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.