ABSTRACT Process-based models of soil-vegetation-atmosphere interactions developed for small plots (points) define vertical transfers of water and energy. One can attempt to scale to larger heterogeneous land units by disaggregating the landscape into a set of elements and applying a vertical SVAT model independently to each element (Running et al., 1989; Pierce et al., 1992). Such applications fail to consider lateral transfers. A distributed parameter, three-dimensional SVAT (Topog-IRM) developed by the CSIRO Division of Water Resources (O'Loughlin, 1990; Hatton and Dawes, 1991; Hatton et al., 1992) is used to examine the importance of lateral transfers of water for prediction of water balance components at the small catchment scale.

Simulations are used to contrast the predicted water balances from a SVAT model with and without considerations of lateral subsurface and overland flow in complex terrain. Components of the catchment water balance are shown to scale linearly except in those cases where transient perched water tables develop in landscapes with sufficient slope and hydraulic conductivity to redistribute water effectively via subsurface lateral flow. In such cases, the prediction of catchment yield and the spatial pattern of soil moisture requires the explicit treatment of lateral transfers.