Rainfall–Discharge Decoupling and Infrastructure Resilience in a Semi-Arid Reservoir: Evidence from 43 Years at Kiri Dam, Nigeria

Reservoirs in semi-arid regions face challenges due to evolving hydrological patterns, land use changes, and climate variability. This study analysed 43 years of precipitation and discharge data at Kiri Dam, Nigeria, employing statistical tests and modelling techniques to assess resilience and sustainability. The rainfall exhibited considerable variability (656.4–1,260.1 mm; mean 952.6 mm) with no statistically significant trend (Z = 1.23, p = 0.217; Sen’s slope = +1.75 mm/year), indicating climatic stability. Conversely, discharge demonstrated a significant annual increase (mean 74,810 m³/s; Z = 3.17, p = 0.0015; Sen’s slope = +628.84 m³/s/year) and seasonal augmentation during July to September (Z = 2.58, p = 0.0099; Sen’s slope = +897.62 m³/s/year). The correlation between rainfall and discharge was weak (Pearson r = 0.241, p = 0.120; R² = 0.058), with rainfall accounting for less than 6% of inflow variability. This suggests that land use change, catchment degradation, and sediment yield exert significant influence on inflows. Seasonal discharge peaks occur in September (~65,000 m³/s), approximately one month after rainfall, thereby increasing sedimentation and spillway stress. These findings have implications for dam safety, capacity, and downstream water supply. Enhancing resilience requires adaptive operational strategies, catchment conservation, sediment management, and revised spillway standards. The integration of monitoring and modelling is essential for the effective management of water infrastructure within semi-arid regions of West Africa.