Using the dual approach of FAO-56 combined with multi-sensor remote sensing for estimating the regional evapotranspiration

The main goal of this study is to evaluate the potential of the FAO-56 dual technique for the estimation of regional evapotranspiration (ET) and its constituent components (crop transpiration and soil evaporation) in the semi-arid region of the Kairouan plain in central Tunisia. The proposed approach combines the FAO-56 technique with remote sensing data (optical and microwave), not only for vegetation characterization, as proposed in other studies, but also for the estimation of soil evaporation, through the use of satellite moisture products. Two vegetation classes are considered in the evapotranspiration estimations. In the case of annual agriculture (cereals), crop coefficients and cover fractions are estimated using relationships established with the Normalized Difference Vegetation Index (NDVI), retrieved from SPOT-VGT data. In order to characterize the soil evaporation, a relationship is established between evaporation and the retrieved soil moisture values, based on the ERS/WSC products developed by the University of Vienna. Finally, a comparison is made between the ISBA-A-gs and FAO approaches, for the same studied site for the period between 1991 and 2007.