RAINFALL VARIABILITY AND LONG-TERM TRENDS IN OWERRI, SOUTHEASTERN NIGERIA

Volume-09 Issue 10, October-2025 ISSN: 2456-9348 Impact Factor: 8.232 International Journal of Engineering Technology Research & Management (IJETRM) https://ijetrm.com/ IJETRM (http://ijetrm.com/) [346] RAINFALL VARIABILITY AND LONG-TERM TRENDS IN OWERRI, SOUTHEASTERN NIGERIA Ugwuegbulam, J.C 1, Osuagwu, J.C 2, Nwoke, H.U 3, 1, Lecturer, Civil Engineering Department, School of Engineering Technology, Federal Polytechnic Nekede, Owerri, Nigeria 2,3, Lecturer, Civil Engineering Department, School of Engineering and Engineering Technology, Federal University of Technology, Owerri, Nigeria [email protected] [email protected] ABSTRACT Rainfall variability critically shapes agriculture, water security, and ecosystem resilience in West Africa, yet southeastern Nigeria remains underrepresented in hydroclimatic analyses. This study evaluates six decades (1961–2020) of monthly rainfall in Owerri to characterize descriptive statistics, seasonal cycles, interannual variability, and long-term trends. After screening and validation, data were analyzed using descriptive metrics, anomaly classification, and trend detection via the Mann–Kendall test and Sen’s slope. Results show a mean annual rainfall of 2178 mm (SD = 103 mm; CV = 4.8%), with extremes ranging from 1736 mm in 1969 (driest year) to 2371 mm in 1962. Classification revealed 6 wet years (>2281 mm), 7 dry years (<2075 mm), and 47 normal years (78%), underscoring the dominance of normal conditions but highlighting the hydrological importance of extremes. Seasonality is unimodal, with ~85% of totals concentrated in April–October, marked by July and September peaks and a distinct August break. Interannual anomalies (–442 to +193 mm) confirm moderate variability (CV = 4.7%), with wetter phases dominating the 1980s–1990s and relative stabilization after 2000. Regression analysis indicated a weak but significant upward trend of 2.02 mm year⁻¹ (R² = 0.11, p = 0.008), amounting to ~120 mm over six decades. Comparison with Lagos and Douala situates Owerri within broader West African rainfall shifts, where local variability reflects both regional climate drivers and microclimatic factors. Overall, extremes rather than mean increases define Owerri’s rainfall regime, with implications for agriculture, hydrology, and adaptation planning, while future integration of climate model projections and ENSO/IOD teleconnections could clarify the anomalies and seasonal peaks evident in Figures 1–3. Keywords: Rainfall variability; Owerri; West Africa; Seasonality; Climate trends; Hydrological extremes. INTRODUCTION Rainfall is a critical component of the hydrological cycle, sustaining agriculture, water supply, and ecological balance in tropical regions. In West Africa, rainfall variability has been linked to recurrent droughts, floods, and food insecurity [1], [9]. Understanding rainfall dynamics is particularly crucial in southeastern Nigeria, where livelihoods and infrastructure are heavily dependent on seasonal rainfall. Owerri, the capital of Imo State, experiences a humid tropical climate with high annual rainfall. Despite the availability of long-term meteorological records, few studies have undertaken systematic analyses of rainfall trends in this region. Broader West African studies suggest significant shifts in monsoon dynamics and associated rainfall variability [2], while recent nationwide analyses reveal complex patterns of rainfall change across Nigeria [5]. This study addresses this gap by examining 60 years of monthly rainfall data for Owerri (1961–2020). Specifically, it Volume-09 Issue 10, October-2025 ISSN: 2456-9348 Impact Factor: 8.232 International Journal of Engineering Technology Research & Management (IJETRM) https://ijetrm.com/ IJETRM (http://ijetrm.com/) [347] evaluates interannual variability, seasonal distribution, and long-term trends. The outcomes will support decision-making in agriculture, flood management, and infrastructure development. MATERIALS AND METHODS Study Area Owerri (5°29′N, 7°02′E) lies in southeastern Nigeria’s humid rainforest belt, with mean annual rainfall exceeding 2,000 mm. The climate is bimodal, peaking in April–July and September–October, separated by an August break. Data Collection Monthly rainfall records (1961–2020) were obtained from the Ministry of Environment’s Meteorological Unit. The dataset comprised continuous, station-based monthly totals (mm). Data Preprocessing (i) Screening: Implausible values (<5 mm in peak wet months) were cross-checked and corrected against regional climatology. (ii) Validation: Missing data were interpolated using adjacent-year monthly means. (iii) Formatting: Records were converted into annual and monthly time series. Analytical Methods (i) Descriptive statistics: mean, standard deviation (SD), and coefficient of variation (CV = SD/mean × 100). (ii) Seasonal decomposition: mean monthly climatology to describe intra-annual distribution. (iii) Trend detection: Ordinary least squares regression and Mann–Kendall test (non-parametric, robust against non-normality) with Sen’s slope estimator for trend magnitude [12], [11]. (iv) Anomaly and variability analysis: Standardized rainfall anomalies were computed as: Where is annual rainfall, the long-term mean, and the standard deviation [8]. Positive and negative anomalies denote wetterand drier-than-normal years. RESULTS The results are presented on Table 1,Figure 1,Figure 2 and Figure 3. Descriptive Characteristics Table 1 shows that annual rainfall in Owerri (1961–2020) averaged 2178 mm (SD = 103.4 mm, CV = 4.8%), with extremes ranging from 1735.8 mm (1969) to 2371.4 mm (1962), indicating moderate interannual variability. Based on ±1 SD thresholds, 6 wet years (>2281 mm: 1962, 1965, 1983, 1985, 1993, 2000), 7 dry years (<2075 mm: 1961, 1966–1969, 1977, 1984), and 47 normal years (78.3%) were identified, showing that while normal conditions dominate, extreme anomalies remain hydrologically significant. Linear regression revealed a weak but significant increasing trend (R² = 0.114; slope = 2.02 mm year⁻¹; p = 0.008), amounting to ~120 mm rise over six decades. Decadal patterns highlight strong variability in the 1960s, relative stability in the 1970s, wetter phases in the 1980s–1990s, and post-2000 stabilization. Overall, extremes rather than the modest trend define the regime, underscoring the need for adaptation strategies that prioritize variability and extremes over mean annual values. Table 1. Summary statistics of annual rainfall in Owerri (1961–2020). Volume-09 Issue 10, October-2025 ISSN: 2456-9348 Impact Factor: 8.232 International Journal of Engineering Technology Research & Management (IJETRM) https://ijetrm.com/ IJETRM (http://ijetrm.com/) [348] Year Annual Rainfall (mm) Year Annual Rainfall (mm) Year Annual Rainfall (mm) 1961 1999.8 1981 2128.3 2001 2182.4 1962 2371.4 1982 2221.4 2002 2160.3 1963 2036.7 1983 2294.1 2003 2208.8 1964 2020.7 1984 2120.1 2004 2134.4 1965 2279.9 1985 2293.2 2005 2179 1966 2046 1986 2291.6 2006 2260.6 1967 1996.1 1987 2125.9 2007 2168.2 1968 1853.2 1988 2273.1 2008 2243.9 1969 1735.8 1989 2215.7 2009 2197.2 1970 2261.5 1990 2206.7 2010 2189.5 1971 2160.1 1991 2208.2 2011 2249.3 1972 2226.9 1992 2174 2012 2163.5 1973 2227.1 1993 2296.3 2013 2191 1974 2157.7 1994 2293.9 2014 2147.9 1975 2208.3 1995 2257.4 2015 2195.6 1976 2172.2 1996 2181.4 2016 2149.3 1977 2109.5 1997 2207.7 2017 2231.1 1978 2129.3 1998 2261.2 2018 2214.7 1979 2126 1999 2169.7 2019 2202 1980 2189.4 2000 2280.8 2020 2205 Period Mean (mm) Minimum (mm) Maximum (mm) Std. Dev. (mm) Coefficient of Variation (%) 1961– 2020 2178 1735.8 2371.4 103.4 4.75 Seasonal Variability Figure 1 demonstrates a distinct unimodal rainfall regime, with minima in December–February (<50 mm), a progressive rise from March, and peaks in July and September (~330 mm), followed by a sharp decline after October. This cycle reflects the seasonal migration of the ITCZ and associated monsoonal dynamics, concentrating rainfall in the main wet season. April–October contributes ~85% of annual totals, whereas the dry season (November–March) provides only ~15%, underscoring the system’s strong temporal asymmetry. Such concentration heightens sensitivity to variability, where shifts in onset, intensity, or cessation directly affect agricultural productivity, hydrological balance, and ecosystem resilience. The pronounced wet–dry contrast therefore emphasizes the importance of adaptive strategies that account for seasonal rainfall distribution rather than annual aggregates. Volume-09 Issue 10, October-2025 ISSN: 2456-9348 Impact Factor: 8.232 International Journal of Engineering Technology Research & Management (IJETRM) https://ijetrm.com/ IJETRM (http://ijetrm.com/) [349] Figure 1. Mean monthly rainfall distribution in Owerri (1961–2020). Interannual Variability Figure 2 highlights marked interannual rainfall variability relative to the 1961–2020 mean (2178 mm). Negative anomalies dominated the 1960s, culminating in the severe deficit of 1969 (–442 mm, the driest year on record). By contrast, the 1980s–1990s were characterized by predominantly positive anomalies, with several years exceeding +100 mm, reflecting a wetter phase likely linked to ENSO and other large-scale climate drivers. Since the 2000s, anomalies have clustered closer to zero, indicating relative stabilization but with fluctuations persisting. Statistically, anomalies ranged from –442 to +193 mm, with a standard deviation of 102.6 mm and coefficient of variation of 4.7%, confirming moderate variability. Of the 60 years assessed, 36 (60%) recorded positive anomalies and 24 (40%) negative anomalies, underscoring a predominance of wetter-than-average years in recent decades. This shift from negative to positive anomalies aligns with the weak but significant long-term upward trend, yet extremes remain the defining feature of the regime, with critical implications for water security, agriculture, and climate adaptation. Figure 2. Annual rainfall anomalies relative to the long-term mean (1961–2020). Long-Term Trend Figure 3 illustrates the long-term trend of annual rainfall in Owerri (1961–2020). Linear regression indicates a weak but statistically significant increase (R² = 0.114, p = 0.008), expressed as Rainfall (mm) = 2118.5 + 2.02 × (Year – 1961), with the slope implying a cumulative rise of ~120 mm over six decades. Rainfall anomalies ranged from –442 mm (1969, driest year) to +193 mm (1993), yielding a standard deviation of 102.6 mm and Volume-09 Issue 10, October-2025 ISSN: 2456-9348 Impact Factor: 8.232 International Journal of Engineering Technology Research & Management (IJETRM) https://ijetrm.com/ IJETRM (http://ijetrm.com/) [350] coefficient of variation of 4.7%, consistent with moderate variability. Of the 60 years analyzed, 36 (60%) exhibited positive anomalies and 24 (40%) negative anomalies, while classification relative to ±1 SD identified 6 extreme wet years (>+103 mm: 1962, 1965, 1983, 1985, 1993, 2000) and 7 extreme dry years (<–103 mm: 1961, 1966–1969, 1977, 1984). This distribution underscores the predominance of wetter-than-average years in recent decades, yet confirms that extremes, rather than the modest upward trend, remain the defining feature with significant implications for hydrology, agriculture, and climate adaptation. Figure 3. Long-term trend of annual rainfall in Owerri (1961–2020). DISCUSSION The analysis of rainfall variability in Owerri over 1961–2020 reveals a regime characterized by moderate interannual variability, pronounced seasonality, and a weak but statistically significant long-term upward trend. These findings align with broader West African hydroclimatic dynamics, where rainfall regimes are shaped by the interplay of the ITCZ, Atlantic Niño, and ENSO teleconnections [13], [6]. The observed unimodal seasonality, with ~85% of rainfall concentrated in April–October, mirrors patterns in other West African cities such as Lagos and Enugu, where studies have reported similar dependence on monsoonal peaks and heightened vulnerability to onset and cessation shifts [7], [10]. The predominance of wetter-than-average years in recent decades in Owerri also reflects a wider regional signal. Ebodé [4] documented increasing rainfall trends across southern Cameroon, while Biasutti [3] highlighted the persistence of Sahelian rainfall recovery since the 1980s, suggesting a broad West African transition toward wetter regimes following the severe droughts of the 1960s–70s. Importantly, extremes remain hydrologically significant: our classification identified six wet years and seven dry years outside ±1 SD, a pattern echoed in nearby humid tropical zones where extreme rainfall years exert disproportionate impacts on agriculture and flood risk [4], [6]. Thus, situating Owerri within the regional literature underscores two key points: first, while long-term rainfall totals show only modest upward trends (~2 mm yr⁻¹), the temporal concentration and episodic extremes are the dominant features of the regime; and second, these features are consistent with hydroclimatic signals observed across the Gulf of Guinea region, where variability and extremes, rather than means, drive the most critical socio-environmental challenges. Effective adaptation strategies must therefore emphasize the management of intraand interannual variability, accounting for both agricultural drought and flood hazards in line with regional climate risk assessments [3], [6]. CONCLUSION This study analyzed six decades (1961–2020) of rainfall dynamics in Owerri, southeastern Nigeria, integrating descriptive statistics, anomaly classification, seasonal distribution, and trend analysis. The results demonstrate that while mean annual rainfall (2178 mm) has remained relatively stable, interannual and seasonal variability— rather than long-term shifts—constitute the defining hydroclimatic feature. Extreme wet (6 years) and dry Volume-09 Issue 10, October-2025 ISSN: 2456-9348 Impact Factor: 8.232 International Journal of Engineering Technology Research & Management (IJETRM) https://ijetrm.com/ IJETRM (http://ijetrm.com/) [351] episodes (7 years) punctuate otherwise “normal” conditions, underscoring the role of variability in shaping agricultural and hydrological outcomes. Seasonality is strongly unimodal, with ~85% of totals concentrated in April–October (Figure 1), amplifying vulnerability to shifts in onset or cessation. Although a weak but significant upward trend of 2.02 mm year⁻¹ was detected (Figure 3), extremes and anomalies (Figure 2) exert far greater impacts than gradual change. Comparisons with broader West African evidence highlight both regional coherence and local deviations driven by microclimatic and land-use factors. Future research should extend this baseline analysis by integrating climate model projections and teleconnection diagnostics (e.g., ENSO, IOD, Atlantic SST anomalies), as these large-scale drivers likely modulate the anomalies (Figure 2) and seasonal peaks (Figure 1) observed. Notably, while Owerri shows relative stabilization post-2000, coastal hubs such as Lagos and Douala have reported stronger ENSO-linked rainfall swings, underscoring the importance of situating Owerri within regional teleconnection frameworks. Embedding such diagnostics into long-term analyses will improve predictive skill and strengthen early-warning systems, ultimately supporting climate-resilient agriculture, water management, and urban planning in southeastern Nigeria. REFERENCES [1] Adefolalu, D. O. (1986). Rainfall trends in Nigeria. 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