Influence of stormwater control measures on watershed hydrology and biogeochemical cycling
Urban development replaces vegetation with impervious surfaces and natural drainage channels with pipe networks that quicken flow paths and alter hydrologic regimes. Additionally, the import of food, application of fertilizer to lawns and gardens, and heightened atmospheric deposition increases nutrient availability in urban landscapes. These excess nutrients are ultimately routed to streams through the pipe networks before it can be processed by the vegetation and microorganisms of the landscape. This combination of physical and chemical disturbances impacts stream ecosystems and degrades their ability to perform valuable services such as removal of nutrients, degradation of pollutants, and provision of recreational and aesthetic value. Stormwater control measures (SCMs) are a management strategy that can mitigate these impacts urbanization, ultimately preserving those valuable stream ecosystems. While the effects of urban development and individual SCMs on water quantity and quality have been well documented independently, studies examining the cumulative influence of SCMs on water quantity and nitrogen cycling throughout entire developed watersheds are lacking. First, this work addresses this gap in knowledge by empirically relating hydrologic regimes at sixteen urban watersheds in Charlotte, NC, USA to a series of metrics that describe the extent of urban development and mitigation with SCMs. Next, water quality data were collected at four of the sixteen sites to determine how SCMs affect stream nutrient and carbon concentrations during storms, and how the extent and distribution of urban development modulates the effects of SCMs. Because of the limited ability for monitoring approaches to capture variability along a continuum of development and mitigation, a modeling approach was used to further understand the role of SCMs on hydrology and water quality. A new model was developed, calibrated, validated, and used to assess uncertainty of the hydrologic and ecological processes that occur in SCMs. Finally, these SCM routines were incorporated into an existing spatially-distributed watershed model to test how varying levels of impervious surface connectivity to SCMs changed hydrologic and water quality regimes in a watershed in Charlotte, NC. The results of the study indicate that the degree of urbanization, as measured by a watershed metric total imperviousness, controlled hydrologic behavior at the storm event time scale across the 16 sites monitored. There is evidence that SCMs are able to effect the hydrologic record flashiness at an annual time scale by temporarily storing runoff and extending hydrograph recession. An analysis of water quality data indicates that SCMs are able to reduce N, phosphorous and dissolved organic carbon concentrations in the stream in watersheds with a homogeneous urban land use. However, in newly developing watersheds (e.g., suburban), the presence of SCMs coincides with the addition of urban impervious surfaces and SCMs are not sufficient to return water quality to pre-development conditions, as reflected in increased in nutrient concentrations. To understand how SCMs are able to affect nutrient concentrations along a continuum of development and mitigation intensity, we explored a hydrologic and water quality model of SCMs. Through calibration, the model was able to match the distribution of outflow water and both nitrate and ammonium concentrations of a single SCM monitored in Charlotte, NC. SCM inorganic N removal and retention increased with temperature and SCM water depth. When the SCM routines were used at the watershed scale, results showed that increased mitigation of urban impervious surfaces with SCMs led to proportional reductions in total runoff volumes, and annual loads of both nitrate and ammonium. These results have implications for watershed managers looking to protect stream ecosystems through the use of SCMs. Treating urban impervious areas with SCMs can reduce hydrologic record flashiness, which is correlated to stream invertebrate health. Mitigating impervious surfaces with SCMs may be able to reduce nitrogen loads by both reducing total water yield and reducing in-stream N concentrations, although the change in concentrations is likely to be dependent on climatic forcing, the distribution of land use, and design of the SCM. Finally, a management strategy as simple as planting trees may also produce similar reductions in runoff and loads, as results showed lower runoff volumes and lower N concentrations in watersheds with greater tree coverage.
McMillan, Purdue University.
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