Estimation of ETp in the southern part of Aras Basin based on CanESM2 model data

Evapotranspiration is one of the important components of water balance. Estimation of evapotranspiration has been the focus of many researchers in Iran and the world. Accurate estimation of evapotranspiration is very important in hydrological modeling, irrigation design, and water resources management. This variable is one of the most important and effective components in the water balance. Evapotranspiration after rainfall is considered the second largest component of the earth's water cycle on a global scale. The assessment of the future climate and climate change and its effects is done through the output of climate models.

In this research, the average daily minimum and maximum temperature data were scaled according to the CanESM2 model under RCP scenarios using the SDSM for 6 synoptic stations in the southern part of the Aras River basin to draw the perspective of ETp using the Hargreaves-Samani method until the 2050s. For this purpose, observational data of stations and reanalysis data (NCEP) in the daily period (1985-2005) as well as historical data of the CanESM2 model (historical-2005) under RCP scenarios (for the period 2006-2100) has been used.

Estimated ETp values for the Aras basin during the coming period based on the downscaled temperature data of the CanESM2 model under RCP scenarios showed that the value of this variable under the RCP2.6 scenario compared to the base period, slightly decreased and under the RCP4.5 and RCP8 scenarios will have a slight increase. The amount of ETp in this basin will have a decreasing change in the Ardabil, Ahar, and Khoi stations and an increasing change in the Pars-Abad and Jolfa stations. The monthly ETp value of the Aras basin in the future period in January, April to June, and August was estimated to increase with a range between 0.1 to a maximum of 24.3 mm compared to the base period. Comparing the estimated ETp values of the future and the past period showed that the ETp estimated by the Hargreaves-Samani method in the past period compared to the evaporation data of stations except Pars-Abad and Khoi was overestimated by more than 100 mm per year, and it was less in other stations. Hargreaves-Samani ETp values, except for Mako, which is higher from 1985 to 2005 than in 1992-2005, in the other stations in the period of 1992-2005 are greater than the values of the base period.

The estimated ETp values for the Aras basin during the coming period showed that the value of this variable at the annual basin level will increase slightly compared to the ETp of the base period (by the Hargreaves-Samani method), which means that the water requirement of plants will increase in the future in the growing season and this increase means an increase in the water requirement of plants in the future in the growing season, a decrease in infiltration and an increase in evaporation of water resulting from rainfall and snow melting, and a decrease in the feeding of aquifers.