Exploring future global change-induced water imbalances in the Central Rift Valley Basin, Ethiopia

Lake Ziway, the only freshwater lake in Ethiopia’s Central Rift Valley basin, has been the source for irrigation, floriculture, fish farming and domestic water supply in the region for the last few decades. This study examined the impacts of the planned future agricultural developments and climate change on the lake water balance by an integrated application of the Soil Water Assessment Tool and Water Evaluation and Planning models. The future projections of precipitation and temperature from the Coordinated Regional Downscaling Experiment, CORDEX-AFRICA, under the Representative Concentration Pathways 4.5 and 8.5 were used for the climate change impact assessment. Nine irrigation development and climate change scenarios were developed and simulated to examine the separate and combined impacts on the lake water balance and supply coverages. The study showed that the planned future agricultural developments could result in a mean annual lake water level decline by about 0.15 m, with a considerable reduction (27% to 32%) in the outflow to the downstream Bulbula River. Climate change could increase evaporation losses from the shallow lake resulting in a drastic decrease in the lake water level, especially during the dry season. It could also significantly reduce (by about 74%) the amount of water flowing out of the lake. The combined impacts of future development and climate change are likely to reduce the supply coverages of most of the competing demands. Approaches need to be studied to minimize the lake water evaporation losses and explore water demand/supply management options.