Water Resources in Lake Tana Basin: Analysis of hydrological time series data and impact of climate change with emphasis on groundwater, Upper Blue Nile Basin, Ethiopia

Ethiopia is a source region of the Nile River and famous for its water resources potential. The available annual average water per person per year is estimated to be 1575 m3. The Lake Tana accounts for 50% of the national fresh water. It has a total catchment area of about 15,321 km2 and hosts more than three million people. The climate is characterized by a high seasonality of rainfall with a rainy season between June and September. However, the scientific understanding of the hydrologic response to intensive agriculture, the interconnection of groundwater and surface water, and future perspectives of the water availability under global climate change is limited. Therefore, the main aim of this study is to improve our understanding of past, present, and future hydrologic conditions in the Lake Tana basin. To this end, long-term time series analysis and hydrological modeling using SWAT (Soil and Water Assessment Tools) and a coupled surface water and groundwater model (SWAT-MODFLOW) were applied. Time series analysis and modelling results revealed that the hydrology of the basin was changed significantly during the last half century and is expected to change during the 21st century mainly due to land use change and climate change. Although projections of annual rainfall did not show a significant change, surface runoff increased, whereas base flow decreased during the past and for mid- and long-term periods in the 21st century. Results from the coupled model revealed a high connectivity of groundwater and surface water systems. Agricultural crops influence the hydrologic components differently. Groundwater recharge was relatively high on agricultural land covered by cereal crops, whereas surface runoff was significantly enhanced on cultivated land covered by leguminous crops like peas. Overall, the results of this dissertation reveal that hydrology of the Lake Tana Basin has changed considerably during the last half century and more changes are to be expected in the future. Consequently, the results of this dissertation can contribute to develop future water management plans in the region and beyond.