Measuring and modelling the radiation balance of a heterogeneous shrubland

The accurate quantification of the energy available for sensible and latent heat transfer from plant canopies is essential for the prediction of impac ts of climate on vegetation water use and growth. Unlike agricultural field s and extensive forests of more humid zones, vegetation growing in semi-ari d climates is usually sparse creating a heterogeneous surface of shrubs, an nuals and bare soil. Under these conditions many of the assumptions of the basic equations used in microclimatology, which assume a uniform vegetated surface, may be violated. It is proposed here that heterogeneous canopies m ay require a formulation of their energy balance that includes a measure of the canopy complexity in order to both interpret field measurements and be used in predictive models. This paper explores the need for a more complex formulation of the vegetation energy balance through a series of experimen ts on a sparse clumped shrubland of Retama sphaerocarpa in the Tabernas Des ert, Almeria, south-east Spain. These experiments investigated the importan ce of the radiative properties of each surface on energy balance of soil, a nnuals and shrubs individually, and the surface as a whole. The study evalu ated the use of the fractional vegetative cover (f) and the radiative chara cteristics of each surface (reflection coefficients and emissivities) for c alculating net radiation partitioning between shrubs and bare soil. Results indicated that partitioning of net radiation between components could be a ccurately calculated from values of fractional vegetative cover, reflection coefficients and emissivities for both bare soil and plant surfaces. A sen sitivity analysis showed the importance of specific radiation properties of each surface. Measurements of horizontal long-wave fluxes between componen ts showed that the location of a plant with respect to other plants made li ttle difference to its long-wave energy balance. The results also emphasize d the importance of soil water content on the energy balance, through its i nfluence on albedo and soil heat storage. This was particularly true when m easurements of soil heat flux were used to measure the available energy for soil under shrubs and bare soil because of strong hysteresis cycles. These cycles were larger in the bare soil than in the substrate under shrubs.