Institute of Astrophysics and Atmospheric Physics, Estonian Academy of Sciences, EE2444 Tõravere, Estonia
Absorption profiles of incoming solar radiation determine incoming fluxes between the vegetated ground and the atmosphere. On the basis of the Nilson-Kuusk canopy reflectance model a procedure for calculating the absorption profiles inside a homogeneous vegetation canopy is proposed. The model considers the absorption of uncollided and single- -scattered radiation including specular reflection on leaves and the hot-spot effect. The multiple scattering is considered in a two-stream approach. Model calculations demonstrate significant deviations of absorption profiles from exponential in the near infrared spectral region and high dependence of profiles on foliage orientation and the solar zenith angle.
The set of input parameters of the model coincides with that of the Nilson-Kuusk canopy reflectance model. These parameters can be estimated from remote measurements of the angular distribution of canopy directional reflectance.
The proposed treatment of solar radiation absorption in a homogeneous vegetation canopy made it possible to calculate the absorption profiles with precision comparable with contemporary CR models. The reported examples of absorption profiles demonstrate rather significant changes of profiles dependent on the changes of the canopy structure and optical properties of foliage. The set of input parameters of the absorption model coincides with that of the Nilson- Kuusk (1989) model. The input parameters of the CR model can be determined by optical remote measurements and inversion of the CR model (Kuusk, 1991a,b), and thus, we can determine the absorption profiles of radiation inside a vegetation canopy on the basis of remote optical measurements.