Quantifying the influences of stormwater control measures on urban headwater streamflow

The Watershed Hydrology Lab will be at the Geological Society of America meeting in November in Baltimore. Anne will be giving an invited talk in the Urban Geochemistry session (T32) on Sunday, November 1st at 9 am in BCC room 308. Here’s what she’ll be talking about:

Quantifying the influences of stormwater control measures on urban headwater streamflow

Anne Jefferson¹, Colin Bell², Sara McMillan², and Sandra Clinton³
1. Department of Geology, Kent State University, 221 McGilvrey Hall, Kent, OH 44242 USA. Phone: 1-330-672-2746 Email: ajeffer9@kent.edu
2. Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana, USA.
3. Department of Geography and Earth Sciences, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, NC 28223, USA.

Stormwater control measures are designed to mitigate the hydrological consequences of urbanization, but their as-built effectiveness in altering patterns of urban streamflow remains poorly quantified. Stream gaging and water stable isotopes were used to understand the effects of stormwater ponds and wetlands on hydrograph characteristics and water sourcing in four urban headwater streams in Charlotte, North Carolina. At the small watershed scale (0.15-1.5 km2), runoff ratio and peak discharge are more strongly related to impervious area than area treated by stormwater controls. For one stream during 10 events, we used stable isotopes to quantify contributions of retention pond discharge to streamflow, taking advantage of the unique isotope signature of pond outflow. The pond, which drains 25% of the watershed’s impervious area, contributed an average of 10% (0-21%) of the streamflow on the rising limb and 12% (0-19%) of discharge at peak flow. During recession, this pond contributed an average of 32% (11-54%) of the stream’s discharge, reflecting the pond’s design goals of temporarily storing and delaying runoff. The isotopic signature of the pond’s discharge also reveals varying water residence times (hours to weeks) within the structure, which may have implications for nutrient and metal fluxes into the stream. Our results suggest that even when individual stormwater control measures are working as designed, they are insufficient to fully mitigate the effects of urbanization on stream hydrology. They also demonstrate the combination of traditional hydrometric and tracer-based techniques can reveal a nuanced view of stormwater influences on urban streams. Such hydrological nuance will be necessary to develop strong mechanistic understanding of biogeochemical processes in urban streams and watersheds.