EFFECTIVE PARAMETERS OF SURFACE-ENERGY BALANCE IN HETEROGENEOUS LANDSCAPE

This paper addresses the problem of estimating surface fluxes at large scale over heterogeneous terrain, and the corresponding determination of effective surface parameters. Two kinds of formulation are used to calculate the fluxes of sensible and latent heat: the basic diffusion equations (Ohm's law type) and the Penman-Monteith equations. The str ategy explored is based upon the principle of flux conservation, which stipulates that the average flux over a large area is simply the area -weighted mean of the contributions from the different patches making up the area. We show that the application of this strategy leads to di fferent averaging schemes for the surface parameters, depending on the type of flux (latent heat, sensible heat) and on the type of formulat ion used to express the flux. It appears that the effective Value of a given parameter must be appraised for each individual application, be cause it is not unique, but differs according to the magnitude being c onserved and the equation used to express this magnitude. Numerical si mulations are carried out to test over contrasted areas the aggregatio n procedures obtained. The areal fluxes estimated from these effective parameters, together with the areal fluxes calculated by means of a s imple areal averaging of the parameters, are compared to the ''true'' average fluxes, calculated as area-weighted means of the elementary fl uxes. The aggregation procedures obtained prove to be much more accura te for estimating areal fluxes and for closing the energy balance equa tion than those based upon simple areal averaging of the parameters.