Regional Hydrological Impacts of Climatic Change—Hydroclimatic Variability (Proceedings of symposium S6 held during the Seventh IAHS Scientific Assembly at Foz do Iguaçu, Brazil, April 2005). IAHS Publ. 296, 2005. 138 Storage capacity and long-term water balance of the Volta Basin, West Africa JAN FRIESEN 1,4 , MARC ANDREINI 1,2 , WINSTON ANDAH 3 , BARNABAS AMISIGO 1,3 & NICK VAN DE GIESEN 4,1 1 Centre for Development Research, Bonn University, Walter-Flex-Str. 3, D-53113 Bonn, Germany j.friesen@uni-bonn.de 2 International Water Management Institute, Sub-regional Office West Africa, PMB CT 112, Cantonments, Accra, Ghana 3 Water Research Institute, PO Box M.32, Accra, Ghana 4 TU Delft, Water Resources Section, PO Box 5048, 2600 GA Delft, The Netherlands Abstract Rainfall in West Africa is often typified as being subject to large inter-annual variability. For the Volta Basin, this variability is actually not very large. The coefficient of variation for rainfall is only 0.08 (1931–1995) with an average of 400 km 3 year -1 . Yet, a much higher coefficient of variation of 0.38 (1931–1995) is found for runoff with an average of 43 km 3 year -1 . The basin shows a nonlinear response that amplifies small changes in rainfall into large changes in runoff. A simple runoff model for the Volta Basin was formulated with a Nash-Sutcliffe efficiency of 72%. This model implies that once rainfall exceeds a threshold of 342 km 3 year -1 , more than 50% of the exceedence runs off. This threshold behaviour makes the water resources of the Volta Basin highly sensitive to both climatic variability and anthropogenic impacts. First, the impact of climate variability is quantified. Subsequently, development of the storage capacity of the Basin is evaluated on the basis of the water balance over the period 1966–1995 with an adapted Thornthwaite- Mather model. Results show that there is no change in hydrological behaviour of the basin that could be attributed to human impact. This lack of a clear anthropogenic signal can partially be explained by two contrary developments, increase in surface water storage in reservoirs and decrease in soil moisture storage due to soil and vegetation decline. Key words anthropogenic impact; basin-wide storage capacity; climate impact; Volta; water balance; West Africa INTRODUCTION Climate and anthropogenic change can be detected in West Africa at both the point and regional levels. Climate change is apparent when looking at rainfall time series, whereas increasing population figures and changes in land use and cover indicate anthropogenic changes. This work investigates to what extent changes are detectable at the level of the Volta Basin, which has an area of 400 000 km 2 (Fig. 1). Previous work for the Nakambe basin (White Volta), a sub-basin of the Volta Basin in northern Burkina Faso, showed that both climate and anthropogenic change had impact on the river flow (Mahé et al., 2002). In this study, we examine whether long-term changes in the runoff regime can be observed at the level of the whole Volta Basin. On the basis of a storage capacity and water balance analysis, it will be shown that climatic changes