Calibration and evaluation of three empirical methods for estimating reference evapotranspiration in deferent climates of Iran

The knowledge of crop water requirements is an important practical consideration to improve water use efficiency in irrigated agriculture. Therefore, accurate estimation of reference evapotranspiration is an important factor in efficient water management. FAO Penman-Monteith method is used as the standard method to estimate evapotranspiration. But its calculation is long and complicated and requires a lot of climatic data. Due to the lack of information needed for this method in all areas, derived equation for the calculation regional evapotranspiration based on minimum meteorological data is necessary. For this purpose, three modified empirical equations (Blaney-Criddle, Hargreaves and Thornthwaite) were selected and calibrated based on FAO Penman-Monteith equation for five different climates using linear regression and nonlinear optimization. It was considered a 10-year period monthly data for selected stations and it was divided into two groups: first 5 years and second 5 years. Equations were calibrated with data from the first 5 years and they were evaluated using data from the second 5 years through F-test and relative root mean square error (RRMSE) criteria. Results indicated that if estimation of evapotranspiration using FAO Penman-Monteith method is not possible due to lack of required data, calibrated equations can be used to obtain similar results to FAO Penman-Monteith method estimations.