An original interpretation of the surface temperature-albedo space to estimate crop evapotranspiration (SEB-1S)

O. Merlin
2013 Hydrology and Earth System Sciences Discussions  
The space defined by the pair surface temperature (T ) and surface albedo (α), and the space defined by the pair T and fractional green vegetation cover (f vg ) have been extensively used to estimate evaporative fraction (EF) from optical remote sensing data. In both space-based approaches, evapotranspiration (ET) is estimated as remotely 5 sensed EF times the available energy. For a given data point in the T − α space or in the T − f vg space, EF is derived as the ratio of the distance
more » ... he distance separating the point from the line identified as the dry edge to the distance separating the dry edge and the line identified as the wet edge. The dry and wet edges are classically defined as the upper and lower limit of the spaces, respectively. When side-by-side investigating the T − α and 10 the T − f vg spaces, one observes that the range covered by T values on the (classically determined) wet edge is different for both spaces. In addition, when extending the wet and dry lines of the T − α space, both lines cross at α ≈ 0.4 although the wet and dry edges of the T − f vg space never cross for 0 ≤ f vg < 1. In this paper, a new ET (EF) model (SEB-1S) is derived by revisiting the classical physical interpretation of the T − α space 15 to make its wet edge consistent with that of the T − f vg space. SEB-1S is tested over a 16 km by 10 km irrigated area in northwestern Mexico during the 2007-2008 agricultural season. The classical T − α space-based model is implemented as benchmark to evaluate the performance of SEB-1S. Input data are composed of ASTER (Advanced Spaceborne Thermal Emission and Reflection radiometer) thermal infrared, Formosat-20 2 shortwave, and station-based meteorological data. The fluxes simulated by SEB-1S and the classical T − α space-based model are compared on seven ASTER overpass dates with the in situ measurements collected at six locations within the study domain. The ET simulated by SEB-1S is significantly more accurate and robust than that predicted by the classical T − α space-based model. The correlation coefficient and slope 25 of the linear regression between simulated and observed ET is improved from 0.82 to 0.93, and from 0.63 to 0.90, respectively. Moreover, constraining the wet edge using air 6278 temperature data improves the slope of the linear regression between simulated and observed ET. 20 1998; Anderson et al., 2007; Cammalleri et al., 2012, e.g.), and to data assimilation procedures of T into coupled energy and water balance models (Caparrini et al., 2004; Olioso et al., 2005; Pipunic et al., 2008, e.g.). The current optical-based ET estimation methods greatly vary in complexity. The main advantages of data assimilation methods into physically-based SVAT (Soil Vege-25 tation Atmosphere Transfer) models are (1) to integrate many ET-related physical processes such as soil water diffusion and vegetation water uptake in the root zone, and 6279 HESSD
doi:10.5194/hessd-10-6277-2013 fatcat:pzlsgji27ffr5eeljxieta2myq