Diminishing evapotranspiration paradox and its cause in the Middle East and North Africa

Global warming has significantly affected different climate variables and potential evapotranspiration (PET), influencing irrigation needs and water balance. However, the influence of climate change on PET trends is inconsistent in different regions due to the PET paradox. The climate change influence on PET can also vary with time due to the changing pattern of climatic variables and their interactions. This study applies sensitivity analysis of PET to various climate factors and PET trends to reveal the dynamics of PET in the Middle East and North Africa (MENA) and decipher its cause. Penman-Monteith (PM) method was used to estimate PET using ERA-5 data for three 30-year periods, 1951-1980, 1971-2000 and 1991-2020. The results showed a decrease in PET over a large part of MENA during 1951-1980, while an increase over most of MENA during 1971-2000 and 1991-2020, indicating demising evapotranspiration paradox in the region over time. Sensitivity analysis showed wind speed (70-80%) as the most influential factor in determining PET in MENA, followed by solar radiation (10-20%) and daily maximum temperature (5-10%) for all periods. However, wind speed and solar radiation have not changed significantly over periods in most areas. In contrast, a large increase in temperature (0.1 to 0.8 degrees C/decade) over most of the region caused a diminishing PET paradox and increased PET over most of the MENA. Overall, a 0.1 to 0.2 mm/day increase in PET per decade was observed. The study indicates the rising temperature would cause an increasing PET which can have severe implications in this global water stress hotspot.