Agriculture Reference
In-Depth Information
illumination, sensing distance - does not supply knowledge about the basic effects
of single parameters. This knowledge can be derived from investigations that allow
a system analysis, in which the effect of varying factors can be controlled.
The graphs in Figs.
6.16
and
6.17
have been obtained by using modern canopy
reflectance models (Clevers et al.
2010
). These models offer the advantage that the
results are independent of site and plant species and thus fairly universally valid. For
details to these models see Jacquemoud and Baret (
1990
) as well as Verhoef (
1984
).
The graphs have been supplemented with data to remote sensing preferences.
Across the whole near-infrared and shortwave-infrared region, a decrease of
the water content causes a rise in the reflectance (Fig.
6.16
). This is important,
particularly when crops grow older. Small differences in the water content due
to short term weather variations in early growth stages are more difficult to
detect. However, the visual inspection of the course of the reflectance curves
does not provide reliable information about the best wavelengths for sensing.
This knowledge can be obtained from calculations of the respective correla-
tions. Details to this cannot be dealt with here, yet important results are indi-
cated in Figs
6.16
and
6.17
.
Accordingly, good spectral information about canopy water content can be
obtained from features centered at either
970
or
1,200 nm wavelength
. These wave-
lengths are approximately at the bottom of the two dips within the near-infrared
spectrum. For best results, either first derivatives or simple
wavelength ratios
from
the slopes of the spectrum just adjacent to these wavelength centers should be used.
Recommended ranges for the derivatives or the simple ratios are either the left or
alternatively the right slope from the 970 nm centre and also the left slope from the
1,200 nm center.
The green framed columns in Fig.
6.17
, bottom, show the recommended slope
ranges. Some of the simple ratios in Table
6.1
correspond closely to these ranges.
For
sensing from satellites
, absorption of radiation due to water vapor in the
atmosphere within some wavelength ranges needs attention. Because sensing in
these regions results in very noisy or inaccurate signals, they should be avoided.
Within the short- wave infrared region (1,300-2,500 nm), there are two ranges that
are affected. In Fig.
6.16
, these ranges are indicated by red columns. It is not acci-
dentally that these ranges are also close to dips within the spectral curves. Because the
dips are the result of high spectral absorption by liquid water in the canopy. And
the red columns indicate high absorption, yet by water vapor. Precisely seen, however,
the centers of
water vapor
absorption are shifted approximately 50 nm to shorter
wavelengths when compared to the dips in the curves for the
liquid water
in the
canopy (Clevers et al.
2008
,
2010
).
In the near-infrared region, there are only two very thin bands of atmospheric
absorption by water vapor. These narrow bands of absorption by water vapor - that
too are radiation barriers - are located rather closely to the recommended slope
ranges for canopy water sensing (see red vertical lines in Fig.
6.17
). The centers of
these bands in the near-infrared - which should be avoided - are at 940 and 1,140 nm
(Gao and Goetz
1990
). Thus “hyperspectral precision” in selecting the wavelengths
for remote canopy water sensing is needed.