Impact of Climate Change on River Flows in the Black Volta River

抄録

This paper assesses the impact of climate change in the Black Volta River by using data output from the atmospheric general circulation model with a 20-km resolution (AGCM20) through the Japanese Meteorological Agency (JMA) and the Meteorological Research Institute (MRI). The Black Volta, which flows mainly in Burkina Faso and Ghana in West Africa, is a major tributary of the Volta River. The basin covers 142,056 km² and has a semi-arid tropical climate. Before applying AGCM20 output to a rainfall–runoff model, the performance of the AGCM20 rainfall data is investigated by comparing it with the observed rainfall in the Black Volta Basin. To assess the possible impact of rainfall change on river flow, a kinematic wave model, which takes into consideration saturated and unsaturated subsurface soil zones, was performed. The rainfall analysis shows that, the correlation coefficient of the monthly rainfall between the observed rainfall and AGCM20 for the present climate (1979–2004) is 0.977. In addition, the analysis shows that AGCM20 overestimates precipitation during the rainy season and underestimates the dry season for the present climate. The analysis of the AGCM20 output shows the precipitation pattern change in the future (2075–2099). In the future, precipitation is expected to increase by 3%, whereas evaporation and transpiration are expected to increase by 5% and by 8%, respectively. Also, daily maximum rainfall is expected to be 20 mm, or 60%, higher. Thus, the future climate in this region is expected to be more severe. The rainfall–runoff simulation is successfully calibrated at the Bamboi discharge gauging station in the Black Volta from June 2000 to December 2000 with 0.72 of the Nash–Sutcliffe model efficiency index. The model is applied with AGCM20 outputs for the present climate (1979–2004) and future climate (2075–2099). The results indicate that future discharge will decrease from January to July at the rate of the maximum of 50% and increase from August to December at the rate of the maximum of 20% in the future. Therefore, comprehensive planning for both floods and droughts are urgently needed in this region.