Estimating Evapotranspiration Values in River Basin Scale Using SWAT Model and SEBAL Algorithm

Estimating actual evapotranspiration in river basins is necessary to use water resources optimally and to improve river basin management. SWAT hydrologic model and SEBAL remote sensing algorithm are among the known methods which have addressed this issue. In the present study, in the first step, the actual evapotranspiration of Karkheh river basin was estimated in dry, normal, and wet years (2008, 2012, and 2015, respectively), using the SWAT model calibrated based on runoff and crop yield and SEBAL algorithm. SWAT model was calibrated and validated using six hydrometric stations for the periods of 1993-2009 and 2010-2013, respectively, in which the , NS and RMSE values were ​​obtained between 0.45 to 0.7, 0.52 to 0.67 and 12.64 to 25.02 (m3/s) for the calibration period and between 0.4 to 0.6, 0.3 to 0.56 and 11.08 to 23.17 (m3/s) for the validation period, respectively. Further, the average observed and simulated yield of the strategic crop (wheat) in the basin were equal to 4.70 and 5.01 (ton/ha), respectively. In addition, the results of SEBAL algorithm and SWAT model were compared together based on the water year status, which the correlations between the results of those methods were equal to 0.74, 0.60, and 0.52 for normal, dry, and wet years, respectively. In the second step, based on the ground data and MODIS, which has a suitable temporal resolution, and OLI which has a suitable spatial resolution, the results of SEBAL algorithm and the variation ranges of main parameters are presented for Pole-dokhtar and Ravansar plains. Ravansar plain has more cultivation areas and lower topography changes compared to Pole-dokhtar plain. The simulation of crop yield by SWAT gave a better result in Pole-dokhtar plain. Based on the results of this study, the values ​​of evapotranspiration obtained from SEBAL algorithm and SWAT model can be reliable and close to the actual values ​​of evapotranspiration in the river basin.