High-Resolution Simulation and Attribution of Future Flood Frequency Changes in the Lower Niger River System Under CMIP6 Scenarios

The Lower Niger River system, traversing Kogi, Anambra, Delta, and Bayelsa states before discharging into the Gulf of Guinea, is among Nigeria's most flood-exposed hydrological systems, and understanding future changes in flood frequency under climate change demands high-resolution physically consistent modelling. This study simulates and attributes future flood frequency changes in the Lower Niger River system under CMIP6 climate scenarios using a dynamically coupled atmospheric-hydrological modelling chain. WRF regional climate model at 9-kilometre resolution was driven by four CMIP6 models under SSP2-4.5 and SSP5-8.5 for the historical (1985-2014) and future (2040-2069 and 2070-2099) periods. WRF precipitation outputs were bias-corrected using quantile mapping calibrated against NiMet station records and used to drive the variable infiltration capacity (VIC) land surface model coupled to a routing scheme. Flood frequency analysis using the generalized extreme value distribution was applied to simulated streamflow series to derive return period flood estimates under each scenario and period. Attribution analysis partitioned future flood frequency changes into contributions from precipitation intensification, temperature-driven evapotranspiration changes, and antecedent soil moisture alteration. Results project a 40 to 65 percent increase in 50-year flood peak discharge at Lokoja confluence by 2070-2099 under SSP5-8.5, with rainfall intensification accounting for 72 percent of the attributed increase. An original probabilistic flood frequency change atlas for 12 Lower Niger gauging stations is produced as a planning resource for the Federal Ministry of Water Resources. Keywords: Lower Niger, flood frequency, WRF, CMIP6, attribution.