Agriculture Reference
In-Depth Information
6.1
Basics of Sensing by Visible and Infrared Reflectance
Crop properties often vary even more than soil properties do. This is, because they
depend not only on the respective soils but on many additional factors as well such
as microclimate, species, variety, growth stage, farming operations, nutrient supply,
weed competitions and pest infestations. It certainly is useful to know, which factors
are responsible for differences in crop development. Yet before such questions can
be pursued, temporal and spatial differences in crop growth must be identified. And
for doing this on a site-specific basis with a sufficiently high resolution - similar to
the situation with soil properties - sensing via electromagnetic radiation offers chal-
lenging opportunities.
There exist fundamental differences in the
temporal variation
or stability of
crop properties compared to most soil properties. The most important soil proper-
ties such as texture, organic matter content and cation-exchange-capacity often vary
spatially, yet remain rather constant during a growing period. The sole exception to
this is the water content of soils. But the main crop characteristics - such as bio-
mass, structure of plants and ingredients - change steadily while a crop develops.
Consequently for crop properties, the respective time of sensing is very important as
it is for soil water.
Sensing of crop properties by means of electromagnetic radiation predominantly
relies on
reflectance
. Theoretically, it could be based on transmittance or even
absorbance as well. However, both the transmitted- and the absorbed radiation can-
not easily or not at all be recorded on-the-go via proximal sensing by farm vehicles
or via remote sensing from aerial platforms or satellites.
But it should be realized that the reflectance itself depends heavily on the absor-
bance as well as on the transmittance. This is because irradiance (= incident radia-
tion) that is absorbed or transmitted is not available any more for reflection.
From visible radiation that hits healthy and growing plants, the dominant part is
absorbed for
photosynthesis
(Fig.
6.1
, left). Consequently, both the reflectance and
the transmittance are rather small in this part of the spectrum. However, there are
differences within the visible region. The reflectance is smaller in the blue and in the
red part than in the green range. Because the blue and especially the red light are
better suited for photosynthesis. The higher reflectance in the green range causes the
color of growing crops.
When dealing with crops, the
near-infrared range
from 700 to 2,500 nm is
subdivided. Because the lower part of this range - 700 to 1,300 nm - has an extremely
low absorbance and consequently a very high reflectance and transmittance. And
contrary to this, the upper part of this range has regions of very high absorbance by
water and thus low reflectance and low transmittance (Figs.
6.1
and
6.2
). Therefore
with crops only the lower part from 700 to 1,300 nm is denoted as near-infrared,
whereas the upper part from 1,300 to 2,500 nm is named
shortwave-infrared
. The
confusing. The respective wavelength range must be noted.
