Prediction of Water Requirement Changes some of Agricultural Products of Mashhad Plain due to Air Temperature Changes

     Air temperature is one of the most important indicators of climate change in global and regional scales, that its changes will affect also on the other climatic parameters and also on other sectors such as agriculture and water use in this section. With increasing air temperature is expected potential evapotranspiration increase. Therefore, in this study using time series were performed modeling and prediction monthly maximum and minimum temperatures of Mashhad plain. Then with the help of empirical formulas Hargreaves - Samani and by selecting crop coefficients were calculated potential evapotranspiration rates and crops water requirement of 5 major products with the largest area under irrigated including wheat, sugar beets, tomatoes, alfalfa, and apples for the base period (1987-2008) and next 10-years period (2009-2018). During the growing season, and were compared with each other. The results show that the maximum and minimum temperatures will increase respectively by 1.4 and 1 ° C. So that the maximum temperature more in warm seasons and minimum temperature more in cold seasons will increase. Prediction of  potential evapotranspiration showed that the changes will be more affected by maximum temperature changes Also the comparison of water requirement of different crops indicated increase evapotranspiration in the future between the future and the base periods. So that among the five selected crops in Mashhad plain, sugar beet has the highest percentage of changes in water requirement equal to2.8% (290 m3/ha).