Future Projections of Potential Evapotranspiration and Fresh Water Availability in India using CMIP6 GCMs

Detection and quantification of climate change impact on hydrological processes (e.g., precipitation, evapotranspiration and streamflow) in different climatic regions is necessity for water resources planning and management in order to ensure water security for various purposes. Globally evapotranspiration is the major cause of water loss, which is about 62% of global land-surface precipitation. Reference or potential evapotranspiration (ET₀) represents the atmospheric water demand, which would be the upper limit of actual evapotranspiration in humid climate. Climate change induced variation in meteorological variables will affect ET₀ or crop water requirements. Increase of ET₀ can intensify dry conditions in the arid and semi-arid regions of the world. In our previous study on historical records, we noted regional increase in ET₀ in south and central India due to increase in net solar radiation and temperature; and decrease in regional ET₀ in north-east and north-west India) due to either global stilling (i.e., decrease in wind speed) or global dimming (i.e., decrease in net solar radiation) during 1958-2013. The decreasing trend in ET₀ despite significant increase evident in air temperature is widely referred to as “evaporation paradox”. The objective of the present study is to determine the regional-scale spatialtemporal variations of ET₀ in future climate conditions using recently released GCMs of CMIP6 (Coupled Model Intercomparison Project Phase 6), as this can provide valuable information for future ET₀ regional trend and fresh water availability in India. For this purpose, the homogeneous ET₀ regions formed over India in our recent study are considered.   We considered three GCMs namely CanESM5, INM-CM4-8 and INM-CM5-0 because of the availability of all required climate variables for all four CMIP6  shared socioeconomic pathways (SSPs; SSP126, SSP245, SSP370, and SSP585). The projected changes were estimated for each GCM for the late 21st century (2015–2100). The results were discerned based on ensemble mean of the projections of climate variables obtained from the three GCMs. The trend analysis of ET₀ as well as climate variables reveal that ET₀ will increase significantly in all the homogeneous ET₀ regions in India. Similarly, maximum and minimum temperatures, and net solar radiation are also projected to increase significantly. The evaporation paradox was not found in any parts of India in the future simulations. Among other climate variables, significantly increasing trend for relative humidity and decreasing trend for wind speed was found in majority of regions for higher SSPs. The ET₀ and precipitation data are used in budyko relationship to obtain the futute fresh water availability in the regions. It is found that, despite increase in ET₀ there is significant increase in futute fresh water availability across all the regions for higher SSPs due to increase in precipitation.