Potential impacts of Climate Variability and Change on Hydrology and Water resources over the Volta Basin

Abstract

The Volta Basin is one of the vital water resources in West African region. Climate variability and change pose serious consequences on water availability in the basin, as a result of its climate-sensitive nature. Therefore, there is the need for information on the expected near future states of the water resources in the basin to support water management planning. Regional climate simulations were performed using a climate version of the Weather, Research and Forecast (WRF) model to assess the potential impact of future climate variability and change on hydrology and water resources over the basin. Two time periods (1976–2005 and 2026–2055) of the ECHAM6 scenario RCP4.5 were dynamically downscaled with WRF in a double nested configuration. The outer domain at 50 km resolution covering the whole of West Africa and the inner domain covers the entire Volta Basin at 10 km. The performance of WRF simulations for the period 1980-2005 was assessed using the ERA-Interim reanalysis data as driving data. Standardized precipitation index (SPI) was computed from the WRF simulation results to project future potential impacts of climate variability and change on the Volta Basin hydrology and water resources. Also, the Hydrologic Model System (HMS) was setup over the Volta Basin and the preliminary results at Lawra and Bui gauge stations for the period 2005-2007 were presented. The present-day results show that generally, GCM biases were transferred into the RCM downscaling simulations, however, there were some additional bias contributed solely by the RCM. The WRF model generally underestimated annual mean temperature over both West Africa and the Volta Basin. For the annual total precipitation, a slight underestimation was simulated over West Africa, while overestimation was produced by the model over the Volta Basin. The general performance of the ECHAM6 model was good, particularly for temperature over both Sahel and Sahara regions, however, the added value to the simulated fields by the WRF model was evident, especially for precipitation over the outer domain on the annual time scale, and over the whole Volta Basin and the Soudano-Sahel for the month of April and spring rainfall respectively where nearly zero bias were simulated. For the future climate projections, whereas simulated temperature changes in all cases showed a clear signal of increase in the future simulations and leaves no doubt about a projected climate change signal, the same cannot be said for precipitation. The WRF model projected a reasonable increase in annual temperature between 0.7 K and 1.6 K over West Africa and between 1.0 K and 1.5 K over the Volta Basin. Over West Africa, precipitation deficit was projected more over the Sahel and the Sahara regions, whereas on the annual scale, precipitation change signal ranging from -6% and 12% was projected over the Volta Basin by the WRF model. Projected changes for precipitation on the seasonal scale over the Volta Basin were quite small. However, the model projected a decrease in April precipitation for the future time period. The SPI results over the basin indicate a high variability in projected soil moisture conditions, streamflow and reservoir levels for the period 2026-2041, but from 2041 to 2055, the projected variability in the various hydrological conditions was rather low. For the 2029-2033 period, a potential hydrological drought over the entire basin is projected due to a projected precipitation deficit for that period. Finally, although the HMS preliminary results of streamflow at the two stations over the Volta Basin showed some bias with those observed, the model performed better at the Bui station than the Lawra station. A change in the future climate of the Volta Basin is therefore, likely to occur, should anthropogenic emissions of greenhouse gases follow the RCP4.5 scenario and furthermore, the projected variability and change in the future climate is likely to impact water availability within the Volta Basin