Annual Evapotranspiration Prediction Using Meteorological Data in some Semi-Arid Areas in Iran

Prediction of reference crop potential evapotranspiration (ETo), and other effective climate parameters, are importance of in planning and management of water resources. Our objectives in this study were to: i) detect the probable trends in the parameters that effect decide the ETo rate, and ii) predict ETo for the different climatic zones of Iran using the multi-differential regressions. The meteorological stations, which had collected long-time reliable data (1951 - 2005) were: Mashhad, Shiraz, Tabriz, Kermanshah, Khorramabad and Orumieh; all of these stations are located in the semi-arid areas. The least mean square error technique was used to show the trends. The first 5- year data (L-5) were used to derive the coefficients of the multivariable functions. The last 5-year data were used to predict the ETo, and to evaluate regression equations. The results indicated that the maximum and minimum temperatures, and the ETo increased, and the relative humidity and rainfall decreased in most stations. Using the multi-variable regression technique revealed the presence of a good agreement between the amount of ETo by our calculations and those obtained using the FAO-Penman-Monteith method. The relative errors had low levels; these were 2.4-4.1 mm for the Mashhad Station, - 1.6 to 0.9 mm for the Shiraz Station, -0.5 to 2.4 mm for the Tabriz Station, -2.3 to - 0.7 mmfor the Kermanshah Station, -0.1 to 2.9 mm for the Khorramabad Station, and -2.3 to 2.8 mm for the Orumieh Station. Therefore, our developed regression equations satisfactorilypredicted the ETo.