Agriculture Reference
In-Depth Information
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of surface roughness variations of real surfaces may be one of the reasons
for this failure. Surface roughness is usually expressed in terms of the root
mean square (r.m.s.) of height or, in the case of the more complex models,
in terms of the autocorrelation function of the surface height variations. It
appears that agricultural and other natural soil surfaces are relatively com-
plex and often have a multiscale structure. That is why simple parameters
such as the r.m.s. of height fail to adequately describe radar backscatter
(Davidson et al., 2000). One way of working around this problem is to use
change detection techniques that only interpret changes in soil moisture
under stable surface roughness conditions.
Vegetation Crop type, crop phenology, and crop condition influence the
architecture and dielectric constant of the canopy and thus radar backscat-
ter. There are numerous studies related to crop discrimination using radar
(Bouman and Uenk, 1992; Brisco et al., 1992; Ban and Howarth, 1999;
Treitz et al., 2000). The ability to discriminate crop type and phenological
stage with radars may be attributed to changes in both canopy geometry
an d plant biophysical parameters. The radar frequency and incident angle
influence the relationship.
Radar backscatter from an agricultural field is normally not influenced
just by the plants, but also by the underlying soil and by soil-plant in-
teractions. As indicated in figure 8.1, there are three main contributions
from a vegetation canopy: direct backscatter from the vegetation elements,
multiple scattering from plants and soil, and the contribution from the soil
surface. Accordingly, total canopy backscatter, σ 0 , is formulated as the sum
of three parts:
[110
Line
——
0.1
——
Norm
PgEn
[110
σ 0
= σ veg +
a t σ soil + σ int
[8.1]
Figure 8.1 The interaction of radar with an agricultural target results in backscatter (1)
directly from the crop, (2) from a combination of crop and soil, and (3) directly from the
so il [from Brisco and Brown, 1998].
 
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