Shortwave radiation budget of sahelian vegetation .2. Radiative transfer models

In semi-arid environments, vegetation canopies can often be represented by two distinct layers; the upper layer of perennial trees or shrubs, and the lower layer of annual or perennial herbaceous species. Two models of radiative transfer, one appropriate for homogeneous vegetation (the SAIL (scattering from arbitrarily inclined leaves) model), and the other for clumped plant stands (the cylinder model), were coupled to simulate photosynthetically active radiation (PAR) radiative transfer in a wide range of semi-arid vegetation canopies. The coupling was based on the assumption that the PAR incident above the herb layer was equal to the quantity of PAR transmitted by the woody layer. The fluxes predicted by the models and their coupling are validated against a data set of PAR interception acquired in small plots during the 1992 HAPEX-Sahel field campaign (Niger, West Africa). It was shown that the SAIL model alone did not adequately describe the interception of clumped vegetation communities, whereas the combined SAIL-cylinder model was able to simulate observed interception efficiencies. The coupled model was then applied to the HAPEX-Sahel West-Central grass fallow, shrub fallow, degraded shrubland and millet crop sites, to estimate daily, intercepted PAR of the sites during the 1992 growing season. The results are compared with hemispherical ceptometer measurements made along transects across the sites.