Aymn Elhaddad 1 Luis A. Garcia 2
Surface energy balance models to estimate actual evapotranspiration using satellite imagery using visible and thermal bands are becoming widely used in water resources management, water rights, and irrigation scheduling. In the last decade several energy balance models were developed for estimating actual evapotranspiration from vegetated areas, These models use both raster (such as satellite imagery and DEM) and point inputs (wind run, reference instantaneous ET and 24 hour reference ET), to solve the surface energy balance equation. However, they only use point data when solving the relation between the temperature gradient and the near surface temperature, which does not take into account weather spatial variability. A fully raster based energy balance model has been develop by the authors (Remote Sensing of ET ReSET Raster). The advantages of using a fully rasterized ET model versus the classical point based remote sensing ET models will presented. Point based ET models can only use one weather station as a reference point in their processing of an area to estimate ET. In case that several weather stations are present in an area, these point models use either an average value of the different weather parameters from all weather stations or a Thiessen polygon approach is used. The first approach can cause inaccuracies in the reference point value used and the second approach can cause discontinuities at the edge of the Thiessen polygons that can lead to errors in estimating ET (up to 17% in the case study used for this presentation). The magnitude of the ET estimation error can be larger depending on the spatial variability of the weather data used in the model. The ReSET-Raster model (Elhaddad and Garcia, 2011) uses a raster data format for all its input data and in all its internal calculations. This presentation will focus on the advantages of using a Raster based ET model versus using single point ET models.
The raster based model used in this work (ReSET-Raster, Elhaddad and Garcia 2011) is a variant of the ReSET model (Elhaddad and Garcia 2008) that builds on the work done by both the SEBAL model (Bastiaanssen et al.1998) and the METRIC model (Allen et al. 2007).
Keywords. Evapotranspiration; remote sensing; heat-flux; ET modeling.
1 Research Scientist, Integrated Decision Support Group, Dept. of Civil and Environmental Engineering (1372), Colorado State Univ., Fort Collins, CO. 80523. 2 Director, Integrated Decision Support Group and Prof., Dept. of Civil and Environmental Engineering (1372), Colorado State Univ., Fort Collins, CO. 80523. Phone: (970) 491-5049, FAX: (970) 491-7626, E-Mail: Luis.Garcia@Colostate.edu.