Inter‐ and Intra‐Annual Variation of Evapotranspiration in the Data‐Sparse Guder River Watershed, Ethiopia

Understanding evapotranspiration (ET) variation is crucial for water management and to comprehend the direction of hydroclimatic fluctuations in a changing world. This study aimed to investigate the inter‐ and intra‐annual variation of ET in the data‐sparse Guder River watershed using the appropriate ET estimation model. We have evaluated the performance of two widely used remote sensing ETs and three temperature and radiation‐based ET models and selected the best‐performing data. We have used innovative trend analysis (ITA) and harmonic analysis to find the general trends and periodic variations in ET. The long‐term mean ET of the Guder River watershed ranged from 701.7 to 841 mm per year. Over the past two decades, there has been a general declining trend in annual ET. The analysis of seasonal variation showed considerable fluctuations in ET, with the lowest value of 26.7 mm in winter and the highest value of 159 mm in autumn. The seasonal data analysis revealed that the declining trend was mainly observed in the wet season, particularly in spring. The harmonic analysis of ET between 1980 and 2000 showed variances of the semiannual cycle of two and four cycles dominating the first harmonic variance, which indicates low interannual variation. However, from 2001 to 2021, the variance of the first harmonic contributed more than the seasonal variances, indicating an increase in the interannual variation of ET. Future research has to emphasize the deriving factor to the observed variability and the effect on streamflow by considering complete watershed variables.