Modelling groundwater-surface water mixing in a headwater wetland: implications for hydrograph separation

Many headwater wetlands contain seasonally or permanently saturated areas a djacent to streams, in small depressions or in regional or local groundwate r discharge zones. Environmental isotopes indicate that pre-event water dom inates the storm period from these headwater wetlands, however, saturated o verland flow usually dominates stormflow at these sites supposedly making g roundwater contributions relatively less important as a runoff mechanism. M ixing of event water with surface storage water transported by saturated ov erland Row has been suggested as an alternative mechanism to account for la rge volumes of pre-event water during saturated overland flow stormflow. In this paper we present results from various model simulations of groundwate r-surface-water mixing in a headwater wetland. In the model we have varied both local and groundwater contributions to the wetland, precipitation inte nsity and the initial 'wetness' of the wetland to better understand the pro cesses controlling both stormflow and water chemistry. Surface water mixing is shown to be an important process during. low, moderate and high intensi ty events. Results from these simulations are used to predict the effect of reduced groundwater input to the wetlands on stormflow and water chemistry . Copyright (C) 2000 John Wiley & Sons, Ltd.