A comparative assessment of SWAT-model-based evapotranspiration against regional-scale estimates

We attempted to assess the actual evapotranspiration (ETa) rates estimated by the Soil and Water Assessment Tool (SWAT) model in an arid region by comparing other regional-based ETa estimates: the Operational Simplified Surface Energy Balance (SSEBop) products, the MODerate Resolution Imaging Spectroradiometer (MODIS) Global Evapotranspiration product (MOD16), and the Generalized Complementary Relationship (GCR) model. The results by the SWAT model clearly showed that ETa rates in the SWAT model are limited by soil water availability. From the comparison of the annual ETa estimates, the GCR model among those ETa estimation methods simulated slightly higher values than the water-balance derived ETa rates (i.e., the difference between annual precipitation and annual stream flow rates). The annual ETa rates of the SSEBop products showed the closest slope of the regression line to 1.0, while the highest R2 (coefficient of determination) value was found from the annual ETa estimates by the SWAT model. The monthly ETa by the GCR model had the highest linear relationship with those of the SSEBop products. However, the SWAT model did not have a strong linear relationship with the monthly ETa of the SSEBop products unlike the comparison against the MOD16 products. This study recommends a further study on more reliable ETa estimates in arid or semi-arid regions by the SWAT model.