VARIABLE BUCKET REPRESENTATION OF TOPMODEL AND INVESTIGATION OF THE EFFECTS OF RAINFALL HETEROGENEITY

In this paper we present a variable bucket capacity (VBC) representati on of TOPMODEL (Beven and Kirkby, 1979), generalizing the Manabe 'simp le bucket' scheme widely used in climate models. In doing this, we hav e been motivated by the variable infiltration capacity (VIC) model pre sented by Wood et al. (1992). For catchments, or for parts of the land surface, whose hydrology fits the TOPMODEL assumptions, this lumped v ersion offers an alternative representation that can be used as the bu ilding block for land surface hydrology models at large spatial scales . The model is first applied, with satisfactory results, to a small ca tchment near Canberra in Australia. Next, using the same model, the ef fects of random spatial variability of rainfall are studied, but in th e context of land surface hydrological parameterizations for global cl imate models. Sub-grid rainfall variability, especially the partial co verage that characterizes rainfall events at such large scales, is fou nd to cause significant biases in the estimation of land surface fluxe s. Other manifestations of the effects of the spatial variability of r ainfall are also investigated, also by means of model simulations, but with a view to gaining insights for developing simple parameterizatio ns of land surface fluxes at large scales. This includes the investiga tion of the dynamics, i.e. spatial and temporal variability, of the so il moisture state variable during and after a rainfall event. In addit ion, empirical relationships linking the variable contributing area, s ubsurface runoff and evaporation, to a soil moisture state variable (e .g. soil moisture storage), are derived based on model simulations, an d the effects of the sub-grid rainfall variability on these relationsh ips are examined. Because of the strong non-linearity inherent in the process descriptions within the model, the above 'constitutive' relati onships are strongly affected by rainfall heterogeneity. (C) 1997 by J ohn Wiley & Sons, Ltd.