Analyzing daily, Monthly and Seasonal Accuracy of Reference Evapotranspiration Equations in a Humid Region

Proper water management in agricultural fields is largely depends on suitable estimation of evapotranspiration (ET), as a major component of hydrological cycle. The FAO-Penman- montieth method has the ability to provide appropriate estimation of ET, however, due its complexity, using simpler methods is preferred for programming and modeling processes. In this study, using the weather data of the Dasht-e- Naz station in Sari, the capability of 14 methods for estimation daily, monthly and seasonal reference ET was evaluated. These methods included three temperature- based methods, four radiation- based methods, three pan evaporation- based methods and four combined methods. The performance of each method was assessed using statistics including coefficient of determination (R2), root mean square error (RMSE), percent error (PE) and Mean Absolute Deviation (MAD). Investigations showed that the radiation- based equations were more appropriate for estimating ET than other methods. The Irmak (with R2=0.87 and RMSE=5.77 mm d-1) and Ritchi (with R2=0.93 and RMSE=12.59 mm d-1) methods were the best methods with the highest accuracy in the autumn and winter seasons. In the spring, the use of the FAO-24 Pen (with an average error of 0.5 mm) and Rohwer (with an average error of 6.8 mm) (equations are recommended for spring and summer. Based on the results, the Irmak, FAO-24-Radiation, Ritchie, Priestley-Taylor and Blaney-Cridlle methods were five best methods in the study area.