Agriculture Reference
In-Depth Information
When regarding the general course of the spectra that are reflected from soil or
from vegetation, some distinct differences show up (Fig.
6.2
). The reflectance from
soils increases rather steadily and uniformly with the wavelength in the visible and
near-infrared range. Contrary to this, the reflectance of plants in the visible regions
is defined by the absorbance for photosynthesis and hence is below that of soil. Yet
within the near-infrared range, the absence of absorbance for photosynthesis allows
a multiplication of the reflectance from crops. The result of this is a steep rise of the
reflectance in the transition zone from the visible to the near-infrared radiation. This
steep rise in the reflectance is generally known as the “
red edge
” because it is
located between the visible red- and the near-infrared radiation.
However, the curves of the spectra in Fig.
6.2
represent
average situations
just
in order to show the principal differences in reflectance between soil and vegetation.
In detail, the reflectance spectra of soils differ according to texture, organic matter
ingredients and chemical composition. The sensing possibilities by means of reflec-
tance rely on these differences.
The most important differences for vegetation result from biomass and chloro-
phyll. The
biomass
always has been a very significant criterion for defining the
development of a crop. For forage crops, the total aboveground plant mass is uti-
lized, hence the biomass in t per ha as well as its ingredients are of interest. With
grain crops, the main objective is not the total aboveground biomass but mainly the
reproductive part of it. Yet for all crops, the leaf area is important since photosynthe-
sis takes place in the leaves. Consequently the
leaf-area-index
of crops is a signifi-
cant criterion in order to assess the development. It is defined as the relation between
the photo-chemically active, one-sided leaf area and the ground surface. In a simpli-
fied way, the leaf-area-index shows how much “factory space” for photosynthesis a
crop supplies. Depending on the development stage, species, variety and growing
conditions, the leaf-area-index can vary widely. Starting with 0 (zero) before emer-
gence, it can go up to 8 (eight) for well developed, lush grain crops before the ripen-
ing process begins.
Within the leaves,
chlorophyll
is the essential driver of photosynthesis and hence
of plant production. Hence information about the leaf-area-index must be supple-
mented by data about the chlorophyll concentration within the leaves, which can be
defined by the mass of chlorophyll per unit of leaf area.
So knowledge about the separate effects of chlorophyll on the one hand and the
leaf-area-index on the other hand on reflectance spectra is helpful. These effects can
be shown separately by using simulation models that have been developed by
Verhoef (
1984
) and Jacquemoud and Baret (
1990
). Looking at the results from these
simulation models (Fig.
6.3
), it is obvious that the effect of the chlorophyll per unit
leaf area is restricted to wavelengths below the red edge. Above this edge, the chlo-
rophyll per unit leaf area does not affect the reflectance. But in the visible region,
the reflectance is the lower, the higher the chlorophyll concentration per unit leaf
area is.
Contrary to this, the effect of the leaf-area-index mainly is above the red edge
inflection point in the near-infrared range. The reflectance here increases very
