Tal Svoray 
  
GVD are the green leaf volumetric densities of the three vegetation formations as formulated in the study of Svoray et 
al. (2000). The use of the green leaf biomass volumetric density rather than the green leaf absolute biomass provides a 
better representation of the backscatter contributors in the different vegetation canopies. Green leaf biomass [kg m?] 
values of the GVD were received from the above mentioned field campaign. The soil moisture coefficients and the 
local incidence angle were considered as described in the general form of the water cloud model. The mixed proposed 
model can be more clearly illustrated and explained using figure 2. 
The difficulties involved with modeling the backscatter 
from a mixed pixel of herbaceous and woody formations 
are mainly related to different scattering mechanisms of the 
two canopies. Due to the low penetration depth at C-band, 
the interaction of the radar beam with the surface includes 
the soil moisture effect in the case of herbaceous vegetation 
but involves only the uppermost layer of the canopy in the 
case of the woody formations. Thus, contribution of soil 
moisture to backscatter of a mixed pixel is only from the 
parts of the pixel that are covered with herbaceous 
vegetation. The other parts of the pixel, covered with 
woody formations, contribute only from the upper part of 
the canopy. The solution provided by us is to include the 
soil contribution only in the areas covered by the  Figure2. An Illustration of backscatter from a 
herbaceous vegetation. mixed pixel. 
  
4 RESULTS AND DISCUSSION 
This section describes the linear relationship that were found between ERS-2 SAR backscatter and volumetric soil 
moisture in the study area; multi-temporal ERS-2 SAR backscatter curves of the dominant vegetation formations in 
the study area; and finally results of applying the water cloud model in homogenous and heterogeneous plots are 
discussed and the predicted values of the inversion models are being assessed. 
Examination of the relationship between volumetric soil moisture and ERS-2 SAR backscatter have shown a strong 
linear correlation between volumetric soil moisture and ERS-2 SAR backscatter. Statistical tests have proven 
(0=0.05) that this relationship did not vary along the climatic gradient. This similarity between the linear soil moisture 
models of a humid and arid sites and other models that were applied to different environments around the world, have 
lead us to conclude that a unified model (figure 3) could be applied for the entire climatic gradient region. 
The results in the unified model illustrate high level 
of scattering in points at the lowest levels of 
volumetric soil moisture (between 0% to 10%) and 
at the highest levels of moisture (between 25% to 
40%). The points in between these ranges are 
characterized by much more closeness to the 
regression line. This result may imply on the 
sensitivity of the ERS-2 SAR backscatter and the 
linear soil moisture model to changes in the soil 
moisture concentrations. Thus, although the results 
in the upper and lower parts of the plot are 
reasonable and the total coefficient of determination 
Backscatter [dB] 
is high (0.9), it appears that the best range to be 0 M 20 an 
represented by the soil moisture model is the Volumetric Soil Moisture [%] 
between 10% to 25% of volumetric soil moisture 
concentration. This range, although is relatively Figure 3. The relationship between the ERS-2 
short, is very meaningful for the analysis of water SAR backscatter and the volumetric soil 
Stress in semi-arid areas. moisture along the climatic gradient. 
A study of ERS-2 SAR backscatter from canopies of the dominant vegetation formations in the study area (trees, 
shrubs, dwarf shrubs and herbaceous vegetation) prove a clear difference between the multi-temporal curves of 
backscatter from woody formations (trees, shrubs and dwarf shrubs) and the multi-temporal curves of herbaceous 
vegetation (grasses and wheat) canopies. In general, the woody formations provide relatively stable multi-temporal 
  
324 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 
  
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