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2 - EXPERIMENT DESCRIPTIONS 
Accurate characterization of the hydrologic cycle and surface energy balance over a wide range of scales is 
vital to the successful implementation of the strategy presented in Figure 1. This is a difficult task in any 
region but the challenge is compounded in arid and semiarid regions due to the relative extremes and large 
spatial and temporal gradients encountered in water and energy balance components. One means of gaming 
a synoptic understanding of hydrologic processes is to conduct large-scale interdisciplinary field campaigns 
which combine traditional ground and atmospheric measurements with remotely sensed measurements. Two 
such experiments- Monsoon’90 and Walnut Guich’92 - have been conducted in a semiarid rangeland 
southeast of Tucson, Arizona, and a third experiment - SALSA-MEX - has been proposed (Figure 2). 
The Monsoon’90 Experiment was conducted in the USDA-ARS Walnut Gulch Experimental Watershed 
(WGEW), primarily during the wet season (July/August) in 1990 (Kustas et al., 1991). The objective of this 
combined ground, aircraft and satellite campaign was to assess the feasibility of utilizing remotely-sensed data 
coupled with water and energy balance modeling for large area estimates of fluxes in semiarid rangelands. 
During the intensive measurement period, an interdisciplinary team of scientists made measurements of 
hydrological, meteorological, atmospheric, edaphic and vegetative conditions simultaneously with aircraft- 
and satellite-based spectral measurements over the north-central portion of WGEW (see dashed outline, Figure 
2). Most of the ground-based measurements were focused on eight sites which covered the main vegetation 
biomes in the mixed grass-brush rangeland. At each site, there were continuous measurements of 
meteorological conditions at screen height, near surface soil temperature and soil moisture, surface 
temperature, incoming solar and net radiation, soil heat flux, and indirect determination of sensible and latent 
heat fluxes. 
The Walnut Gulch’92 Experiment (WG’92) was conducted as a follow-up to Monsoon’90, designed to 
capitalize on preliminary research results and existing hydrologic and meteorologic instrumentation at WGEW 
(Moran et al., 1993). WG’92 was conducted during the dry, early-monsoon, mid-monsoon, post-monsoon 
and "drying" seasons from April through November 1992. During this period, eight Landsat Thematic 
Mapper (TM) scenes, six ERS-1 Synthetic Aperture Radar (SAR) images, and three SPOT High Resolution 
Visible (HRV) scenes were obtained with clear-sky conditions. Each satellite overpass was accompanied by 
extensive ground- and aircraft-based measurements of surface and atmospheric conditions. The overall 
research goal was to investigate the seasonal hydrologic dynamics of the region and to define the information 
potential of combined optical-microwave remote sensing. 
Figure 2. Map of location and boundaries of the USDA-ARS Walnut Gulch Experimental 
Watershed in Arizona, relative to the San Pedro River Basin. Taken from Kustas and 
Goodrich (1994).