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Title:   Author  item
                                            TIMMERMANS, WIM - INT. INST. OF GEO-INFORM.item
                                            Kustas, William

                                                         - *Bill*item
                                            Anderson, Marthaitem
                                            FRENCH, ANDREW - USDA-ARS
  Submitted to: Remote Sensing of Environment
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 3/5/2007
                                    Publication  Date: 5/15/2007
                                    Citation: Timmermans, W.J., Kustas, W.P., Anderson, M.C., French, A.N. 2007. An intercomparison of the surface energy balance algorithm for land (SEBAL) and the two-source energy balance (TSEB) modeling schemes. Remote Sensing of Environment. 108: 369-284.Interpretive Summary: An intercomparison of output from two models estimating spatially distributed surface energy and water fluxes from remotely sensed imagery is conducted. A major difference between the two models is whether the soil and vegetation components of the scene are treated separately (Two-Source Energy Balance; TSEB approach) or as a lumped composite (one-source approach; Surface Energy Balance Algorithm for Land; SEBAL) in the parameterization of water and energy exchanges with the overlying air. Comparisons are performed using data from two large scale field experiments covering sub-humid grassland (Southern Great Plains ’97) and semi-arid rangeland (Monsoon ’90) having very different landscape properties. The intercomparisons of model output over the full modeling domains yielded large discrepancies in the water and energy fluxes that are related to land cover type. Modifications to SEBAL inputs that reduced discrepancies with TSEB and observations for some land cover classes tended to increase differences for others. In particular, ground-truth data to better define some of the inputs to the SEBAL model tended to exacerbate errors with respect to observed and TSEB modeled fluxes.  These results suggest that some of the simplifying assumptions in SEBAL may not be strictly applicable over the wide range in conditions present within these landscapes.  An analysis of TSEB and SEBAL sensitivity to uncertainties in primary inputs indicated that errors in surface temperature or surface-air temperature differences had the greatest impact on flux estimates.  Inputs of secondary importance were fractional vegetation cover for TSEB, while for SEBAL, the selection of pixels representing wet and dry moisture extremes significantly influenced flux predictions.  The models were also run using input fields derived from both local and remote data sources, to test performance under conditions of varying ancillary data availability.  In this case, both models performed similarly under both constraints.  Since both models have the potential for assessing crop water use and vegetation stress operationally with satellite data, development of a hybrid model combining the strengths of the two modeling approaches is under investigation.
                                Technical Abstract: An intercomparison of output from two models estimating spatially distributed surface energy fluxes from remotely sensed imagery is conducted. A major difference between the two models is whether the soil and vegetation components of the scene are treated separately (Two-Source Energy Balance; TSEB approach) or as a lumped composite (one-source approach; Surface Energy Balance Algorithm for Land; SEBAL) in the parameterization of radiative and turbulent exchanges with the overlying air. Comparisons are performed using data from two largescale field experiments covering sub-humid grassland (Southern Great Plains ’97) and semi-arid rangeland (Monsoon ’90) having very different landscape properties. In general, there was reasonable agreement between flux output from both models versus a handful of flux tower observations. However, spatial intercomparisons of model output over the full modeling domains yielded relatively large discrepancies (on the order of 100 W m-2) in sensible heat flux (H) that are related to land cover. In particular, bare soil and sparsely vegetated areas yielded the largest discrepancies, with TSEB fluxes being in better agreement with tower observations. Modifications to SEBAL inputs that reduced discrepancies with TSEB and observations for bare soil and shrub classes tended to increase differences for other land cover classes. In particular, improvements to SEBAL inputs of surface roughness for momentum tended to exacerbate errors with respect to observed fluxes.  These results suggest that some of the simplifying assumptions in SEBAL of may not be strictly applicable over the wide range in conditions present within these landscapes.  An analysis of TSEB and SEBAL sensitivity to uncertainties in primary inputs indicated that errors in surface temperature or surface-air temperature differences had the greatest impact on H estimates.  Inputs of secondary importance were fractional vegetation cover for TSEB, while for SEBAL, the selection of pixels representing wet and dry moisture end-member conditions significantly influenced flux predictions.  The models were also run using input fields derived from both local and remote data sources, to test performance under conditions of varying ancillary data availability.  In this case, both models performed similarly under both constraints.


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