Assessment of Climate Change Impacts on the Hydrological Cycle of the Anambra River Basin Using Statistical Downscaling

The Anambra River Basin in Southeast Nigeria sustains agriculture, fisheries, and water supply for millions of people across Anambra and Enugu states, yet its hydrological future under climate change scenarios remains insufficiently analyzed. This study assesses climate change impacts on the hydrological cycle of the Anambra River Basin using statistical downscaling of CMIP6 general circulation model outputs combined with the SWAT semi-distributed hydrological model. Daily precipitation and temperature projections from five CMIP6 models under SSP2-4.5 and SSP5-8.5 scenarios were statistically downscaled using the Bias Correction and Spatial Disaggregation (BCSD) method calibrated against NiMet station records. The SWAT model was calibrated and validated against observed streamflow records at Aguleri gauging station for the period 2010 to 2020. Ensemble-mean projections under SSP2-4.5 indicate a 12 to 18 percent increase in mean annual rainfall by 2050-2070, associated with a 9 percent increase in mean annual streamflow but a 23 percent increase in flood frequency for the 20-year return period event. SSP5-8.5 projections indicate more severe alterations with substantially elevated drought risk during the dry season months. Multi-model uncertainty analysis reveals that rainfall projections carry substantially higher uncertainty than temperature projections, reinforcing the need for ensemble-based impact assessments. Recommendations focus on climate-resilient water allocation planning and revision of flood design standards for infrastructure in the basin. Keywords: climate change, hydrological modelling, Anambra River, SWAT, statistical downscaling.