Estimation of Actual Evapotranspiration Using SEBAL Algorithm and Comparison with Improved FAO 56 Standard Evapotranspiration with KC-NDVI Relationship

Monitoring the spatial and temporal variations of evapotranspiration is crucial for irrigation management and the crop water requirement, especially in arid and semi-arid areas. The purpose of the present study is to estimate the actual evapotranspiration using the SEBAL algorithm and compare it with the FAO 56 standard evapotranspiration to determine pistachio orchards under drought stress in Yazd province. To do so, Landsat 8 satellite images time series with 15 images in 2015 were used. At first, actual evapotranspiration was calculated in 15 days of pistachio phenology and then by summation of evapotranspiration in 15 days, total evapotranspiration was determined in four main stages of pistachio phenology covering the whole period of annual growth. The FAO 56 standard evapotranspiration was also obtained by using the KC-NDVI relationship as the standard for comparing with actual evapotranspiration. Based on the results, SEBAL algorithm has an acceptable capability to determine the evapotranspiration rate in the study area. However, due to lack of valid Lysimeter data in the study area, It was not possible to validate the results of the SEBAL algorithm. But comparing the results with the FAO 56 standard method showed that the two methods are in good agreement with each other. In average, the coefficient determination, RMSE and MAE between the results of SEBAL algorithm and FAO KC-NDVI approach were 0.8, 16.7 mm and 14.5 mm, respectively, for the 15-day of pistachio phenology stages. The average of actual and standard evapotranspiration rates during a pistachio growing season at the study area were 950 and 1086 mm, respectively. Comparison of actual and standard evapotranspiration shows that in most of the study area actual evapotranspiration is lower than standard conditions.