EFFECT OF LAND-SURFACE REPRESENTATION ON FOREST WATER AND CARBON BUDGETS

Forested landscapes often show very well-pronounced heterogeneity in t he factors that control evapotranspiration, runoff production and carb on assimilation at a variety of length scales. In hilly or mountainous environments, strong contrasts in net radiation, available soil water , soil structure and stand characteristics can produce a large varianc e in both the meteorological drivers and surface resistance to carbon and water exchange with the atmosphere over distances measured in tens of metres. Because of the strong nonlinearities characterizing the in fluence of the environmental variables on surface resistance (particul arly available soil water), the parametrization of surface process mod els with mean values of the environmental variables and no distributio n often leads to significant bias in areal average carbon and water fl ux. However, it is often not feasible to incorporate directly the full distribution and patterns of the landscape for regional-scale models. Continental- and subcontinental-scale vegetation data sets currently being collected by synoptic-level satellites (e.g. the Advanced Very H igh Resolution Radiometer, AVHRR) do not capture the large proportion of landscape variability that exists below the resolution of the senso rs. This paper explores the impacts of various landscape representatio n schemes that retain a range of detail in the description of land sur face form and processes on simulated areal average evapotranspiration, runoff production and net carbon exchange with the atmosphere. Specif ic comparison is made of schemes that attempt to incorporate the topog raphic structure, soil and vegetation distributions of a region with s chemes that sample the surface at levels similar to current coarse-res olution satellites. For strongly heterogeneous basins (mountainous top ography), it is found that spatial variations in available soil water can have significant effects on areal averaged carbon and water flux r ates, particularly under drying conditions, whereas the spatial variat ions in radiation, temperature and humidity over the terrain appear to have a lesser impact.