STORM-RUNOFF GENERATION IN AN URBANIZED WATERSHED: EXAMINING THE CONTRIBUTIONS OF GROUNDWATER TO RUNOFF USING TWO-COMPONENT HYDROGRAPH SEPARATION

The traditional view that water losses from urban areas during a storm event are so small that they do not significantly affect the runoff volume is disputed. Considerable uncertainty about stormflow generation and groundwater recharge exists in urban watersheds. A two-component hydrograph separation technique using oxygen-18 and electrical conductivity to quantify event and pre-event components of an individual precipitation event in a highly urbanized glaciated catchment is explored. Isotopic separation results are used to estimate direct groundwater recharge.

This exploratory study was undertaken in a 7.6 km2 highly urbanized catchment to investigate isotopic and chemical variations during a summer storm event and to evaluate the importance of various hydrological processes. It was found that only about 15% of the rainfall amount appeared as overland flow and that groundwater accounted for 21% of the total stormflow. Higher electrical conductivity values, possibly associated with flushing of urban contaminants during the storm, demonstrated a 9% higher groundwater contribution to total stormflow. The saturated zone thickness increased by 31 mm following the storm. More than half of this increase occurred within one and a half hours following the onset of rain at an average rate of almost 11 mm/ hour. It was estimated that 35% of the total rainfall amount contributed to direct groundwater recharge.

The findings have far-reaching implications in our understanding of groundwater surface water interaction and contaminant flow paths in urban catchments. Study results demonstrate rapid groundwater recharge and delivery to stream channel following a storm event as well as the importance of groundwater in stormflow generation in urbanized catchments.