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

Climate change affects meteorological variables and reference evapotranspiration (ETo), altering irrigation demands and water availability. ETo and its climatic drivers show global and regional variations due to the dynamic nature of ETo and complex interactions of climatic variables. However, the influence of climatic factors on ETo in Iraq remains unclear. Therefore, this study investigates the impact of climate change on the patterns and trends of ETo 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 ETo (0.10-0.19 mm day(- 1)/decade) over the past 63 years, particularly in crucial agricultural lands. Wind speed (U-2) 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 (degrees C/decade) across Iraq except for the northern region, is identified as the primary driving factor behind the rising ETo. U-2 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 U-2 in some regions, has led to a notable rise in ETo rates from 1997, demising the ETo paradox in the study area during the late 1990s due to global climate change.