Agriculture Reference
In-Depth Information
1.0
740
1.0
740
0.8
0.8
730
730
0.6
0.6
720
720
0.4
0.4
proximal sensing
satellite sensing
NDVI: r
2
= 0.44
REIP: r
2
= 0.62
710
710
NDVI: r
2
= 0.45
REIP: r
2
= 0.61
0.2
0.2
crop: wheat
crop: wheat
site: Negev, Israel
site: Negev, Israel
700
0.0
0.0
700
012345 678
012345 678
leaf area index, dimensionless
leaf area index, dimensionless
Fig. 6.7
Sensing the leaf-area-index by using the normalized difference vegetation index (NDVI)
or the red edge inflection point (From Herrmann et al.
2010
, altered)
point from reflectances at just three or four wavelengths in the red edge region have
been developed (Dash and Curran
2007
; Guyot et al.
1988
; Herrmann et al.
2010
).
Details to these reflectance indices used for approximating the position of red edge
inflection point are in the literature cited.
The prime crop property that farmers are interested in is the plant productivity per
unit of field area. For this, valuable control signals can be expected from crops itself,
since its properties reflect the environment and human activities. Important is the time
span between the recording of crop properties and respective control operations. Signals
that are derived from real yield monitoring at harvest time imply long time spans and
represent conditions that existed within the whole preceding growing season.
In case the signals are from the still growing crop that is to be controlled, the
question is, which plant properties deliver suitable data. Prime candidates for such
a control system that is oriented at the yield and takes place within the growing
season are the
leaf-area-index
and the
chlorophyll concentration
.
The leaf-area-index as well as the chlorophyll concentration within the leaves
can be estimated by means of reflectance sensing. When doing this for lush crops, it
is important to use a reflectance index that can differentiate between different
leaf-area-indices at high levels. The normalized difference vegetation index gener-
ally is not able to do this. As can be seen in Fig.
6.7
, the estimation by this reflec-
tance index is admittedly very distinct at the start of the growing period, but then
flattens out when the leaf-area-index exceeds the value 2. The
saturation
of this
present day standard vegetation index at about this crop growth stage has been
observed at several places (Serrano et al.
2000
; Mistele et al.
2004
; Sticksel et al.
2004
). This limitation is serious when taking into account that a well developed
