Because soil water content and energy fluxes are key parameters in several of the linked models for SRBEX, we are focussing on evaluating, and trying to enhance, techniques for extracting these parameters from the data returned by several remote sensing instruments in the context of soil-vegetation-atmosphere transfer (SVAT) models.
Scattergrams of NDVI against the apparent surface temperature derived from thermal IR for partially vegetated regions permit extraction of a value which is a measure of soil surface water content (Carlson et al., 1994). Attemps to compare measured soil water (SW) profiles with SW derived from various types of remote sensing, however, have raised significant questions as to exactly which layers of the soil contribute to the radiation measured by various remote sensing instruments (Carlson et al, 1995). A collaborative effort with members of the Beltsville, MD office of the USDA/ARS (Agricultural Research Service) has been initiated to use available field measurements of SW profiles, including those from Walnut Gulch, AZ and the planned Little Washita field program in Oklahoma, to further address this problem. An existing soil hydrology model is also being modified to permit more precise modeling of SW in thin soil layers near the surface.
The SAR effort is focussing on the Mahantango Creek Watershed, for which the NASA/JPL AIRSAR instrument collected backscatter data as part of the MACHYDRO 1990 mission. The study area is relatively hilly, with contour plowing methods used for most row crops. Existing software from NASA sources has been implemented on local computers.
Exploratory data analysis has revealed significant differences in backscatter between landcover types for all frequencies and like polarizations as well as between wet and dry conditions for soils under like cover types. It is apparent, however, that corrections for terrain slope and aspect are of major importance. To investigate topographic effects in greater detail, we are currently assembling higher resolution digital elevation data and additional software, and seeking additional SAR data.