How Will Combined Changes in Water Demand and Climate Affect Water Availability in the Zambezi River Basin?

Many of the world's large international river basins, which are the principal sources of freshwater for billions of people and nature, are likely to experience increasing stress over the coming decades. In many cases this stress will be due to changes both on the water supply (because of climatic changes) and demand side. Only few studies have examined the implications of water demand and climatic changes jointly. We combine a comprehensive set of water demand scenarios and climate change projections with a hydrological model to estimate future water availability in key parts of the Zambezi river basin (ZRB) until 2050. We focus on the ZRB because it is both substantively important and analytically challenging in terms of demonstrating the value of our methodological approach: it is one of the largest freshwater catchments in Africa and worldwide; it has a complex hydrological and political geography; and it experiences only minor water stress today. The results show that our approach is feasible and can produce valuable results. Our results indicate that current water abundance in most parts of the ZRB is unlikely to last. While, perhaps surprisingly, climatic changes are likely to have only relatively small effects on water availability, population and economic growth as well as expansion of irrigated agriculture and water transfers are likely to have very important transboundary impacts. Such impacts involve drastically reduced runoff in the dry season at key locations and changing (relative) shares of ZRB countries in the basin's total runoff and water demand. These results imply that effective governance mechanisms for water allocation and for dealing with flow variability should be set up within the next few years in order to manage the situation cooperatively