Modification of Jensen- Haise Equation for Estimating Evapotranspiration in Farce province

Accurate estimation of crop water requirement based on evapotranspiration calculation has an important role in improvement of field water management. The Penman-Monteith equation is the most common method for estimating reference crop potential evapotranspiration. However, this equation needs full weatherr data, but a few stations with complete weather data exist in Fars province. On the other hand, the Jensen- Haise equation is a more simple method for estimating reference crop potential evapotranspiration. In this study, using the weather data of some synoptic stations in Fars province, the Jensen- Haise equation was calibrated and then evaluated based on Penman-Monteith method for every month of the year. For this purpose, the calibrated coefficients of the Jensen- Haise equation were determined for all months and all stations, separately. Also, the monthly effective temperature (combinatio of minimum and maximum monthly temperature with a constant coefficient) was used instead of monthly mean temperature in calibration stage, and the results indicated the appropriateness of this replacement for estimating evapotranspiration. Furthermore, the estimated sunshine hours (based on minimum and maximum monthly temperature) were used instead of measured sunshine hours in validation stage in Jensen- Haise equation, and the results indicated the appropriateness of this replacement for good validation of this equation in Fars province. Finally, the results indicated that the adjusted Jensen- Haise equation was appropriate in Fars province.