Modelling the Coupled Effects of Climate Change and Land Use Change on Water Quality in the Kaduna River Basin

The Kaduna River Basin, spanning North Central and North West Nigeria, faces dual pressures from intensifying agricultural land use and projected increases in rainfall intensity associated with climate change that will alter the character and concentration of dissolved and particulate contaminants reaching the Shiroro and Kainji reservoirs serving millions of people. This study models the coupled effects of projected climate change and land use change on water quality in the Kaduna River Basin using a physically based, spatially distributed catchment modelling approach. The Soil and Water Assessment Tool was calibrated and validated for the Kaduna River Basin using fifteen years of observed discharge and limited water quality data at three gauging stations. Land use scenarios were developed from integrated assessment model outputs representing agricultural expansion under economic growth trajectories, digitised at local government area scale. Climate projections from a four-member regional climate model ensemble downscaled from CMIP6 global models provided future precipitation and temperature inputs. Coupled simulation runs for the 2030 to 2060 and 2060 to 2090 periods generated projections of suspended sediment load, total nitrogen, total phosphorus, and faecal coliform concentrations under each scenario combination. The study disaggregates future water quality deterioration into components attributable to climate change and land use change separately to guide the relative prioritisation of adaptation measures. An original contribution is made in the development of a scenario weighting scheme that accounts for model uncertainty in projecting extreme rainfall events. Keywords: water quality modelling, climate change, Kaduna River Basin, SWAT, land use change.