Evaluation and calibration of Blaney–Criddle equation for estimating reference evapotranspiration in semiarid and arid regions

Penman–Monteith (FAO-56 PM) equation is suggested as the standard method for estimating evapotranspiration by the International Irrigation and Drainage Committee and FAO. On the other hand, the Blaney–Criddle (BC) temperature-based equation is an alternative and simple method compared with the FAO-56 PM equation. In the present study, the original coefficients BC equation (a and b) were calculated and calibrated spatial and temporal calibration at each station for each month based on the FAO-56 PM method for estimating reference evapotranspiration (ET0) from 15 meteorological stations in central Iran (about 170,000 km2) under semi-arid and arid conditions. The values of a and b in BC equation were negative and positive for all months of any station, respectively. Highest and lowest a values were obtained in December and August, respectively. December showed the lowest b values while August showed the highest. Therefore, the values of a and b were greater in cold and warm months of the year, respectively. After calibration, the root mean square error, mean bias error and percentage error values were obtained lower than 0.50, 0.015 mm day−1 and 10 % for the whole stations and months, respectively. The calibrated b values (bcal) were proportional and inversely to the calibrated a values (acal). The ET0 values based on the calibrated Blaney–Criddle equation were better than the results of the BC equation when compared to the FAO-56 PM equation as the reference model.