Abstract

Water resource management, hydrologic simulation, drought monitoring, and environmental assessment rely on the accuracy of precipitation and reference evapotranspiration (ETo) data on a global scale. However, achieving precise estimations across a vast spatial network of weather stations is a formidable challenge. Tana watershed is one of the Ethiopian river basins facing ground data scarcity. This study assessed the performance of the WaPOR portal and the ERA5-Land reanalysis model in estimating daily precipitation and ETo at the basin level, in Ethiopia. Additionally, the spatiotemporal variability of precipitation and reference evapotranspiration in the Tana basin were analyzed. Ten weather stations were provided to collect ground data, and two open-source datasets, WaPOR and ERA5-Land, were utilized. The Modified Penman–Monteith method was applied to calculate ETo. The results indicated the WaPOR outperforms ERA5-Land in ETo estimation in terms of coefficient of determination (R²) and index of agreement (IOA). However, considering RMSE, ERA5-Land showed the closest fit with actual ETo in 7 out of 10 weather stations, surpassing WaPOR. Both datasets demonstrated similar performance in 6 out of 10 weather stations when considering the index of agreement. This implies that the choice of dataset for daily ETo estimation may vary depending on the station characteristics. Moreover, the WaPOR portal exhibited superior accuracy in estimating daily precipitation (R² = 0.77, IOA = 0.89, and RMSE = 3.25mmday⁻¹) compared to ERA5-Land (R² = 0.44, IOA = 0.55, and RMSE = 5.95mmday⁻¹) in the Tana basin. Spatial and temporal variations in ETo and precipitation across the Tana Basin were observed, indicating variability in weather patterns and water availability, which is crucial for water resource management. The study underscored the reliability of WaPOR and ERA5-Land for water management, hydrological modeling, and irrigation system planning. This study offers valuable insights for policymakers, water resource managers, hydrologists, and irrigation scheme planners in data-scarce regions. This study concentrated on a single basin. Future research should consider additional basins, utilizing a larger number of weather stations and a wider geographic distribution.