Monthly Evapotranspiration Modeling in the Absence of Climatic Data Using the Soft Computing Methods in West and Northwest of Iran

Evapotranspiration (ET0), a major component of the hydrologic cycle, is important in water resources development and irrigation planning. The ET0 for west and northwest of Iran was estimated using the FAO Penman-Montieth method (FAO-56) as the reference. The performance of four different data-driven methods, namely the Artificial Neural Networks (ANN), Adaptive Neuro Fuzzy Inference System (ANFIS) with grid partition (GP), ANFIS with subtractive clustering (SC), and Gene Expression Programming (GEP) were investigated on the ET0 estimation. Latitude, longitude and altitude of stations, and the periodicity component were used as inputs to the applied models to predict the long-term monthly ET0 using the data from 12 stations in the west and northwest of Iran. The maximum coefficients of determination (R2) were found to be 0.994, 0.998 and 0.997 for the ANN, ANFIS-GP and ANFIS-SC models at the Sanandaj Station, respectively. The highest R2 (0.982) of the GEP model was calculated for the Khoy Station. The root mean squared error ranged 0.311-1.551, 0.368-1.319, 0.450-1.80 and 0.435-0.833 mm/day for the ANN, ANFIS-GP, ANFISSC and GEP models, respectively. The results revealed the high accuracy of the GEP in the prediction of ET0. Therefore, the GEP model can be applied as a simple method in future studies as an alternative to the use of empirical relationships for the ET0 estimation.