In an agricultural setting vegetation indices should accurately track the state of crop growth under conditions in which the radiometer field of view includes both active vegetative, senesced, and non-vegetative components. Ideally, they should also do so regardless of variation in viewing and background conditions. The authors field evaluate 5 vegetation indices based on their ability to respond linearly to two distinct rates of crop growth. The indices resistance to moisture related and site-to-site variation in soil reflectance were also evaluated, but those results are not discussed in detail. Thus of the general criteria outlined above only resistance to variation in viewing conditions are not considered, as no attempt has been made to test the effects of solar or viewing angle, or resistance to atmospheric distortion. Index evaluation here is based on the response to the development of fractional vegetated area (FVA) and Leaf Area Index (LAI) measured over cotton (Gossypium hirsutum L.) plots grown under both irrigated and dryland conditions. Their emphasis on FVA and LAI is based on the central role of leaf area in both crop growth and remote sensing. LAI has traditionally been cited as a key property influencing a canopy's cross-sectional area to incident photosynthetically active radiation (PAR), and thus its ability to assimilate light energy into biomass. During the remote sensing era LAI has been widely considered as the biophysical variable linking canopy reflectances and vegetation index (VI) values to absorbed PAR (APAR) and agricultural yield. Many studies have compared LAI variation with that of the Normalized Difference Vegetation Index (NDVI).
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