Investigation and estimation of effective rainfall in wheat cultivation Case Study: Isfahan Kabootarabad Agrometeorological Research Station

Isfahan district has been faced with limited water resources in recent years because of its special geographical location and highly dependent on the Zayandeh-rood River and groundwater for various uses, including agriculture. Meanwhile, according to available statistics, more than 90% of the country's water consumptions are allocated to the agricultural sector. Calculating effective rainfall, especially in arid and semi-arid regions that face with limited water resources is very important. In this study, effective rainfall in wheat cultivation in the 2015-2016 crop year was estimated by direct (field) method of Ramdas and obtained results from six empirical methods of Renfro, US Bureau of Reclamation (USBR), Evapotranspiration to precipitation ratio, US Soil Conservation Organization (SCS) method, FAO "(FAO/AGLW)", and percentage were compared with Ramdas technique by root-mean-square error (RMSE) tests, normalized root-mean-square error (NRMSE) and the mean absolute error (MAE). The results indicated that the ratio of evapotranspiration to precipitation and the method of the US Soil Conservation Organization (SCS) with RMSE and NRMSE 0.31 and 0.7, MAE 0.11 and 0.45 respectively could be recommended as empirical approaches in Kabootarabad region (Isfahan district).