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.
Estimation of regional evapotranspiration is of major importance in hydrological modeling, where the partitioning of available energy into sensible and latent heat fluxes is crucial. Point-based measurements are routinely obtained with micrometeorological methods through a combination of radiometers and eddy-covariance instruments. Notwithstanding closure problems, they are considered to yield reliable flux point values. However, when dealing with heterogeneous semi-arid terrain, these point estimates are not representative for regional values.
In this paper the results are presented of an analysis where MODIS images are used for the mapping of energy and water balances of a heterogeneous land surface in a savannah environment on the southern fringe of the Okavango Delta (Maun, Botswana). Despite its semi-arid character, fresh floodwaters arrive through the Delta seasonally and therefore part of the area’s vegetation is always transpiring at a potential rate.
The model we implemented is governed by remotely sensed values of surface temperature, reflection and vegetation density. The availability of MODIS data provided an opportunity to test the new algorithm by determining the energy balance components on a regional scale for a heterogeneous area and then comparing the results with energy flux measurements using a meteorological flux tower situated in a woodland savannah environment.
The results indicate good estimates of net radiation, soil and turbulent fluxes. However, if energy closure problems are neglected, latent heat estimates show significant deviations.