Causes for the Changes in Reference Crop Evapotranspiration Over Ethiopia During 1980–2021

Abstract

Considering the great significance of reference crop evapotranspiration (ET_o) for various sectors (i.e., biodiversity, agriculture, forestry development and water resources), this study comprehensively investigated its changes during 1980–2021 based on the Food and Agriculture Organisation-56 Penman-Monteith equation and the five popular meteorological datasets. As for the whole of Ethiopia, both the annual and monthly ET_o significantly increased, but the increasing trends varied among months with larger values during January–April and September–December. Spatially, the annual and monthly ET_o changes exhibited evident differences, characterised by increases over more than 88% of areas, especially for January–April and September–December, with an area percentage exceeding 78%. Attribution analyses based on a joint-solution method with multiple sensitivity experiments suggested that for Ethiopia, the annual and monthly ET_o increases were determined by increased mean temperature (T_a). However, the annual and monthly dominant factors varied spatially. On an annual scale, the dominant factors were net radiation (R_n) and T_a, accounting for 21.2% and 78.8% of areas, respectively, mainly in northwestern, southwestern and southeastern Ethiopia and the remaining regions. Although different spatial distributions of dominant factors existed among months, the dominant factor of T_a always had the most extensive area percentage (> 47%), followed by R_n (> 18%) during January–April, June, August, October and November, but wind speed at 2 m (> 19%) during May, July, September and December. This complete analysis of ET_o changes and the related physical mechanisms can partly fill the research gap in Ethiopia. Moreover, this study provides essential information for better understanding climate change, protecting biodiversity and sustaining regional development (e.g., agriculture and water resources).