THE IMPACT OF USING AREA-AVERAGED LAND-SURFACE PROPERTIES - TOPOGRAPHY, VEGETATION CONDITION, SOIL WETNESS - IN CALCULATIONS OF INTERMEDIATE SCALE (APPROXIMATELY 10 KM(2)) SURFACE-ATMOSPHERE HEAT AND MOISTURE FLUXES

It is commonly assumed that biophysically based soil-vegetation-atmosp here transfer (SVAT) models are scale-invariant with respect to the in itial boundary conditions of topography, vegetation condition and soil moisture. In practice, SVAT models that have been developed and teste d at the local scale (a few meters or a few tens of meters) are applie d almost unmodified within general circulation models (GCMs) of the at mosphere, which have grid areas of 50-500 km(2). This study, which dra ws much of its substantive material from the papers of Sellers et al. (1992c, J. Geophys. Res., 97(D17): 19033-19060) and Sellers et al. (19 95, J. Geophys. Res., 100(D12): 25607-25629), explores the validity of doing this. The work makes use of the FIFE-89 data set which was coll ected over a 2 km x 15 km grassland area in Kansas. The site was chara cterized by high variability in soil moisture and vegetation condition during the late growing season of 1989. The area also has moderate to pography. The 2 km x 15 km 'testbed' area was divided into 68 x 501 pi xels of 30 m x 30 m spatial resolution, each of which could be assigne d topographic, vegetation condition and soil moisture parameters from satellite and in situ observations gathered in FIFE-89. One or more of these surface fields was area-averaged in a series of simulation runs to determine the impact of using large-area means of these initial or boundary conditions on the area-integrated (aggregated) surface fluxe s. The results of the study can be summarized as follows: 1. analyses and some of the simulations indicated that the relationships describin g the effects of moderate topography on the surface radiation budget a re near-linear and thus largely scale-invariant. The relationships lin king the simple ratio vegetation index (SR), the canopy conductance pa rameter (del(F)) and the canopy transpiration flux are also near-linea r and similarly scale-invariant to first order. Because of this, it ap pears that simple area-averaging operations can be applied to these fi elds with relatively little impact on the calculated surface heat flux . 2. The relationships linking surface and root-zone soil wetness to t he soil surface and canopy transpiration rates are non-linear. However , simulation results and observations indicate that soil moisture vari ability decreases significantly as an area dries out, which partially cancels out the effects of these non-linear functions. In conclusion, it appears that simple averages of topographic slope and vegetation pa rameters can be used to calculate surface energy and heat fluxes over a wide range of spatial scales, from a few meters up to many kilometer s at least for grassland sites and areas with moderate topography. Alt hough the relationships between soil moisture and evapotranspiration a re non-linear for intermediate soil wetnesses, the dynamics of soil dr ying act to progressively reduce soil moisture variability and thus th e impacts of these non-linearities on the area-averaged surface fluxes . These findings indicate that we may be able to use mean values of to pography, vegetation condition and soil moisture to calculate the surf ace-atmosphere fluxes of energy, heat and moisture al larger length sc ales, to within an acceptable accuracy for climate modeling work. Howe ver, further tests over areas with different vegetation types, soils a nd more extreme topography are required to improve our confidence in t his approach.