Comparison of ETo calculated by Hargreaves-Samani and standard FAO 56 methods in regional values in Grand Karkheh Basin

The aim of the present research is to generate the daily regional map of reference crop evapotranspiration (ETo) using minimum meteorological parameters for Grand Karkheh basin. To conduct this study, meteorological data from 24 synoptic stations located in Karkhe basin and surrounding sites were applied and the daily ETo for each site were calculated by Hargraves-Samani and FAO-56 methods in RefET Software. Furthermore, different interpolation methods were fitted on the results of both mentioned models. The best interpolation method was selected according to coefficient of the determination and Root Mean Square Error criteria. Results indicate that Ordinary Kriging is more accurate and reliable than other interpolation approaches for months of growth season (April-September) for generating the regional ETo maps in the study regions. The regional maps of ETo were derived and classified for months of growth season by ArcMap software using the selected interpolation method. Moreover, the area allocated to each classification obtained for each maps of ETo and in each month of the growth season both ETo methods in region scale were compared with each other. The discrepancies of ETo maps from two methods were more significant in the North and North-west of the study region. Regional map differences between two methods were found to be more significant in July and August.