An Integrated Modeling Framework for Water Accounting Assessment in the Lake Tana Sub-Basin, Ethiopia: Impacts of Interbasin Water Transfer

Study Region: Lake Tana Sub-Basin, Ethiopia

Study Focus: The Lake Tana sub-basin plays a vital role in Ethiopia’s hydropower generation and irrigation development. However, the recent operation of an interbasin water transfer has intensified competition for water resources, raising concerns about long-term hydrological sustainability and downstream ecological flows. To evaluate these impacts, this study developed an integrated modeling framework that couples the HBV Light rainfall-runoff model, a lake water balance model, and the Water Accounting Plus (WA+) approach to assess water availability, consumption patterns, and downstream ecological flow conditions for 2010–2020.

New hydrological insights for the Region: The HBV Light model was unable to accurately simulate the natural lake outflow, but its coupling with the lake water balance model significantly improved model performance, resulting in NSE of 0.79 and R² of 0.92. The mean annual inflow to the lake was estimated at 6.9 km³ , with 55% contributed by the Gilgel Abbay catchment. The rainfall and evaporation over the lake was estimated at 4.1 km³ yr⁻¹ and 5.1 km³ yr⁻¹ , respectively. Total annual outflow averaged 5.8 km³ , with 3.1 km³ yr⁻¹ diverted through the interbasin water transfer and 2.7 km³ yr⁻¹ outflow at the natural outlet. The interbasin water transfer now exceeds lake's natural outflow and has increased the frequency of unmet environmental flow requirements from 6% (pre-transfer period) to 27% during 2010–2020. In terms of consumption, rainfed agriculture dominates water consumption at 5.7 km³ yr⁻¹ , while irrigation accounts for only 0.4 km³ yr⁻¹ . Green evapotranspiration (ET) constitutes 68% of total water consumption, with blue ET making up the remaining 32%. These results demonstrate the hydrological implications of interbasin water transfer on lake outflow and downstream ecological conditions. The integrated modeling framework offers a scalable approach for hydrological assessment and water allocation in data-scarce basins.