| Abstract | The amount of vegetation absorbed photosynthetically active radiation (APAR) is useful for climatic and ecological studies. Monitoring APAR on large scale requires the use of satellites. The methods
proposed so far for measuring APAR from space use the absorptance of the vegetation canopy, FPAR, and an estimation of the incoming PAR at the top of the canopy. We propose a method that uses the reflected PAR flux at the top of the atmosphere to
evaluate how much PAR is absorbed at the surface. The whole approach is presented in two parts. In the previous part, it was demonstrated how the amount of PAR absorbed at the surface, APARs, can be obtained from the visible reflectance at the top of
the atmosphere measured by satellites. It is not necessary to know the optical thickness of the clouds or the surface albedo. In the second part, presented in this paper, we show how to distinguish the absorbed PAR by the vegetation, APAR, from the
total PAR absorbed by the surface, APARs, using a parameter that we call RPAR. RPAR is the ratio APAR/APARs. Based on radiative transfer simulations, we show how RPAR, in the same way as FPAR (the absorptance of the canopy in the visible) in the
traditional methods, can be estimated from a vegetation index. The effects of the soil reflectance, the solar zenith angle, the diffuse fraction of the incoming light and the optical properties of the vegetation are examined. |