A vegetation radiative transfer scheme in the ISBA-A-GS interactive vegetation model

Vegetation surfaces play an important role in the Earth´s energy balance and have a significant impact on the global carbon cycle. The fraction of solar radiation that is absorbed by a vegetation canopy will assess the rate of photosynthesis and besides the amount of carbon flux further fixed or released by this same canopy layer. The radiative transfer scheme within the canopy of ISBA-Ags interactive vegetation model was defined in 1998 by Calvet et al [2] according to a self-shading approach. The incident fluxes at the top of the canopy go through a multilayer vegetation cover. Then, the attenuated flux in the PAR wavelength domain of each layer is used by the Jacobs Model [3] to evaluate the net assimilation (An) of the leaf. Net assimilation of the leaf estimated for each layer is mixed together to derive the average An quantity of the total vegetation cover. A detailed description of the vegetation radiative transfer scheme within the canopy is given in Appendix B of Calvet et al. [2]. The objective of the present study is the improvement of the current scheme and its evaluation. The impact of the new absorbed radiation on the photosynthesis module is considered, in order to obtain better simulations of the Leaf Area Index (LAI) and Gross Primary Production (GPP). These evaluations sustain the value added of an advanced representation of the radiative transfer within the canopy in order to estimate the photosynthesis.