Temperature-emissivity separation with ASTER and LANDSAT 7 imagery validation on the fringe of the Okavango Delta, Botswana

Land surface temperatures are important in global change studies, in estimating radiation budget, heat balance studies and as control for climate models. A new algorithm for estimating land surface temperature and emissivity spectra for multi spectral thermal infrared ranging from 8 to 12mm images has been developed recently (Schmugge et al., 2002) for use with data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on the TERRA platform. Similar methods are also used with the MODIS instrument. In this study, the method developed by Ogawa et al. (2002) was adopted to estimate the broadband emissivity from the narrow band emissivities of the five TIR channels of ASTER instrument in an area on the southern fringe of the Okavango Delta (Botswana). MODTRAN 4 was used to determine the necessary atmospheric corrections while software was developed to facilitate MODTRAN pre- and post-processing. The results were compared with field data, with a LANDSAT 7 image of the same day, and finally also with reported ASTER surface temperature and emissivities for the same image (high level ASTER product). Results indicate that the surface temperature depends rather sensitively on atmospheric transmissivity. No relation was found between broad-band emissivity and NDVI, contrary, for example, to earlier findings in Botswana by Owe and Van de Griend (1993). Using the TES method it becomes possible to obtain more reliable solutions to the energy balance and evapotranspiration problem, especially in semi-arid areas.