EVALUATION OF THE EFFECTS OF GENERAL-CIRCULATION MODELS SUBGRID VARIABILITY AND PATCHINESS OF RAINFALL AND SOIL-MOISTURE ON LAND-SURFACE WATER-BALANCE FLUXES

Many existing general circulation models (GCMs) use so-called 'bucket algorithms' to represent land-surface hydrology. Wood et al. (1992) pr esented a generalization of the simple bucket representation, based on their variable infiltration capacity (VIC) model. The VIC model, in e ssence, assumes a statistical distribution of bucket sizes within the grid square. In this way, it provides a simple, computationally effici ent and yet physically realistic model of land-surface hydrology. A pr eliminary attempt is made here, using this simple model, to evaluate t he effects of the spatial variability of rainfall intensities and the resulting soil moisture within a hypothetical GCM grid square. Rainfal l is assumed to be patchy, with only a fraction of the grid square bei ng wetted by rainfall at any given time. Within the wetted area, howev er, rainfall is allowed to vary randomly in space. Evaporation during interstorm periods is estimated from the rest of the grid square by a simple non-linear function of the available soil moisture. The soil mo isture is also assumed to be patchy and spatially variable due to the antecedent rainfall that caused it. A number of simplifying assumption s have been made in the model about the redistribution of soil moistur e at the end of storm and interstorm periods, and the effects of a veg etation canopy have been ignored. The model is applied, under a variet y of conditions, to estimate the biases in the modelled water balance fluxes if the assumed spatial heterogeneity is neglected. The model is also used to simulate the long-term water balance dynamics under assu med hypothetical storm and inter-storm climatic inputs, to see how the steady-state hydrological regime is affected by spatial variability. These simulations are relevant to current efforts towards developing s imple parameterizations of land-surface hydrology that explicitly inco rporate subgrid heterogeneity.