According to the importance of this issue in present study first using of time series data of years 2006-2012 and RCM-PRECIS simulation model the impacts of greenhouse gases emission on climatic variables temperature and precipitation under different scenarios in watersheds of Qazvin province was investigated. Then, in order to assess the impacts of climatic variables temperature and precipitation on selected products yield from ordinary least squares (OLS) method and regression analysis was used. In followed, by consider the result of regression analysis in positive mathematical programing (PMP) model the amount of the created variation in supply and demand of irrigation water and agricultural output in watersheds of Qazvin province was investigated. The results showed that emission of greenhouse gases under scenarios A, B and C affects the climatic variables temperature and precipitation about 0/43 to 1/27 °C and -14/1 to 1/31 mm. This case changes selected products yield in the surface of each river basin of Qazvin province. Change in yield affects acreage of agricultural crops about -10/51 to 3/17 percent, the amount of irrigation water supply about -10/4 to 1/64 percent and the amount of irrigation water demand about 1/60 to 7/35 percent. Also, the results showed that maximum and minimum decrease in the gap between supply and demand of irrigation water happens in Kharroud and Shahroud watersheds and about 9/20 and 1/82 percent.