Climate change impacts on potential evapotranspiration and crop water demand in the Tana Subbasin, upper blue Nile Basin, Ethiopia

Abstract

Climate change is intensifying atmospheric evaporative demand, posing growing challenges for irrigation planning in data-scarce river basins. This study quantifies historical (1990–2023) and projected changes in reference evapotranspiration (ET0) and crop water requirements (CWRs) in the Lake Tana Subbasin, Upper Blue Nile Basin, Ethiopia. Multiple temperature-based ET0 models were systematically evaluated against the benchmark FAO-56 Penman–Monteith method using statistical performance metrics (root mean square error, bias, and efficiency indices). The locally calibrated Enku model demonstrated the highest accuracy and lowest projection uncertainty, establishing its suitability for ET0 estimation under data-limited conditions in this basin. Using bias-corrected CMIP6 projections, basin-average ET0 is projected to increase by 9–15% by the late century under SSP2-4.5 and SSP5-8.5, with the strongest intensification in central and southern zones. CWRs are correspondingly expected to rise by 10–20%, particularly for high-demand crops such as tomato and wheat, indicating substantial expansion of irrigation deficits. By validating a locally appropriate ET0 model and linking projected evaporative demand to crop-specific irrigation requirements, this study provides a strong analytical basis for climate-resilient irrigation planning and adaptive water allocation in the Lake Tana Basin.