Calibrate and Evaluation of the Three Experimental Methods Estimating Source Evapotranspiration. (Case study: Three Different Climates of Six Main Climates in Iran)

Knowing about plant's water requirement is one of the most important considerations in reforming water productivity in agriculture. So precise estimating of source evapotranspiration is one of the most important factors in improving water management. Penman Mantis FAO is used as a standard formula to estimate the source evapotranspiration. But this detailed and complicated method requires lots of information. Due to the lack of certain required information in all the regions, regional equations for calculating evapotranspiration based on the least of the meteorological data seems necessary. To this end, three experimental equation" Blany Criddle, Hargreaves and Torrent White" was selected and based on Penman Mantis FAO equation for three different climates by the linear regression and nonlinear optimized method was calibrated. A ten year period's data for the three stations of thoroughly humid, semi-arid and dry climate was calculated and divided into two groups: the first 5 years and second one. The data for the first 5 years the equations was calibrated and evaluation of the equations was conducted through F test and RPMSE test, using the second 5 years. Based on the results, using of equations Blany Criddle , and Hargreaves, for the thoroughly humid climate and the Torrent White equation calibrated for semi-arid and dry climate using the regression calibration method is recommended to estimate source evapotranspiration. In the calibrating process of equations, linear regression performed more properly than the optimized non-linear method.