1 Spatial Modelling of the Terrestrial Environment - Georeferencial

68 Spatial Modelling of the Terrestrial Environment
look-angle (degs)
look-angle (degs)
Figure 4.3 Opacity coefficient derived for the soyabean canopies in (a) Figure 4.1c and (b)
Figure 4.1d as a function of look angle and polarization
found at look-angle 10 ◦ (0.36 for drying period 2, and 0.43 for drying period 3) fall within
the range of those found by Burke et al. (1999) using the simple model for the same dataset,
i.e., 0.31–0.39 for drying period 2, and 0.44–0.52 for drying period 3. They are also similar
to those found by Burke et al. (1999) using the discrete model of Wigneron et al. (1993),
i.e., 0.36 and 0.49. However, Figure 4.3 also shows that the opacity coefficient is a distinct
function of both look angle and polarization. Moreover, the opacity coefficient is greater
for drying period 3 when the canopy is denser, implying that the opacity coefficient might
also be a function of vegetation water content, as suggested by Le Vine and Karam (1996)
and Wigneron et al. (1996; 2000). Wigneron (2002, personal communication) also found a
dependence of the optical depth on look angle using the discrete model of Ferrazzoli et al.
(2000).
To quantify the errors introduced by assumptions about the vegetation made in soil
moisture retrieval algorithms, field experiments are required that focus on the dependence
of the opacity coefficient on look-angle and polarization, canopy size and type and ambient
temperature. Quantifying these dependences will provide useful information relevant to a
potential soil moisture product, in particular, on its reliability when used in data assimilation
systems. Burke et al. (2003) quantified the errors in the distributed optical depth
estimated using a distributed measure of NDVI and the relationship shown in Figure 4.2,