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2. EXPERIMENTAL DESIGN 
2.1. The Walnut Gulch 92 (WG' 92) experiment: 
To investigate the use of combined optical-microwave remote sensing, an experiment was conducted during the 
1992 dry and monsoon (wet) seasons in the USDA-ARS Walnut Gulch experimental watershed (southeastern 
Arizona). Rainfall typically ranges from 250 to 500 mm/year in this region with almost 2/3 occuring during the 
summer "Monsoon" season in July and August. Nine Landsat TM scenes, three SPOT HRV scenes and five 
ERS-1 SAR images were acquired to monitor the seasonal surface changes. Eight subsites (METFLUX 1 to 8) 
were selected within the watershed to assess the spatial variability of vegetation, soil and meteorological 
properties, with a particular monitoring of METFLUX 1 (Lucky Hills) and 5 (Kendall). For each of the 
METFLUX (MF) sites, a 5x3 TM pixels target (150x90 m) was defined and used for ground based spectral 
measurements as well as for vegetation and soil measurements. The vegetation is basically composed of grass 
and bushes in variable proportions as shown in the table below: 
sites 
dry 
biomass 
(g/m2) 
% shrub 
biomass 
litter 
(g/m2) 
vegetation 
cover 
(%) 
rock surface 
cover 
(%) 
MF1 
228.7 
99 
96.8 
28 
46 
MF2 
289.9 
72 
94.8 
51 
48 
MF3 
228.0 
89 
32.3 
42 
45 
MF4 
224.8 
24 
48.6 
62 
59 
MF5 
102.3 
33 
57.4 
44 
54 
MF6 
458.6 
92 
57.1 
38 
52 
MF7 
140.2 
67 
155.2 
32 
10 
MF8 
1033.0 
99 
95.8 
40 
58 
Table 1: mean surface characteristics of the eight metflux sites 
Systematic gravimetric surface soil moisture samples (0-5 cm) were collected throughout the 
monsoon season at four representative metflux sites (1,3,5,6) with some additional sites depending on satellite 
overpasses and continuous TDR and tensiometric cubes measurements at Kendall and Lucky Hills. Volumetric 
soil moisture were then deduced from bulk density measurements. This volumetric soil moisture will be later 
expressed in percent soil moisture (% sm). 
2.2. ERS-1 images 
Five ERS-1 scenes were obtained during the experiment around 18h00 UT (llhOO local): days of year (DOY) 
135, 170, 240, 275, 310 (35 days repeat orbit) covering only a west part of the watershed (MF 1,2,3,7,8). The 
characteristics of this spacebome Synthetic Aperture Radar (SAR) are 5.3 GHz (C band), W polarization, 23° 
incidence and 2.5 dB radiometric resolution at -18 dB. We used the standard PRI products provided by the 
European Space Agency (ESA) that is amplitude images processed as 3-looks (reduced noise) ground range 
(equal size pixels of 12.5x12.5 m) calibrated digital images. 
The images were georeferenced using 1:5000 scale orthophotos of the US Department of 
Agriculture-Agricultural Research Service. Because of the speckle effect, it was sometimes difficult to define 
enough ground control points, or to locate them, to perform accurate geometrical rectification. However final 
transformation RMS error was around 2 pixels (i.e. 25 m) which was quite satisfactory to locate the MF targets. 
A multitemporal SAR image has thus been produced, showing contrasted pattern in terms of dry to wet 
conditions evolution. Backscattering coefficients o° were then extracted on each of the images for all the 
150x90m targets using the formulation below: 
a°(dB) = 10 log <DN 2 > -K + p (1) 
P= 10 log Sm (2) 
sm a nf