Assessing the impact of global changes on the surface water resources of Southwestern Nigeria

Abstract

Understanding the relative impact of land use, land cover and climate change (LULCC) on basin runoff is necessary in assessing basin water stress. This assessment requires long-term observed rainfall time series and land-use/land-cover (LULC) spatial data. However, there are challenges with the availability of spatio-temporal data, particularly limited range of available historical hydro-meteorological measurements. In order to assess the likely water stress, the study used long-term (1961–2007) rainfall data to drive the Pitman monthly rainfall-runoff model to assess changes to the water resources of three selected basins in Nigeria—Asa, Ogun, and Owena. Three CGCMs—CSIRO Mark3.5, MIROC3.2-medres and UKMO-HadCM3 dynamically downscaled to a 60 km by 60 km grid using the Conformal-Cubic Atmospheric Model (C-CAM)—are used to simulate impacts of future climate changes on water resources. These three models were found suitable for simulating rainfall-runoff based on the insignificant differences of models’ mean with mean of observed rainfall and temperature for pre-2010 data compared to other downscaled C-CAM models (GFDL-CM2.0, GFDL-CM2.1, and ECHAM5/MPI-Ocean model). The model results show increases in the runoff coefficient with decreases in forest cover between 1981 and 2007 with average runoff coefficients of 5.3%, 12.0% and 6.4% for Asa, Ogun and Owena basins respectively. Based on CSIRO, MIROC, and UKMO predicted annual reduction in rainfall trend, the future scenarios revealed a low runoff coefficient for the three basins—Asa (CSIRO 6.0%, MIROC 6.0%, and UKMO 5.9%), Ogun (CSIRO14.6%, MIROC 14.6%, and UKMO 14.4%), and Owena (CSIRO 8.5%, MIROC 8.7%, and UKMO 8.9%). In all scenarios Asa basin has lower runoff coefficient when compared to Ogun and Owena basins, indicating that future water stress in Asa basin would be much higher.