Date of this Version
Transactions of the ASABE Vol. 58(5): 1265-1285
Large-scale quantification of crop evapotranspiration (ETc) from various vegetation surfaces can aid in planning, managing, and allocating water resources. Field measurement of surface energy fluxes, including ETc, remains (and should remain) a crucial process for calibration and validation of satellite/remote sensing-based methods, which can provide important supporting information for water balance assessments and for analyzing the spatial distribution of energy fluxes on large scales. The Surface Energy Balance System (SEBS) was evaluated in estimating surface energy fluxes in south central Nebraska using Landsat imagery and meteorological data. SEBS-estimated surface energy fluxes were compared to Bowen Ratio Energy Balance System (BREBS) flux data measured over tall (maize) and short (winter wheat and rainfed grass) vegetation surfaces at Nebraska Water and Energy Flux Measurement, Modeling, and Research Network (NEBFLUX) tower sites. A total of 54 cloud-free Landsat 5 Thematic Mapper and Landsat 7 Enhanced Thematic Mapper Plus images that were available for both path 29 row 31 and path 29 row 32 were analyzed for the spatial distribution of ETc over the study area. On an all-vegetation-average basis (pooled data from all surfaces), the correlation between estimated and measured surface energy balance components had R2 values of 0.88, 0.90, 0.63, and 0.32 for ETc, net radiation (Rn), sensible heat flux (H), and soil heat flux (G), respectively. SEBS overestimated Rn considerably by 46 W m-2, and estimates for G were also poor. Results were somewhat improved when comparisons were made on an individual vegetation surface basis. In addition to detailed analyses of ETc and other surface energy fluxes of irrigated maize, winter wheat, and rainfed grassland, the spatial distributions of ETc for ten other surfaces (rainfed maize, sorghum, soybean, winter wheat, alfalfa, open water, developed/open space, deciduous forest, grassland/pasture, and woody wetlands) were mapped and evaluated. Substantial variability in ETc was observed over the study area, which was mainly due to the diverse cropping systems and management practices across the area. The SEBS performance was poor and unsatisfactory during days with precipitation events. Additional research is needed to investigate the performance of SEBS for various vegetation surfaces and to develop algorithms to improve the performance of the model to estimate surface energy fluxes for different periods of the growing season and during days with precipitation events.