REMOTE-SENSING OF SOLAR-RADIATION ABSORBED AND REFLECTED BY VEGETATED LAND SURFACES

The problem of remotely sensing the amount of solar radiation absorbed and reflected by vegetated land surfaces was investigated with the aid of one-and three-dimensional radiative transfer models. Desert-like vegetation was modeled as clumps of leaves randomly distributed on a bright dry soil with a ground cover of generally less than 100%. Surface albedo (ALB), fraction of photosynthetically active radiation absorbed by the canopy (FAPAR), fractions of solar radiation absorbed by the canopy (FASOLAR) and soil (FASOIL), and normalized difference vegetation index (NDVI) were calculated for various illumination conditions. A base case was defined with problem parameters considered typical for desert vegetation in order to understand the dynamics of NDVI and ALB with respect to ground cover, leaf area index, soil brightness, and illumination conditions. The magnitude of errors involved in the estimation of surface albedo from broad-band monodirectional measurements was assessed through model simulations of SPOT, AVHRR, and GOES sensors. The nature of the relationships between NDVI vs. FASOLAR, FAPAR, FASOIL, and ALB, and their sensitivity to all problem parameters was investigated in order to develop simple predictive models. Finally, the relationship between NDVI measured above the atmosphere to that above the canopy at the ground surface was studied to characterize atmospheric effects in the remote sensing problem.