An intercomparison of techniques to determine the area-averaged latent heat flux from individual in situ observations: a remote sensing approach using the European Field Experiment in a Desertification-Threatened Area data

A knowledge of the area-averaged latent heat flux (lambdaE)is necessary to validate large-scale model predictions of heat fluxes over heterogeneous land surfaces. This paper describes different procedures to obtain (lambdaE) as a weighted average of ground-based observations. The weighting coefficients are obtained from remote sensing measurements. The remote sensing data used in this study consist of a Landsat thematic mapper image of the European Field Experiment in a Desertification-Threatened Area (EFEDA) grid box in central Spain, acquired on June 12,1991. A newly developed remote sensing algorithm, the surface energy balance for land algorithm (SEBAL), solves the energy budget on a pixel-by-pixel basis. From the resulting frequency distribution of the latent heat flux, the area-averaged latent heat flux was calculated as (lambdaE) = 164 W m-2. This method was validated with field measurements of latent heat flux, sensible heat flux, and soil moisture. In general, the SEBAL-derived output compared well with field measurements. Two other methods for retrieval of weighting coefficients were tested against SEBAL. The second method combines satellite images of surface temperature, surface albedo, and normalized difference vegetation index (NDVI) into an index on a pixel-by-pixel basis. After inclusion of ground-based measurements of the latent heat flux, a linear relationship between the index and the latent heat flux was established. This relationship was used to map the latent heat flux on a pixel-by-pixel basis, resulting in (lambdaE) = 194 W m-2. The third method makes use of a supervised classification of the thematic mapper image into eight land use classes. An average latent heat flux was assigned to each class by using field measurements of the latent heat flux. According to the percentage of occurrence of each class in the image, (lambdaE) was calculated as 110 W m-2. A weighting scheme was produced to make an estimation of (lambdaE) possible from in situ observations. The weighting scheme contained a multiplication factor for each measurement site in order to compensate for the relative contribution of that site to (lambdaE). It was shown that (lambdaE) derived as the arithmetic mean of 13 individual in situ observations leads to a difference of 34% ((lambdaE) = 104 W m-2), which emphasizes the need for improved weighting procedures.