SCALING EFFECTS ON MOISTURE FLUXES AT UNVEGETATED LAND SURFACES

As part of a larger study on spatial variability of land surface proce sses, the authors explore the sensitivity of land surface modules for climate models to the method of simulating the unsaturated subsurface flows. By examining the behavior of a number of different subsurface m odules, it is shown that the surface fluxes, and consequently the wate r balance throughout the year, vary widely for different simulations o f subsurface conditions. Typical results are presented for a specified climates and soil types. In order to reduce the complexity and comput ation time for the subsequent sensitivity studies, it is shown that a linearized module displays the range of behavior expected in practice. For given forcing functions of precipitation and potential evaporatio n, varying the depth of the modelled soil layer and changing the lower boundary conditions greatly influence the annual values of the compon ents of the water balance, Monte Carlo simulations are used to demonst rate that spatial variation in soil properties produces large variatio n in runoff and compensating variations in deep drainage with a much s maller variation in evaporation. Finally, it is shown that for a given coefficient of variation in soil scaling properties, the effect on th e effective large-scale sorptivity is insensitive to the type of stati stical distribution used to describe the variation.