USE OF REMOTE-SENSING FOR EVAPOTRANSPIRATION MONITORING OVER LAND SURFACES

Monitoring evapotranspiration (ET) at large scales is important for as sessing climate and anthropogenic effects on natural and agricultural ecosystems. This paper describes techniques used in evaluating ET with remote sensing, which is the only technology that can efficiently and economically provide regional and global coverage. Some of the empiri cal/statistical techniques have been used operationally with satellite data for computing daily ET at regional scales. The more complex nume rical simulation models require detailed input parameters that may lim it their application to regions containing a large database of soils a nd vegetation properties. Current efforts are being directed towards s implifying the parameter requirements of these models. Essentially all energy balance models rely on an estimate of the available energy (ne t radiation less soil heat flux). Net radiation is not easily determin ed from space, although progress is being made. Simplified approaches for estimating soil heat flux appear promising for operational applica tions. In addition, most ET models utilize remote sensing data in the shortwave and thermal wavelengths to measure key boundary conditions. Differences between the radiometric surface temperature and aerodynami c temperature can be significant and progress in incorporating this ef fect is evident. Atmospheric effects on optical data are significant, and optical sensors cannot see through clouds. This has led some to us e microwave observations as a surrogate for optical data to provide es timates of surface moisture and surface temperature; preliminary resul ts are encouraging. The approaches that appear most promising use surf ace temperature and vegetation indices or a time rate of change in sur face temperature coupled to an atmospheric boundary layer model. For m any of these models, differences with ET observations can be as low as 20% from hourly to daily time scales, approaching the level of uncert ainty in the measurement of ET and contradicting some recent pessimist ic conclusions concerning the utility of remotely sensed radiometric s urface temperature for determining the surface energy balance.