Estimating root-zone moisture and evapotranspiration with AVHRR data[Advanced Very High Resolution Radiometer]

The parameterized subgrid-scale surface fluxes (PASS) model uses satellite data and limited surface observations to infer root-zone available moisture content and evapotranspiration rate with moderate spatial resolution over extended terrestrial areas. The ultimate goal of this work is to produce estimates of water loss by evapotranspiration, for application in hydrological models. The major advantage to the method is that it can be applied to areas having diverse surface characteristics where direct surface flux measurements either do not exist or are not feasible and where meteorological data are available from only a limited number of ground stations. The emphasis of this work with the PASS model is on improving (1) methods of using satellite remote sensing data to derive the essential parameters for individual types of surfaces over large areas, (2) algorithms for describing the interactions of near-surface atmospheric conditions with surface processes, and (3) algorithms for computing surface energy and water vapor flux at a scale close to the size of a satellite-derived image pixel. The PASS approach is being developed and tested further with observations from the 1997 Cooperative Atmosphere-Surface Exchange Study (CASES-97) at the Atmospheric Boundary Layer Experiments (ABLE) site in the Walnut River Watershed (WRW), an area of about 5,000 km{sup 2} in southern Kansas. Here the authors describe some of the progress made since the previous report