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3.2 Brightness Temperature and Soil Moisture Mapping 
Data collected using the ESTAR were processed to produce brightness temperatures at four beam positions 
which were identical to those of the PBMR (Schmugge et al., 1994). The two flights provided data over a wide 
range of brightness temperatures, even on a single date (August 3). Meteorological conditions during the 
experimental period resulted in ground moisture values that produced the full range of brightness temperatures 
observed in the 1990 experiment. Prior to the August 1 flight there was a localized rainfall event on July 30 
that was centered in the south central portion of the area. On August 2, prior to the second flight on August 
3, there was a large cellular rainfall event that was centered in the northwest. Brightness temperature images 
have been presented in a paper by Jackson et al. (1993). 
As stated at the outset, the primary goal of this experiment was the verification of the ESTAR 
instrument as a soil moisture sensor. This verification was provided by comparing the observed surface soil 
moisture with the values predicted using a passive microwave simulation model (Jackson, 1993) and the 
observed brightness temperatures at those sites. The a priori model relationship is plotted in Figure 1 along with 
the observed brightness temperature and soil moisture data from the 10 sampling sites. Using this model the 
standard error of estimate for the ESTAR observations was estimated as 2.9% soil moisture and leads to the 
conclusion that the ESTAR can be used to accurately estimate soil moisture. 
The ESTAR data were also compared to the PBMR data collected in 1990. As described in Schmugge 
et al. (1994), this was an extensive data set that covered a wide range of moisture conditions. Using the PBMR 
data, a linear regression equation was developed for the prediction of soil moisture from the brightness 
temperature. This curve and the PBMR data are shown in Figure 1. Its standard error of estimate is 2.5%. 
When this linear regression was used to predict soil moisture from the ESTAR brightness temperatures, the error 
was determined to be 2.6%. 
4 - OKLAHOMA 1992 
The work described here was part of the Washita'92 experiment (Jackson and Scheibe, 1993). The final goal 
of the experiment is to test the usefulness of remotely sensed data in hydrologic modelling. The primary goal 
during the experiment was to, collect a time series of spatially distributed hydrologic data, focusing on soil 
moisture and evaporative fluxes, using both conventional and remotely sensed methods. Other specific goals 
during the experiment included the testing and verification of several new remote sensing devices and the 
development of data bases for target-sensor interaction algorithms. Washita'92 was the most ambitious 
experiment of this type that had been conducted to date, focusing on an area nearly an order of magnitude larger 
than previous studies. Another new aspect of this experiment was that it was conducted in a subhumid 
environment. 
4.1. Washita'92 Experiment Description 
The Little Washita watershed was selected because of the extensive hydrologic research that has been conducted 
there in the past (32 years of data collection), the ongoing data collection by the Agricultural Research Service, 
and the complementary nature of the region to previously conducted large scale remote sensing experiments. 
The watershed is located in southwest Oklahoma in the Great Plains region of the United States and 
covers an area of 603 km 2 . The climate is classified as subhumid with an average annual rainfall of 75 cm. 
The topography of the region is moderately rolling. Soils include a wide range of textures with large regions 
of both coarse and fine textures. Land use is dominated by rangeland and pasture (63%) with significant areas 
of winter wheat and other crops concentrated in the floodplain and western portions of the watershed area. 
Vegetation water content was generally less than 0.5 kg/m 2 on rangeland areas. Additional background 
information on the watershed can be found in Allen and Naney (1991) and Jackson and Schiebe (1993). 
The NASA C-130 was used in the Washita'92 experiment. Three different sets of flightlines were 
selected that included water calibration, control site verification, and mapping.. The mapping flightlines, referred 
to as the high altitude coverage lines, were flown at a nominal altitude of 2200 m and were designed to provide 
overlapping coverage by the ESTAR instrument of a large study area. 
Test sites used in Washita'92 were selected to provide spatially distributed soil moisture information 
and to include a representative sampling of the various soil textures and land cover conditions. In most cases, 
these were fields that were at least a quarter-section (0.8 km x 0.8 km) of homogeneous land cover. Within each 
of these fields, sixteen gravimetric 0-5 cm soil moisture samples were collected. A total of 32 sites were 
sampled each day.