Bioretention Outflow: Does It Mimic Nonurban Watershed Shallow Interflow?

Abstract

Bioretention, a key structural practice of low impact development (LID), has been proved to decrease peak flow rates and volumes, promote infiltration and evapotranspiration, and improve water quality. Exactly how well bioretention mimics predevelopment (or “natural”) hydrology is an important research question. Do bioretention outflow rates mirror shallow groundwater interevent stream recharge flow associated with natural or nonurban watersheds? Streamflow from three small, nonurban watersheds, located in Piedmont, part of central North Carolina, was compared with bioretention outflow from four cells also in North Carolina’s Piedmont region. Each benchmark watershed drained to a small stream, where flow rate was monitored for an extended period of time. After normalizing the flow rates and volumes by watershed size, data were combined to form two data sets: bioretention outflow and stream interevent flow. Results indicate that there is no statistical difference between flow rates in streams draining undeveloped watersheds and bioretention outflow rates for the first 24 h following the commencement of flow. Similarly, there is no statistical difference between the cumulative volumes released by the two systems during the 48 h following the start of flow. These results indicate that bioretention cells behave comparably to watersheds in natural or nonurban conditions, with respect to both flow rates and flow volumes, and suggest that bioretention outflows may mirror post–storm event shallow groundwater interevent stream recharge flow. Solely considering bioretention outflow as a conjugate to runoff may be a misinterpretation of a flowrate that actually resembles shallow interflow.