RUNOFF MECHANISMS IN A FORESTED GROUNDWATER DISCHARGE WETLAND

Hydrometric measurements and geochemical hydrograph separation techniques were used in experimental plots to determine the processes that govern stormflow production in a forested swamp situated in a regional groundwater discharge zone. Hydrometric measurements suggested that saturated overland flow from permanently saturated areas created by discharging groundwater was the major storm runoff mechanism. Chemical separation analysis, using a portable irrigation system, indicated that over 70% of the runoff was pre-event water. Groundwater ridging did not contribute significantly to wetland runoff, whereas preferential flow through pipes accounted for a minor component of the pre-event water at peak streamflow and the majority of the pre-event flow in the recession. Rapid mixing of event water with a significantly larger pool of pre-event water standing in the saturated areas is responsible for the rapid appearance of the pre-event water in surface generated stormflow. The rapid groundwater-surface water mixing in the saturated areas has implications for the interpretation of runoff mechanisms based solely on chemical separation.