Agriculture Reference
In-Depth Information
are capable of producing primary energy through the process of photosynthesis.
However, a fraction of the total production or Gross Primary Production (GPP) is
utilized by the primary producer to meet their respiratory demand and balance is
Net primary production (NPP), which accumulates as plant biomass (Odum
1960
;
Whittaker and Likens
1975
). NPP is one of the most significant characteristics of
ecosystems. However, estimation of NPP in an ecosystem is one of most confusing
assignment because it depends on multiple variables such as light, temperature,
soil moisture, plant nutrients, plant characteristics, etc. However, in agriculture not
only NPP but mostly the agronomic yield, the fraction of NPP which assimilates in
harvestable components such as seeds, tubers, leaves, is important.
Though it is simple in concept, in terrestrial ecosystems accurate estimation of
NPP is difficult. Measurement based on gas exchange, sampling biomass or carbon
flux methods have been devised to measure NPP (Milner and Hughe
1968
). How-
ever, remote sensing of vegetation indices, such as normalized difference vegetation
index (NDVI), promises to provide a means for frequent, non-destructive measure-
ment of NPP at a landscape scale (Tucker and Sellers
1986
; Wang
2004
).
2 Remote Sensing
With remote sensing technology, vegetation can be inferred through its reflectance
signals which are within the visibility and are near-infrared (NIR). In general, green
vegetation canopies have a unique feature of reflectance spectra among the green,
red, and NIR bands (Fig.
4.1
). Green reflectance peak due to chlorophyll, causes
peak reflectance in the green band and reflection valleys in the red band. However,
leaf tissue gives maximum reflection in NIR band. Hence, this range of reflec-
tion spectrum explains the reflection and absorption of green and red bands by the
vegetation and the variation can be used as a tool for studying distribution, health
and productivity of plants (Buschmann and Nagel
1993
). Basically, the shift of the
reflection spectrum from the far red towards the near-infrared (red edge) is taken
as an indicator of stress or damage to plants (Gates et al.
1965
; Chang and Collins
1983
). However, the spectral characteristics of a plant canopy largely depend on
the composite spectral response of leaves and soil background (Richardson and
Wiegand
1977
).
As mentioned elsewhere, remote sensing is a widely used tool in studying veg-
etation. In 1972, Monteith pioneered the concept of calculating NPP based on av-
erage photosynthetically active radiation (APAR) and this production efficiency
model was tested by using satellite-based and ground data. First global level NPP
model was proposed by Hemimann and Keeling in 1989. However, a majority of the
satellite-based yield prediction studies are built on correlating yield to various crop
parameters like leaf area index, wet biomass, light use efficiency, etc., which are re-
trieved using various vegetation indices like the Normalized Difference Vegetation
Index (NDVI) (Groten
1993
).
