URBANIZATION WITH LOW IMPACT DEVELOPMENT AND EFFECTS ON STREAM BASE FLOW

A novel form of urbanization, Low Impact Development (LID), aims to develop urban areas that mimic natural hydrologic functioning. This type of urban development includes the preservation of near-natural groundwater recharge via infiltration close to impervious surfaces where stormwater is generated. The instrumented study watershed is 1.11 km² and located in Clarksburg, Maryland. Development occurred from 2004 to 2010 and resulted in the placement of 73 infiltration-focused stormwater facilities, including bioretention facilities, dry wells, and dry swales. We examined changes to annual and monthly streamflow during and after urban development (2004—2014) in comparison to nearby forested and urban control watersheds. We show that total flow and base flow increased in the study watershed during development as compared to control watersheds. We also found that the urbanizing watershed had slower storm recessions after development and less seasonality in base flow. These changes may be due to a combination of urban processes occurring during development, including a decrease in evapotranspiration and the increase in point sources of recharge. Precipitation that fell on a forested landscape pre-development would have been stored in soil moisture and eventually transpired by plants; in an urban landscape, precipitation may now be recharged to groundwater and contribute to base flow. A transfer of evapotranspiration to base flow is a possible unintended alteration to the urban water budget from use of infiltration-focused stormwater facilities.

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