ON THE RELATION BETWEEN NDVI, FRACTIONAL VEGETATION COVER, AND LEAF-AREA INDEX

We use a simple radiative transfer model with vegetation, soil, and at mospheric components to illustrate how the normalized difference veget ation index (NDVI), leaf are index (LAI), and fractional vegetation co ver are dependent. In particular, we suggest that LAI and fractional v egetation cover may not be independent quantities, at least when the f ormer is defined without regard to the presence of bare patches betwee n plants, and that the customary variation of LAI with NDVI can be exp lained as resulting from a variation in fractional vegetation cover. T he following points are made: i) Fractional vegetation cover and LAI a re not entirely independent quantities, depending on how LAI is define d. Care must be taken in using LAI and fractional vegetation cover ind ependently in a model because the former may partially take account of the latter; ii) A scaled NDVI taken between the limits of minimum (ba re soil) and maximum fractional vegetation cover is insensitive to atm ospheric correction for both clear and hazy conditions, at least for v iewing angles less than about 20 degrees from nadir; iii) A simple rel ation between scaled NDVI and fractional vegetation cover, previously described in the literature, is further confirmed by the simulations; iv) The sensitive dependence of LAI on NDVI when the former is below a value of about 2-4 may be viewed as being due to the variation in the bare soil component. (C) Elsevier Science Inc., 1997.