Seasonal changes in evaporation and potential evapotranspiration under different scenarios of climate change on the territory of Ukraine

Evaporation and potential evapotranspiration are components of hydrological cycle that represent the loss of water from the surface and vegetation to the atmosphere. Potential evapotranspiration is a theoretical index that demonstrates the maximum evaporation and transpiration rates assuming sufficient water availability in soil and canopy. Six identical Regional Climate Models (RCMs) of Euro-CORDEX project were selected in order to obtain a unified ensemble for both characteristics for estimation under RCP2.6, RCP4.5 and RCP8.5 scenarios for the middle (2021-2050) and the end of the 21^st century (2071-2100) for Ukraine. ERA5 observational dataset is used as a baseline climate normal (1991-2020) for tracking the future changes. In this study we applied a quantile mapping approach for bias correction for smoothing systematic errors between observational and simulated datasets. For the baseline period, the sums of evaporation varied mainly between 20-30 mm in winter to 250-290 mm in summer, with the exception of the Carpathians and southern regions near marine coastal areas (more than 300 mm). Climate normals of evapotranspiration were zonally distributed with the exception of mountainous region and varies from 20-50 mm in winter to 290-550 mm in summer. The most tremendous changes of evaporation are expected to occur in winter. In general, during the following 30-year period of 2021-2050, the most significant increase by 8-18% (compared to 1991-2020 baseline) would be expected for RCP4.5 with more pronounced increase during 2071-2100, reaching its highest values up to 40% under RCP8.5 The maximum rates are observed in the Carpathians and the northeast of Ukraine. In contrast, evapotranspiration in winter is expected to increase only by 1-6% during 2021-2050 for all RCPs and 12-22% by the end of the century. The Carpathians will face even a decrease by -4%. Changes in evaporation will be lower for the spring season, with changes by 2-4% in 2021-2050 and 6-12% by the end of the century. The highest spring changes up to 28% also will occur in the Carpathians. The same rates are estimated for evapotranspiration, for which the sharpest changes are 10-16% under RCP 8.5 for 2081-2100 In comparison to winter and spring, summer and autumn seasons will face much slower changes. Moreover, summer season will be characterized by a decrease in evaporation at a rate up to -2..-4% under RCP2.6 and varying within ±1% for other scenarios by the mid-century, showing the typical tendencies for so called “evaporation paradox”. In 2071-2100, the decrease can reach by up to-6% for RCP4.5 and RCP8.5. It must be noted the different tendency for evapotranspiration with an increase by 1-6% in general for all RCPs in 2021-2050, and maximum up to 14% by the end of the century. For autumn the most typical increase in both parameters is within 2-6% for all RCPs, with the highest rates of evaporation in the Carpathians up to 15%.  The obtained results show the importance of considering evaporation in future water management, agriculture and food security in Ukraine, highlighting the seasons and regions with it significant changes.