EFFECTS OF SPATIAL VARIABILITY IN TOPOGRAPHY, VEGETATION COVER AND SOIL-MOISTURE ON AREA-AVERAGED SURFACE FLUXES - A CASE-STUDY USING THE FIFE 1989 DATA

A modified version of the simple biosphere model (SiB) of Sellers et a l. (1986) was used to investigate the impact of spatial variability in the fields of topography, vegetation cover, and soil moisture on the area-averaged fluxes of sensible and latent heat for an area of 2 x 15 km (the First International Satellite Land Surface Climatology Projec t (ISLSCP) Field Experiment (FIFE) testbed area) located within the FI FE area. This work builds on a previous study of Sellers et al. (1992a ) but makes use of a superior data set (FIFE 1989 rather than FIFE 198 7) and has a sharper focus on the nonlinear effects of soil wetness on evapotranspiration. The 2 x 15 km testbed area was divided into 68 x 501 pixels of 30 x 30 m spatial resolution, each of which could be ass igned topographic, vegetation condition, and soil moisture parameters from satellite and in situ observations gathered in FIFE-89. One or mo re of these surface fields was area averaged in a series of simulation runs to determine the impact of using large-area means of these initi al/boundary conditions on the area-integrated (aggregated) surface flu xes. Prior to these simulations some validation work was done with the model. The results of the study can be summarized as follows: (1) SiB was initialized with satellite and airborne remotely sensed data for vegetation condition and soil wetness, respectively. The surface fluxe s calculated by SiB compared well with surface-based and airborne flux observations. (2) Analyses and some of the simulations indicated that the relationships describing the effects of moderate topography on th e surface radiation budget are near linear and thus largely scale inva riant. The relationships linking the simple ratio (SR) vegetation inde x, the canopy conductance parameter V-F, and the canopy transpiration flux are also near linear and similarly scale invariant to first order (see also Sellers et al., 1992a). Because of this it appears that sim ple area-averaging operations can be applied to these fields with rela tively little impact on the calculated surface heat fluxes. (3) The re lationships linking surface and root-zone soil wetness to the soil sur face and canopy transpiration rates are nonlinear. However, simulation results and observations indicate that soil moisture variability decr eases significantly as the study area dries out, which partially cance ls out the effects of these nonlinear functions. (4) The near-infrared surface reflectance rho(N) estimated from atmospherically corrected s atellite data may be a better predictor of vegetation condition than a two-band vegetation index, such as the SR, at least for the grassland s represented in the FIFE area. These results support the use of simpl e averages of topographic and vegetation parameters to calculate surfa ce energy and heat fluxes over a wide range of spatial scales, from a few meters up to many kilometers. Although the relationships between s oil moisture and evapotranspiration are nonlinear for intermediate soi l wetnesses, the dynamics of soil drying act to progressively reduce s oil moisture variability and thus the impacts of these nonlinearities on the area-averaged surface fluxes. These findings indicate that we c an use mean values of topography, vegetation condition, and soil moist ure to calculate the surface-atmosphere fluxes of energy, heat, and mo isture at larger length scales to within an acceptable accuracy for cl imate-modeling work.