Assessing the effect of climate change on spatiotemporal variations in reference evapotranspiration across Iraq

Climate change affects meteorological variables and reference evapotranspiration (ET_o), altering irrigation demands and water availability. ET_o and its climatic drivers show global and regional variations due to the dynamic nature of ET_o and complex interactions of climatic variables. However, the influence of climatic factors on ET_o in Iraq remains unclear. Therefore, this study investigates the impact of climate change on the patterns and trends of ET_o in Iraq using the FAO Penman-Monteith method and a validated ERA5 global reanalysis dataset (1959‒2021). Sensitivity and contribution analysis assessed the influence of each climatic variable on trends. The key findings reveal a significant increase in annual mean ET_o (0.10–0.19 mm day^− 1/decade) over the past 63 years, particularly in crucial agricultural lands. Wind speed (U₂) emerges as the most influencing climatic factor (30–74%), followed by mean air temperature (TM) (6–21%). The TM, with statistically significant changes ranging from 0.17 to 0.56 (°C/decade) across Iraq except for the northern region, is identified as the primary driving factor behind the rising ET_o. U₂ has also changed significantly from − 0.28 to 0.02 (ms^− 1/decade) in the west, southwest, and northeast. However, solar radiation, relative humidity, and surface pressure exhibit no significant changes (P < 0.05), except for a narrow strip in the far north. The study concludes that the considerable increase in TM since 1993, coupled with a moderate change in U₂ in some regions, has led to a notable rise in ET_o rates from 1997, demising the ET_o paradox in the study area during the late 1990s due to global climate change.