CLIMATE CHANGE IMPACT ASSESSMENT AND INTERCOMPARISON ON STREAMFLOW IN THREE WEST AFRICAN RIVER BASINS

Abstract

The harsh effects of a changing climate will be experienced more by the developing countries in Africa because of their inadequate adaptation capacity. To offer accurate results for stakeholders, Climate change impacts on hydrological processes in river basins should be modelled using validated models and various emission scenarios. Climatic impact assessments have been carried out for some river basins around West Africa, using different climate scenarios and models. Yet, these findings are not comparable and do not give a complete picture of the effects. As a result, a systematic impact comparison is carried out using state-of-the-art models. The impacts assessment and intercomparison were performed for three river basins. The selected basins all have their gauges located within Nigeria and covering the major climatic settings. To this end, the PCR-GLOBWB hydrological model was set up, forced with meteorological data obtained from the Climatic Research Unit (CRU), and validated for the selected stations. Streamflow data from the Global Runoff Data Centre (GRDC) were used to validate the model. Due to the unavailability of sufficient discharge data for one of the basins (the Ogun), a proxy-basin validation effort was performed to evaluate discharge dynamics over the area. Seven model Evaluation statistics were used to assess the PCR-GLOBWB model performance over the selected basins. The impact assessment was performed by driving the model with climate projections from five global climate models for the representative concentration pathways 4.5 and 8.5. The hydrological model was found to be suitable in all three basins but yielded better performance at the Niger and the HJKY compared to the Ogun basin, where a proxy validation approach was followed. Still, model performance was satisfactory in the Ogun. PCR-GLOBWB performances across the three basins validate its reliability as a tool applicable for water resources management strategies and the investigation of climate variations' impacts on river dynamics. The five GCMs were used to drive the PCR-GLOBWB for impact assessment. The median and range of expected changes in seasonal discharge in the far and near future (2070-2099 and 2040-2069) were examined. Results show that the impacts of climate change cause a reduction in discharge volume at the beginning of the high flow period (typically from May to June) and an increase in discharge towards the ending of the high flow period (typically from July to November). This decrease is explained by a delayed start of the rainy season, which agrees with the findings of previous studies. Projected river discharge seasonality is amplified under the high-end emission scenario (RCP8.5). This finding supports the potential advantages of reduced greenhouse gas emissions for the seasonal river discharge regime. In two of the three basins (not for the Ogun), a tendency for higher streamflow was detected. This study is anticipated to provide helpful information to policymakers and river basin development authorities within the context of changing climate. Guided by these results, it is expected, based on these findings, that increasing high flows in the face of Africa's predominant water scarcity poses a concern poses a concern.