Agriculture Reference
In-Depth Information
2.1 Vegetation Indices (VI)
As already mentioned, remote sensing methods provide valuable tools for crop
canopy assessment and these tools will provide improved information for agricul-
ture applications. The main advantage of remote sensing is the ability of repeated
measurements over the time without damaging the vegetation. Therefore, in remote
sensing methods, various VIs are used to generate important information to enhance
assessment of different agronomic parameters. However, the purpose of vegetation
indices is to enhance the vegetation signal while minimizing the solar irradiance
and soil background effects.
The ratio of the near-infrared (NIR)/Red was the first VI which was proposed
by Jordan in 1969. He relates this index to Leaf Area Index (LAI) of plants. As
mentioned earlier, NDVI is the most commonly used VI which is calculated as the
difference between near infrared and red reflection normalized by the sum of the
two
(
IR
−
R
)/(
IR
+
R
)
]
.
[NDVI
=
Basically, the wavelength region from 750 to 850 nm is considered as near-infrared
(NIR) and 625-675 nm range is considered as red(R) (Sellers
1985
; Tucker and
Sellers
1986
). However, major problem in vegetation studies is reflectance from
soil; hence Soil-Adjusted Vegetative Index (SAVI) was described by AR Huete in
1988.
SAVI
=
(R
NIR
−
R
red
) (1
+
L)
/
(R
NIR
+
R
red
+
L)
Adjusted parameter L is typically 0.5. However, this index was further advanced to
derive an Enhanced Vegetative Index (EVI).
EVI
=
2
.
5(R
NIR
−
R
red
)
/
(R
NIR
−
7
.
5R
blue
+
6R
red
+
1)
Enhanced vegetative index uses reflectance in blue region of the spectrum. Accord-
ing to Hatfield et al. (
2008
), this is important to differentiate soil from vegetation at
low amount of ground cover. Among the various VIs, assessment of the chlorophyll
content of crop canopies has included the Normalized Pigment Chlorophyll Ratio
Index described in Merzlyak et al. in
1999
and it was defined as (Red 660 - Blue
460)/(Red 660 + Blue 460). Apart from those indices, Plant Senescence Reflectance
Index was proposed as being sensitive to the senescence phase of plant develop-
ment. Renormalized Difference Vegetative Index (RDVI), Modified Triangular
Vegetation Index (MTVI) and Optimized Soil-Adjusted Vegetation Index (OSAVI)
are also use to study crop yield.
As mentioned earlier, yield is a multivariate function; factors like different soil
types, agricultural inputs and adoption of improved technology affect it, since
the spectral reflectance is a manifestation of all important factors affecting the
crop. Therefore, stratification of crop area on the basis of crop vigour as reflected
