Hydrologic dynamics and linkages in a wetland-dominated basin

Abstract

Hydrologic dynamics are examined across a range of spatial scales in a 78 km 2 wetland-dominated drainage basin in south central Ontario, Canada. The basin typically receives ∼800 mm of precipitation annually (∼20% as snowfall), although precipitation during the 1998 study year was 28% greater due to increased snowfall. Water table elevation, outlet stream discharge and local groundwater flow patterns were monitored in a representative interdrumlin headwater wetland sub-basin. A rising water table during snowmelt sustained surface saturation in the wetland and at the slope–wetland interface, and saturation overland flow generated the bulk of annual runoff. Despite considerable till thickness on the drumlin slopes, minor groundwater fluxes from slopes to the wetland could not maintain near-surface water tables. These fell below the ground surface shortly after snowmelt and led to cessation of stream flow. Vertical water movement in response to precipitation, evapotranspiration and recharge of deeper groundwater was more important than lateral groundwater fluxes in explaining the wetland's hydrologic behavior. Spatial and temporal dynamics of variable runoff source areas in the basin were examined based on stream flow response from 15 headwater wetland sub-basins. Peak basin stream flow resulted from delivery of sub-basins runoff to a spatially linked drainage network during spring snowmelt. However, sub-basins became decoupled from the basin outlet as summer progressed, and their runoff during significant summer and fall storms entered ephemeral stream channels where most was lost to evaporation and infiltration or held in surface storage. Nevertheless, hydrologic connectivity was more protracted in 1998 than would be likely during a ‘normal’ precipitation year. Hydrologic processes in this drumlinized landscape are inconsistent with some aspects of the variable source area concept of stream flow generation, and caution should be used when extrapolating knowledge and models from well-integrated drainage networks to wetland-dominated basins such as that examined here.