Quantifying New Water Contributions to Stormflow in an Urban Watershed Using Electrical Conductivity and Isotopic Tracers
Abstract
Decreased permeability associated with urbanization is known to alter the hydrologic response of streams and rivers to rainfall. We suggest that electrical conductivity (EC) is a simple and cost-effective tracer for determining the direct contribution of precipitation and surface runoff (e.g. new water) to urban streams during rainfall events. Urban watersheds may be ideally suited for EC-based hydrograph separations because (1) storm runoff is often dominated by two sources of water (groundwater and surface runoff) and (2) end-members are typically characterized by large differences in EC values. We therefore evaluate the use of EC as a tracer for two-component hydrograph separation during 13 rainfall events (2001 and 2002) in a 3.9 km2 urban catchment in Massachusetts (25% impervious). EC-based results are compared against isotopic (deuterium) hydrograph separation results for two storms as validation. Precipitation EC values for our 13 rainfall events were significantly lower (12-46 uS/cm) than stream baseflow EC values (520-1297 uS/cm), contributing to less than 8% uncertainty in our hydrograph separations. The direct input of new water, presumably as direct runoff from impervious surfaces, accounts for 70-90% of the elevated discharge during most storms. Since using EC allows for characterization of a large number of storms, we were also able to assess the role of precipitation characteristics and watershed moisture on new water runoff variability between events. Total rainfall volume explains 72% of the variability in new water runoff volumes and is only slightly improved by including an indicator of antecedent moisture in a multiple regression (r2=0.77). In all but one event, 4-11% of the total storm rainfall volume appeared as new water in the stream, suggesting that less than half of the watershed impervious area is hydrologically connected to the stream. The remaining impervious surface runoff either infiltrates pervious surfaces or is exported out of the watershed via the storm drainage network. Understanding the role of impervious surfaces in delivering rapid runoff to streams is critical for hydrologic modeling in urban areas, as well as for assessing the fate of urban pollutants.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.H13B..02P
- Keywords:
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- 1803 Anthropogenic effects;
- 1854 Precipitation (3354);
- 1860 Runoff and streamflow