The sensitivity of simulated flow and water quality response to spatial heterogeneity on a hillslope in the Tarrawarra catchment,Australia

Models are widely used to simulate hydrological response and the generation and transport of constituents such as salt, phosphorus, and nitrogen from catchments to streams. Several models use a spatial representation with catchments divided into subcatchments. Variations in land use and other characteristics within subcatchments are represented by spatially lumped hydrologic response units (HRUs) or functional units instead of using fully distributed models. This approach disregards any spatial interaction between HRUs, including their connectivity to each other and to the stream and the influence of these interactions on water and constituent export. A spatially explicit hydrological model (Thales) was used to simulate a variety of theoretical catchments with soils dominated by combinations of infiltration excess, saturation excess, and subsurface stormflow processes and different soil constituent concentrations that were spatially interacting (i.e. located along a hillslope sequence). The modelling results show that the response of both runoff and concentration is sensitive to varying spatial arrangements due to interactions of runoff, infiltration, and chemical processes between the different soil types in many but not all situations. Results highlight the importance of considering connectivity of pathways when modelling hydrological response and constituents export. This is achieved by comparing pairs of simulations and the corresponding differences in the exported loads. Copyright © 2009 John Wiley & Sons, Ltd.