Comparisons of Four Methods for Evapotranspiration Estimates in Jordan

Abstract:

We compared evapotranspiration (ET) estimates in Jordan calculated by four theoretically-different methods. The first method was the FAO Single Crop Coefficient method. Our calculation took into account 20 dominant crop species in Jordan, utilized the global Climate Forecast System Reanalysis (CFSR) data set, and generated spatially heterogeneous crop coefficients. The second approach was the Surface Energy Balance Algorithms for Land (SEBAL) method. It was used with Landsat TM/ETM+ images to calculate instantaneous ET at the moment of satellite overpass, and the results of multiple images were combined to derive seasonal and annual ET estimates. The third method was based on the 1-km land surface ET product from MODIS, which was calculated using MODIS-observed land cover and photosynthetically active radiation. The fourth method was based on the SWAT model, which combines the Penman-Monteith equation and vegetation growth to estimate daily ET rates at the watershed scale. The results show substantial differences in both magnitude and spatiotemporal patterns of ET estimates across different regions from the four methods. Such differences were particularly evident in the Highlands region, where irrigation plays a critical role in local water balance. Results also suggest that land cover data is a major source of uncertainty in estimating regional ET rates. Although it is difficult to conclude which method was more reliable due to the limited availability of validation data, the results suggest caution in developing and interpreting ET estimates in this arid environment.