Estimation of the Reference Evapotranspiration Using the Projections of CORDEX Project and Investigation of the Meteorological Variables Contribution in its Changes (Case Study: Lake Urmia Basin)

The reference evapotranspiration (ETo) is one of the most important effective factors in hydrological cycle and has been used in the calculation of many drought and aridity indices and the studies related to the projection of climate change impacts. The investigation of climate change impact on ETo is necessary for longterm scheduling and management of water resources. In this study, the downscaled projections of two climatic models of EC-EARTH and GFDL-ESM2M (based on RCP8.5 scenario) under CORDEX project were used and using the modified method of FAO Penman-Monteith and by considering the effects of the atmospheric CO2 concentration on plants stomatal resistance, the ETo was estimated for the historical period (1976-2005) and future period (2006-2100) in lake Urmia basin. The results of investigation of ETo changes in three near-future (2011-2040), mid-future (2041-2070) and far-future (2071-2100) periods in comparison to the historical period revealed an increasing trend of ETo especially for far-future. The contribution analysis of the meteorological variables in ETo changes showed that the maximum temperature (with the average contribution of 91.54 %), the minimum temperature (with the average contribution of 37.68 %) and the atmospheric CO2 concentration (with the average contribution of 41.40 %) will have the most contribution in ETo changes. However, because of increasing trend of the agricultural development, the effects of irrigation on the climate and ignoring the irrigation effects in climatic models, projection of ETo changes based on climatic models may be along with some uncertainties.