Jr. Dymond et Jg. Qi, REFLECTION OF VISIBLE-LIGHT FROM A DENSE VEGETATION CANOPY - A PHYSICAL MODEL, Agricultural and forest meteorology, 86(3-4), 1997, pp. 143-155
Monitoring vegetation condition by remote sensing is made difficult by
the variation of canopy brightness (radiance) with view direction and
sun position. In this paper, we develop a physical model of canopy ra
diance that models this variation as a product of three functions, eac
h being expressed as a simple analytical formula. The first function,
the S function, represents the proportion of canopy seen as sunlit, ex
cluding the hotspot. The second function, the hotspot function, correc
ts the S function for the higher probability of seeing sunlit leaves n
ear the anti-solar point. The third function represents the average ra
diance of sunlit leaves. The model was fitted to densely-sampled, mult
i-view radiance measurements of pine forest (Pinus radiata) and pastur
e (Lolium spp.), taken at a large (similar or equal to 60 degrees) sun
zenith angle. Radiance predictions for multiple views were then made
for a small sun zenith angle (similar or equal to 20 degrees). Compari
son of predicted with measured radiances showed that the model out-per
formed the linear Roujean et al. model (J. Geo. Res., 97: 20,455-20,46
8) and the non-linear Verstraete et al. model (J. Geo. Res., 95: 11,75
5-11,765). The improvement is due to two factors: The hotspot function
has a sun zenith angle dependence; and the S function allows the aver
age projected leaf area to vary with off-nadir view angle. (C) 1997 El
sevier Science B.V.