Selection of the best empirical equations to predict reference evapotranspirationand their coefficient calibration in Rasht region

Regardingthe climate change and recent droughts, the proper management of irrigation systems and water resources is very important. However, achieving the optimum crop yields due to lack of water requires, the accuracy estimating the amount of water by the plants to prevent water loss and or water stress in plants is needed. In this study, the reference evapotranspiration using lysimeter was measured during three cropping seasons. Applying Ref-ET software, the reference evapotranspiration with 16 empirical equations using meteorological data were estimated, the significant difference between measured and estimated was determined using SPSS software. The empirical equations Hargreaves, Priestley-Taylor and Penman (FAO) were determined as the most suitable methods for the studied area. For the obtained data from two growing seasons and by applying nonlinear regression in SPSS software, optimized coefficients for the two Priestley-Taylor and Hargreaves equations were estimated, and then by using obtained data in the third year, thevalidation was done and RMSE as well as nRMSE values were calculated to assess the equations that for optimized Hargreaves were 0.20 mm/day and 4.60% and for optimized Priestley-Taylor equation were 0.22 mm/day and 4.5%, respectively.