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

Abstract

Many headwater wetlands contain seasonally or permanently saturated areas adjacent to streams, in small depressions or in regional or local groundwater discharge zones. Environmental isotopes indicate that pre-event water dominates the storm period from these headwater wetlands, however, saturated overland flow usually dominates stormflow at these sites supposedly making groundwater contributions relatively less important as a runoff mechanism. Mixing of event water with surface storage water transported by saturated overland flow has been suggested as an alternative mechanism to account for large volumes of pre-event water during saturated overland flow stormflow. In this paper we present results from various model simulations of groundwater- surface-water mixing in a headwater wetland. In the model we have varied both local and groundwater contributions to the wetland, precipitation intensity and the initial wetness' of the wetland to better understand the processes controlling both stormflow and water chemistry. Surface water mixing is shown to be an important process during low, moderate and high intensity events. Results from these simulations are used to predict the effect of reduced groundwater input to the wetlands on stormflow and water chemistry.