Agriculture Reference
In-Depth Information
small grain crop can attain a leaf-area-index of 8. But in areas where the climate
does not allow for lush crops, the use of the NDVI may be a satisfactory approach.
When instead of the NDVI the red edge inflection point was used, this saturation
effect did not show up (Fig.
6.7
). Furthermore, the correlation (r
2
) along the whole
range of leaf-area-indices was better.
It should be noted that the results of proximal recording with a height of 1.5 m
above the ground on the one hand and of sensing from a satellite are very similar
(Fig.
6.7
, left and right). However, these results were obtained in an arid climate
with normally clear skies. In areas with humid climate, such results often cannot be
expected because of limitations in the radiation transfer.
6.2.1
Precision in Sensing of Chlorophyll
The higher the chlorophyll content of the leaves, the smaller their reflectance is
(Fig.
6.3
, bottom). Therefore, if for a particular wavelength λ instead of the straight
reflectance R
λ
, its inversion or its reciprocal is used, a more convenient relation is
obtained. Because this inversion (R
λ
)
−1
just rises and falls as the chlorophyll content
does.
Indicating the chlorophyll effect can be further improved if any interfering with
the leaf-area-index is taken care of. Because tendentially, a high leaf-area-index
affects the reflectance in the visible range in a similar way as a high chlorophyll
content within leaves. However, the result of the leaf- area-index on the spectrum
mainly shows up in variations of the near-infrared reflectance (Fig.
6.3
, top).
Consequently, a further logical step is to subtract the inverted reflection of this near-
infrared range - abbreviated (R
NIR
)
−1
- from the inversion of the respective reflection
in the visible range. The thus created difference (R
λ
)
−1
− (R
NIR
)
−1
supplies an optical
index that is linearly proportional to the chlorophyll content in spectral bands of the
green and red edge range (Gitelson et al.
2003
).
It is finally suggested to correct for variations in the leaf structure of crops
(Gitelson et al.
2003
). These variations are taken care of by multiplying the differ-
ence (R
λ
)
−1
− (R
NIR
)
−1
with the non inverted reflectance in the near-infrared range
R
NIR
.
This product can be simplified:
−
(
)
−
1
−
1
{(
R
)
R
}
×
RRR
=
(
/
)
−
1
λ
NIR
NIR
NIR
λ
So from these logical deductions, there resulted three indices for estimating the
chlorophyll effect, namely
• the simple
inverted reflectance:
:
(R
λ
)
−
1
• the
difference of the inversions
:
(R
λ
)
−
1
−
(R
NIR
)
−
1
• the
reflectance ratio minus one
:
(R
NIR
/R
λ
)
−
1
.
The ability of these three indices to estimate the chlorophyll in leaves is shown
in Fig.
6.8
by means of their coefficients of determination (r
2
) from experiments in
