 1991–1992 hydrologic year; (B) total recharge to the study area resulting from the hydrogeological model. Results depend highly on the variable yearly rainfall contribution, surface reservoir management, antecedent conditions and local decisions. Water 2015, 7 2959 Worldwide, the most used method for rainfall diffuse recharge evaluation at a large scale, which is needed for groundwater planning, is the soil water balance. Daily rainfall values are strictly needed in the case of arid and semiarid areas to get reliable results. Several calculation codes are available, using different approaches to the different processes involved. Results, even in relatively homogeneous areas, may be highly biased, as the parameters are often poorly known and sometimes reduce to best guesses. To refine the results, they have to be compared to groundwater data, mainly water table fluctuations and spring and river base flow records, to improve parameter values through calibration. This calibration, and the following validation, if enough data are available, along a sufficiently long time period, can be done to try to reproduce real data as best as possible [19], assuming that no significant changes in the aquifer state along time have been produced. To estimate the associated uncertainty, a sensitivity analysis is needed, but it is still poorly systematized and tedious, so it is rarely done. The main obstacle is getting reliable information on the variables’ variance, which needs tests and a relatively high number of detailed studies in diverse parts of the area. On the Canary Islands, the code Visual Balan [20] or the simpler one Easy-Bal developed by the “Curso Internacional de Hidrología Subterránea” have been used occasionally. On Tenerife Island, less detailed water balances have been applied in a dense network covering the whole island, similarly to the SIMPA technique of the Centro de Estudios Hidrográficos (CEDEX, Madrid, Spain). Calibration capability is a serious limitation on the Canary Islands, as continuous long records of frequently-measured water table changes are not available and probably will be not available due to the difficult local conditions. The water table is often very deep, which means that its reaction to recharge has a long delay and is highly smoothed, besides an important disturbance due to groundwater extraction and the effect of short-circuiting produced by the uncased wells. This makes recharge evaluation very uncertain, independent of the territorial detail and the efforts to improve monitoring. Some preliminary attempts to evaluate recharge uncertainty through sensitivity analysis of daily water balance in the soil have been attempted in northern Gran Canaria [21]. Three catchments have been considered, and each of them has been divided into a low part (precipitation < 400 mm/year), a mid-part (precipitation between 400 and 600 mm/year) and a high part (precipitation > 600 mm/year). Natural recharge was calculated for the period 1980–2013 using Easy-Bal 3.0, which considers soil thickness, maximum soil water reserve and wilting point, daily precipitation and a threshold for runoff generation. Results are shown in Figure 3. Recharge is especially sensible to the maximum soil water reserve and the threshold for runoff generation. Defining averaged soil characteristics in a rather extensive area is difficult not only because of the high spatial variability, but also the thin, poorly-developed soils or their absence in parts of the water balance areas. This cannot be done by measurement, but through calibration, provided data are available. A deep water table and geological variability do not help, so uncertainty has to be coarsely estimated by comparing with other methods and, in the end, assumed as a reality to be incorporated in management and governance. Water 2015, 7 2960 Figure 3. Preliminary recharge estimations in mm/year in the northeast of Gran Canaria Island, extending from the coast to the top [21]. Recharge ranges shows the uncertainty that can be expected in each area according to the two most sensible parameters: maximum soil water reserve (in italics) and threshold for runoff generation (underscored). The third figure in brackets refers to the estimated average ratio of yearly recharge to rainfall, as a fraction. N2, N3 and N4 refer to catchment areas of the Water Plan. Another method to estimate average multiannual recharge, independent of the soil water balance, is the chloride atmospheric deposition balance in the soil. This is a method for a large scale that lumps land properties to obtain averaged recharge values in the long term, but that has to be applied under steady conditions, without land use and climate changes in addition to no changes in the chloride concentration and storage along the flow path through the unsaturated zone [22]. A long sampling time is needed to get the needed data when they are not previously available. Although they are not always available for Water 2015, 7 2967 Workshop on the Study, Use and Management of Water in Volcanic Terrains and Islands, held in Las Palmas de Gran Canaria in January 2014, is acknowledged. Two unknown reviewers and the editor have contributed very useful comments and corrections to improve the text and to extend it to comments not considered at the beginning, but maintaining the objective of contributing experience instead of contributing structured and detailed analysis of uncertainty. Author Contributions Emilio Custodio developed the MASE Project for the Canary Islands and wrote the paper with the help of María del Carmen Cabrera. Roberto Poncela, Luis Puga, Tatiana Cruz-Fuentes, Gema Naranjo and María del Carmen Cabrera have provided data, comments and advice about the Canary Islands hydrogeological, social and economic circumstances. Conflicts of Interest The authors declare no conflict of interest. References 1. Beven, K. Facets of uncertainty: Epistemic uncertainty, nonstationarity, likelihood, hypothesis testing, and communication. Hydrol. Sci. J. 2015, doi:10.1080/02626667.2015.1031761. 2. Elshall, A.S.; Tsai, F.T.C. Constructive epistemic modeling of groundwater flow with geological structure and boundary condition uncertainty under Bayesian paradigm. J. Hydrol. 2014, 517, 105–119. 3. Guillaume, J.; Qureshi, M.; Jakeman, A. A structured analysis of uncertainty surrounding modeled impacts of groundwater-extraction rules. Hydrogeol. J. 2012, 20, 915–932. 4. 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