An appropriate method for estimating potential evapotranspiration in the absence of meteorological data(The case study of Khorrambid Township in Fars Province)

Potential evapotranspiration (PET) is an important parameter of the hydrologic cycle whose estimation is essential for irrigation and water facility schemes as well as drainage and hydrologic studies. The parameter represents potential losses from moist surface soil and vegetation cover. The present study uses different evapotraspiration estimation methods and compares the results obtained from each against those obtained from the FAO-Penman-Monteith method used as the reference in the environment of the Ref-ET software to determine the accuracy of each and to select the best method for estimating PET in Khorrambid Township, Fars Province. All the methods are evaluated in terms of their efficiency using the statistical parameters of Normalized Root Mean Square Error (NRMSE), Efficiency of Model (EF), Agreement Index (d), and Mean Bias Error (MBE). The results are found to rank the investigated methods in the following order: ASCE stPM, ASCE PM, ASCE PMrs, KPen (1972), FAO 24Pn, FP 17 Pen, KPen (1996), Pen (1948), Harg (1985), FAO 24Rd, FAO 24BC, Prs –Tylr, Makk (1957), Turc (1961), and FAO Pan in terms of the lowest MBE and NRMSE but the highest EF and d values recorded. Although the Penman-based equations are found capable of yielding more accurate estimates of evapotranspiration rates by using temperature, relative humidity, wind speed, and sunny hours, in the absence of such data, however, it is the Hargreaves-Samani empirical method that can be effectively used to estimate evapotranspiration in Khorrambid Township with a lower error level than other empirical methods.