HYDROLOGIC IMPACTS OF CLIMATE AND ANTHROPOGENIC CHANGES ON AN ETHIOPIAN RIFT VALLEY LAKE

Abstract

Ethiopia, the Sub-Saharan second populous country in Africa, has been challenged by prolonged droughts, water stresses, and human activities for the last few decades. The Central Rift Valley basin of Ethiopia has faced significant water resources related issues because of the changing environment. Agriculture, fish farming, aquatic and terrestrial bio-diversities are being affected by climate variability and excessive water abstractions. The impacts are more profound in the semi-arid climate region of Lake Ziway sub-basin, where the competition for water is also immense. The present study attempted to explore the impacts of anthropogenic activities and climate variability and changes on the hydrologic processes of Lake Ziway sub-basin. The Soil and Water Assessment Tool (SWAT) hydrologic model and the Water Evaluation And Planning (WEAP) model were used to quantify the impacts of the environmental changes on the water resources. The physical model-based hydrologic evaluation mainly relies on the temporal and spatial representativeness of input data. Precipitation datasets from four gridded products were compared statistically and evaluated hydrologically in Ketar (3282 km2) and Meki (2224 km2) watersheds for applicability in streamflow simulation. All the satellite-based precipitation products (CHIRPS, PERSIANN-CDR, and TRMM) performed well for monthly streamflow simulations. Performance of the CHIRPS precipitation product in capturing daily and monthly streamflow showed that it can provide valuable precipitation estimates in data-sparse regions of developing countries such as Ethiopia. Land-use changes over the last three decades have increased the annual surface runoff and water yield of Ketar watershed and the annual ET of Meki. Similarly, climate variability resulted in a decrease in the annual ET, surface runoff, and water yield for Ketar watershed and a reduction in ET for Meki. Overall, the impacts of climate variability on the monthly streamflow were greater compared to land-use change impacts. The ensemble average future annual streamflow simulated using the RCMS from the Coordinated Regional Downscaling Experiment (CORDEX-AFRICA) dataset showed that streamflow are expected to increase towards the end of the century under both the RCP 4.5 and RCP 8.5 scenarios. However, the ensemble average dry season (from October to January) streamflow are projected to have a decreasing trend during the mid- and end-periods of the century. The projected decrease in streamflow during the dry and pre-wet seasons (October to May), along with excessive abstractions of water from Lake Ziway, would exacerbate water scarcity in the basin. The impacts of planned future agricultural developments and climate change on Lake Ziway water balance were also examined by an integrated application of the SWAT and WEAP models. Results showed that the planned future agricultural developments could marginally decline the mean annual lake water level, but with a considerable reduction in the outflow to the downstream river. The evaporation loss from the shallow lake is estimated to increase under the climate change scenarios resulting in a drastic decrease in the lake water level, especially during the dry season. It could also significantly reduce the amount of water flowing out of the lake through Bulbula River. The combined impacts of future development and climate change are likely to reduce the water supply coverages of most of the competing demands. Water resources management approaches need to be developed to capture the increase in water availability during the wet season to alleviate the water scarcity during the dry season and to minimize the evaporation losses from the lake. Future studies could incorporate groundwater and surface water interactions and sediment load estimation to understand the impacts of environmental changes on the lake water system. Application of more numbers of Global Climate Models (GCMs) could assist in future water resources management under changing climatic conditions and their related impacts on the hydrologic processes.