The relative importance of surface and aerodynamic resistances in a multi-source energy-CO 2 model

Abstract

An interactive multi-source SVAT model, describing the fluxes of heat, water vapour and CO 2, has been developed. The parameterisation of the canopy conductance is based on a physiological photosynthesis-conductance model. The energy partitioning between the various sources is calculated using the Penman Monteith equation and a resistance network, involving aerodynamic and surface resistances. The model has been tested using micro-meteorological measurements over a sparse Sahelian savannah consisting of scattered shrubs with an understorey of grasses, herbs and bare soil. The relative importance of the surface and the aerodynamic resistances in obtaining predictions of the energy balance, CO 2 flux and surface temperatures has been investigated. It appeared that the influence of most of the aerodynamic resistances on fluxes and surface temperatures was small (generally less than 10%). The aerodynamic resistance between the soil surface and the mean canopy source height, however, had a large influence on sensible heat flux, CO 2 flux and soil surface temperature. The largest influence on sensible and latent heat fluxes and surface temperatures was exerted by the surface resistances, especially the one for the savannah shrubs. Halving or doubling these resistances changed the latent and sensible heat fluxes by up to 50%. The CO 2 flux, however, was only slightly influenced (< 10%) by imposed changes in aerodynamic or surface resistances, except for the ones related to the soil, because of the large contribution of soil respiration to the net CO 2 flux of this savannah.