Agriculture Reference
In-Depth Information
The question may arise why in the main absorption regions - the blue and
especially in the red range - the chlorophyll prediction is so poor (Fig. 6.8 ). The
explanation for this just is the high absorption of the incoming light in these ranges.
As a result of the very high absorption in these ranges, the depth of light penetra-
tion into the leaf or leaves is rather low. Because even low amounts of chlorophyll
suffice to saturate the absorption. And when saturation is attained, a further increase
in pigment content influences neither absorbance nor reflectance. In the green and
especially in the red edge region, the absorption of the light by chlorophyll is much
lower. Therefore the light penetrates deeper (Ciganda et al. 2012 ) and the sensitivity
of absorbance as well as reflectance to the chlorophyll content of the canopy volume
is much higher.
This context also explains to a large extent, why the normalized difference
vegetation index (NDVI) saturates when it is used for sensing the leaf-area-index
(Fig. 6.7 ). Because the NDVI too relies on red reflectance. Logically the same dis-
advantage applies for simple ratios of near-infrared to red radiation.
6.3
Sensing Yield Potential of Crops by Reflectance
A key criterion is the crop productivity potential per unit of field area. If this poten-
tial can be sensed during the growing season on a site-specific basis, it can supply
information for an adequate control of fertilizing, plant protection and irrigation
operations.
The chlorophyll content per unit area of the leaves alone cannot serve as an
indicator of crop productivity potential because it ignores the leaf area that is avail-
able. Likewise, the leaf-area-index alone is not sufficient since it does not provide
information about the chlorophyll that is involved. However, it seems reasonable to
create the product of the leaf-area-index and the chlorophyll mass per unit leaf
area. This product is the chlorophyll mass per unit of field area. It is commonly
named chlorophyll index or canopy chlorophyll and can be regarded as a key
criterion for estimating the productivity of crops.
This key position of the canopy chlorophyll for estimating the productivity of
crops results from the fact that it controls the photosynthetic process. And following
the original logic of Monteith ( 1972 ) it can be deduced that the gross primary pro-
ductivity (GPP) of crops is linearly related to the amount of photosynthetically
active radiation that is absorbed. It should be noted that this gross primary produc-
tivity (GPP) takes into account all plant parts plus roots as well as respiration energy.
So it always overestimates the yield that farmers are interested in. However, this
should not exclude the use of the GPP as a relative measure for the assessment of
the site-specific situation regarding productivity.
The hypothesis is that a connection between reflectance sensing, absorbed radia-
tion and finally the crop productivity exists. The relation between reflectance and
absorbance is evident when regarding the fact that these two quantities together with
the transmittance add up to 100, and that the transmittance in the visible range
hardly counts within a well grown crop (Fig. 6.1 , left).
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