Empirical Analysis of Rainfall Trend Variability and Implications for Water Resource Management in the Sokoto-Rima River Basin

Long-term rainfall trends in Nigeria's semi-arid North West zone carry significant implications for water resource availability in the Sokoto-Rima River Basin, a critical source of irrigation and domestic water supply for Sokoto, Kebbi, and Zamfara states. This study empirically analyzes spatiotemporal rainfall variability and trends across the Sokoto-Rima Basin using daily rainfall records from nine NiMet stations for the period 1981 to 2022. The Mann-Kendall trend test, Sen's slope estimator, and the innovative trend analysis method were applied to detect monotonic and non-monotonic trends at annual, seasonal, and monthly timescales. The Standardized Precipitation-Evapotranspiration Index was computed to characterize drought and surplus periods within the basin. Wavelet analysis identified a dominant 11-year cycle and a 3-5 year cycle consistent with ENSO variability. Results document a statistically significant declining trend in basin-wide seasonal rainfall at Sokoto, Birnin Kebbi, and Gusau stations, with Sen's slope values ranging from minus 2.8 to minus 4.1 millimetres per year. Water balance modelling using the Thornthwaite method indicates that declining rainfall has reduced basin inflows to the Goronyo Dam by an estimated 18 percent compared to the 1981-2000 reference period. Findings provide an empirical foundation for revising the basin's water allocation framework and inform NiMet's regional climate services portfolio for North West Nigeria. Keywords: rainfall trend, Sokoto-Rima Basin, Mann-Kendall, water resources, wavelet analysis.