Bioretention Outflow: Does It Mimic Non-Urban Watershed Shallow Interflow?

Bioretention, a key structural practice of Low Impact Development (LID), has been proven to decrease peak flow rates and volumes, promote infiltration and evapotranspiration and improve water quality. Exactly how well bioretention mimics pre-development (or "natural") hydrology is an important question that continues to be researched. Do bioretention outflow rates mirror shallow groundwater inter-event stream recharge flow associated with natural or non-urban watersheds? Streamflow from three small, non-urban watersheds, located in the piedmont of central North Carolina, was compared to 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 inter-event 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 hours following the commencement of flow. Similarly, there is no statistical difference between the cumulative volumes released by the two systems during the 48 hours following the start of flow. These results indicate that bioretention cells behave comparably to watersheds in natural or non-urban conditions with respect to both flow rates and flow volumes and that bioretention outflows somewhat mirror post storm event shallow groundwater recharge. Solely considering bioretention outflow as a conjugate to runoff may be a misinterpretation of a flowrate that actually resembles shallow interflow.