A remote sensing surface energy balance algorithm for land (SEBAL) - 2. Validation

The surface fluxes obtained with the Surface Energy balance Algorithm for L and (SEBAL), using remote sensing information and limited input data form t he field were validated with data available from the large-scale field expe riments EFEDA (Spain), HAPEX-Sahel (Niger) and HEIFE (China). In 85% of the cases where held scale surface flux ratios were compared with SEBAL-based surface flux ratios, the differences were within the range of instrumental inaccuracies. Without any calibration procedure, the root mean square error of the evaporative fraction \DeIta (latent heat flux/net available radiati on) for footprints of a few hundred metres varied from Lambda(RMSE) = 0.10 to 0.20 Aggregation of several footprints to a length scale of a few kilome tres reduced the overall error to five percent. Fluxes measured by aircraft during EFEDA were used to study the correctness of remote sensed watershed fluxes (1,000,000 ha):The overall difference in evaporative fraction was n egligible. For the Sahelian landscape in Niger, observed differences were l arger (15%), which could be attributed to the rapid moisture depletion of t he coarse textured soils between the moment of image acquisition (Is Septem ber 1992) and the moment of in situ flux analysis (17 September 1992). For HEIFE, the average difference in SEBAL estimated and ground verified surfac e fluxes was 23 W m(-2), which, considering that surface fluxes were not us ed for calibration, is encouraging. SEBAl estimates of evaporation from the subsealevel Qattara Depression in Egypt (2,000,000 ha) were consistent wit h the numerically predicted discharge from the groundwater system. Tn Egypt 's Nile Delta, the evaporation from a distributed held scale water balance model at a 700,000 ha irrigated agricultural region led to a difference of 5% with daily evaporative fluxes obtained from SEBAL. It is concluded that, for all study areas in arid zones, the errors average out if a larger numb er of pixels is considered. Part 1 of this chapter deals with the formulati on of SEPAL. (C) 1998 Elsevier Science B.V. All rights reserved.