Spatio-temporal variation of reference evapotranspiration and its contributing factors over the Indian subcontinent under historic and future climate change

The estimation of reference evapotranspiration (ETo) holds significant importance for the hydrological cycle, necessitating an extensive understanding of the various climate variables and their influence on ETo variability. This study aims to examine spatio-temporal variations in Penman Monteith based ETo estimations and the factors contributing to their changes over the Indian subcontinent in the historic and future climate change. Using climate variables from the ERA5 reanalysis and CMIP6 simulations this study focuses on the changes in ETo across different aridity zones in the study area. Further, the partial least squares (PLS) regression was employed to determine the relative contribution of different climate variables on ETo trends. Results show that the majority (70%) of the areas in the subcontinent exhibited decreasing ETo trends in the historical past. Zonal analysis of ETo trends revealed all zones except the humid zone exhibited a significant decreasing trend for ETo. Contribution analysis shows that, across the study area, temperature and radiation are the most significant factors influencing ETo, followed by wind speed and relative humidity. Further, temperature and ETo were found to be having opposing tendencies, highlighting an “evapotranspiration paradox” that encompasses the majority of the study area. CMIP6 simulations show that ETo is projected to increase significantly across the Indian subcontinent, especially in the semi-arid and arid regions with temperature and radiation being the dominant factor contributing to increases in ETo.