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Remote sensing for resources development and environmental management
Damen, M. C. J.

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— 0 -
Figure 2: Schematic presentation of a simplified
reflectance model for vegetation and soil combined.
Figure 1: Schematic presentation to illustrate aspects
of the new definition of soil cover.
C = covered soil; N = soil not covered or illuminated,
a: Soil illuminated by the sun.
b: Soil visible to a sensor,
c: Illuminated soil visible to this sensor.
illuminated by the sun as well as directly detectable
by the sensor) will be classified as the fraction of
soil that is not covered (figure lc). The complemen
tary fraction will now be defined as soil cover
("apparent soil cover"). In the special situation of
the sensor looking vertically downwards, this
definition of soil cover is equivalent to the relative
vertical projection of green vegetation, the relative
area of the shadows included.
In order to ascertain whether there is a useful
relationship between infrared reflectance and LAI for
green vegetation, the former should be corrected for
soil background, because it may influence infrared
reflectance independently of the LAI. The infrared
reflectance is then calculated for the situation of
the visible background being completely black and
not reflecting any radiation. This corrected infrared
reflectance value is then used to estimate LAI.
Let us consider the simple situation of a surface,
partly covered with green vegetation and partly bare
(figure 2). The fraction of the surface covered with
vegetation is called soil cover, B. If the reflectance
of the soil is called r and the reflectance of the
vegetation r , then the total measured reflectance,
r, will equaY:
r = r v . B + r s . (1-B) (1)
A green band will be denoted by the subscript g
and equation (1) is then written as:
r =
s, g
( 2)
The reflectance of vegetation in a green band (r )
may be regarded as being independent of the numbed
of leaf layers, because leaf transmittance in the
green is assumed to be negligible. Hence equation (2)
describes the linear relationship between the
reflectance in a green band and soil sover, if the
soil reflectance can be considered to be constant
(constant soil moisture content).
Analogously, by attaching the subscript r to a red
band reflectance, we have:
B + r
( 3)
Because r can also be regarded as being independent
of the number of leaf layers, this equation describes
the linear relationship between red reflectance and
soil cover.
For an infrared band the subscript ir will be used
and equation (1) is then written as:
r . . B + r
v,ir s,ir
( 4)
In this equation r . is not independent of tne number
of leaf layers, so V ife r may not be regarded as a con
stant. This means that the reflectance measured in
an infrared band (r. ) is not a linear function of
• i lr
soil cover.
For estimation of LAI the corrected reflectance r'
could be used. It is (according to equation 1) defined
r' = r - r . (1-B) = r . B (5)
s v
The corrected reflectance is the reflectance one would
have obtained with a black background.
In order to obtain the corrected infrared reflectance
equation (5) first has to be applied to the infrared
r !
s, ir
( 6)
B can be ascertained by means of equation (2) or (3) .
However, the reflectance of bare soil and of vegeta
tion in a green or red band should be known. Although
it is quite often possible to ascertain a good estimate
for the reflectance of vegetation (complete cover),
estimating the reflectance of bare soil poses greater
difficulties. The reflectance of a soil may change
very rapidly, according to soil moisture content.
Also, very large local differences in soil moisture
content may occur. At low soil cover this may cause
large inaccuracy if neither the soil moisture content
nor the actual reflectance of the soil are known. To
obtain an accurate estimate of LAI one either has to
know or to measure the reflectance of the bare soil,
or one has to derive a relation that is less dependent
on differences in soil moisture content.
For many soil types, reflectance in the different
spectral bands does not differ very much (e.g. Condit,
1970); often there is a slight increase in reflectance
with increasing wavelength. However, often the ratio
of the reflectance in two spectral bands is independent
of the soil moisture content: