During periods of drought conditions, physiological changes within vegetation may become apparent. Satellite sensors are capable of discerning many such changes through spectral radiance measurements and manipulation of this information into vegetation indices, which are sensitive to the rate of plant growth as well as to the amount of growth. Such indices are also sensitive to the changes in vegetation affected by moisture stress (Das, 2000).
The visible and near infrared (IR) bands on the satellite multispectral sensors allow monitoring of the greenness of vegetation. Stressed vegetation is less reflective in the near IR channel than nonstressed vegetation and also absorbs less energy in the visible band. Thus the discrimination between moisture stressed and normal crops in these wavelengths is more suitable for monitoring the impact of drought on vegetation.
The NDVI varies with the magnitude of green foliage (green leaf area index, green biomass, or percentage green foliage ground cover) brought about phenological changes or environmental stresses. The temporal pattern of NDVI is useful in diagnosing vegetation conditions
Moisture stress in vegetation, resulting from prolonged rainfall deficiency is reflected by lower NDVI values. Such a decrease could also be caused by other stresses, such as pest/disease infestation, nutrient deficiency, or soil geochemical effects. Discrimination of moisture stress from other effects does not present a problem in coarse resolution data over large areal units, as neither pest/disease attack nor nutrient stress is selected in terms of area or crop type.
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