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Vegetation Indices

Color of healthy vegetation appears green because it reflects most of the green light and absorbs most of the red and blue channel of Electromagnetic spectrum.  This reflectance of green light has been found to proportional to the chlorophyll content of the vegetation. Healthy vegetation contains large quantities of chlorophyll.

 

In referring to healthy crops, reflectance in the blue and red parts of the spectrum is low since chlorophyll absorbs this energy. In contrast, reflectance in the green and near-infrared spectral regions is high. Stressed or damaged crops experience a decrease in chlorophyll content and changes to the internal leaf structure. The reduction in chlorophyll content results in a decrease in reflectance in the green region and internal leaf damage results in a decrease in near-infrared reflectance. These reductions in green and infrared reflectance provide early detection of crop stress. Examining the ratio of reflected infrared to red wavelengths is an excellent measure of vegetation health. This is the premise behind some vegetation indices, such as the Normalized Differential Vegetation Index (NDVI).

Healthy plants have a high NDVI value because of their high reflectance of infrared light, and relatively low reflectance of red light. Phenology and vigour are the main factors in affecting NDVI. An excellent example is the difference between irrigated crops and non-irrigated land. The irrigated crops appear bright green in a real-colour simulated image. The darker areas are dry rangeland with minimal vegetation. In a CIR (colour infrared simulated) image, where infrared reflectance is displayed in red, the healthy vegetation appears bright red, while the rangeland remains quite low in reflectance.

This vegetation index is a ratio based VI calculated by the difference of the infrared and red bands as ratio to their sum:

Where:

NDVI = Normalised Difference Vegetation Index
IR = Infrared band of the electromagnetic spectrum
R = Red band of the electromagnetic spectrum.

NDVI values vary between -1 to +1. NDVI is extensively used in assessment of vegetation health world-over. The NDVI Map is used for scouting and the creation of management zones. This long established vegetation index works well under intermediate conditions when plant growth is not too abundant. However, during the early stages of the crop, when the green leaf area is still small, it is highly sensitive to soil background effects induced by tillage, crop residues, or moisture. It also tends to saturate once the heavy crops have reached canopy closure.

In addition to NDVI there are many other indices that have developed by the scientists. Some of these are:

Vegetation Condition Index (VCI)
Leaf area index (LAI)
General Yield Unified Reference Index (GYURI)
Temperature crop index (TCI)
Enhanced Vegetation Index(EVI)

Date Requirements:

Vegetation indices  are available as frequently as possible for a given area. Therefore sensors capable of acquiring high temporal resolution data are preferred. SATPALDA provides data from SPOT 2-5, DMC Constellation and SPOT 6. These are sensors specifically tailored to covered large area in a single scene.

The Normalized Difference Vegetation Index (NDVI) shows the distribution of vegetation across Central Africa. The most densely vegetated areas are dark green, and regions with little or no vegetation are gold.

Image Credit: NASA and MODIS teams.