Crop reference evapotranspiration (ET0) is an important component of hydrological cycle for determination of crop water requirement in agriculture and water resources mangement. In the present study, we generate regional maps of monthly reference evapotranspiration (ET0) in Grand Karkheh Basin, by using the minimum numbers of meteorological parameters from 24 synoptic stations in Iran. Experimental weather data were collected over the Grand Karkheh Basin (Iran) and its surrounding areas during 1998 to 2009. To determine the monthly ET0, we used FAO 56 method (Ref-ET) and regression analysis. In this work, ET0 maps which generated by standard FAO 56 method (Ref-ET) are called “direct method” and the ET0 from regression relations and independent variables (Tmax, Tmin, U2, RH) called “indirect method”. The FAO 56 is used as an standard method for validation of ET0 from regression analysis. The highest correlation coefficients between ET0 and independent variables were found for the maximum temperature (Tmax), minimum temperature (Tmin), wind speed at 2 meters (U2) and relative humidity (RH), respectively. Regression equations were then derived based on the four aforesaid weather parameters. The best fitted interpolation method for each parameter was selected and the monthly regional maps of ET0 were generated for Grand Karkheh Basin. Using the coefficient of determination (R2), the mean absolute error (MAE), model efficiency (EF) and the relative error (RE), the accuracy of ET0 from regression method were evaluated against FAO 56 method. Results indicate that the regional ET0 maps which generated by the minimum numbers of weather parameters (indirect method) are reliable compare to the direct method (FAO 56 method). The generated regional ET0 maps are useful tools to estimate reference evapotranspiration rates for any certain agricultural location, particularly for the sites facing lack or shortage of weather data.
