**10. Climate change and increasing water-use requirement of plants**

The physical process of converting liquid water into vapor and entering it into the air is called evaporation. This process is also carried out from the free surface of the water, soil surface, or wet surfaces of the plant. On the contrary, the process of transpiration is called water removal in the form of vapor from the leaves and inside the plant. Practically, the categorizing of transpiration and evaporation in two groups is difficult. Therefore, they are combined and used as evapotranspiration. The sum of evapotranspiration and the water needed for leaching soil is called the water-use requirement of the plant. Since the need for leaching is negligible compared to evapotranspiration, evapotranspiration is also defined as the water-use requirement of the plant [56]. The main factors affecting evapotranspiration depend on climatic elements such as temperature, precipitation, humidity, wind speed, and solar radiation. So, any changes in climatic elements will affect the amount of evapotranspiration and water required by the plant [57]. Due to the changes in climatic elements that have occurred following global warming, it can be expected that the amount of evapotranspiration in different geographical regions can also be changed.

Assessments conducted by various researchers indicate that the occurrence of climate change, evapotranspiration, and water-use requirements of plants will undergo serious changes [58]. Tao et al. [59] by investigating the effect of climate change on reference plant evapotranspiration in China's Xiangjiang basin, under RCP scenarios and using the SDSM model for downscale, concluded that in future periods, the amount of reference plant evapotranspiration will increase under all scenarios and the rate of this increase will vary depending on the region and scenario and the highest incremental rate will be observed under the RCP8.5 scenario..

Heidari Tasheh Kabood and Khoshkhoo [60] predicted future changes of reference evapotranspiration in western Iran based on RCP emission scenarios. They used the FAO-Penman-Monteith method to estimate evapotranspiration, canESM2 global circulation model to simulate climatic conditions, and for downscale of the data in this model, the SDMS method was used. The results showed that in all future periods and under all scenarios for all stations, the mean of reference evapotranspiration in annual scales and for autumn and winter will increase significantly at 0.01 level compared to the base period. In a similar study conducted using the CanESM2 model under RCP2.6, RCP4.5, and RCP8.5 scenarios in Mazandaran province in Iran, the results illustrated that the percentage of changes in evapotranspiration per different months varies from −16.1 to 25.7%. The highest and lowest percentage of reference evapotranspiration changes are in October and March respectively [61].
