| Description |
Bioretention systems have become an increasingly well accepted element of low impact development within stormwater management plans, but much of the research on these systems has taken place in mesic climates. This study examined the effect of vegetative cover on the performance of three 10 m2 bioretention gardens designed to receive the average annual runoff from a 220 m2 impervious surface in Salt Lake City, UT for one year beginning January, 2012. Three vegetation options were assessed: no vegetation, wetland vegetation that received irrigation, and an upland plant community that received no irrigation. Each garden was lined and runoff inflow and outflow were measured directly. Gas exchange measurements were taken before and after simulated runoff events and used to quantify transpiration volume. In the upland plant community, shrubs were shown to have higher overall transpiration rates and contributed 60% of the total annual transpiration volume. Grass species demonstrated strong transpiration rate increases in response to simulated runoff events in the summer, especially during the driest months. Total annual transpiration from the upland garden was estimated to be 7% of the inflow volume. The wetland plants were able to transpire a greater volume than the upland plants, roughly 15% of the inflow, but this came at the cost of irrigation demand. ET runoff reductions were measured at 15%, 29% and 39% for the unvegetated, wetland, and upland bioretention gardens, respectively. The high reduction determined for the upland garden may be due to a tear in the liner that resulted in unmeasured outflow. Maximum ET for the upland bioretention garden was deduced using transpiration measurements and data from the unvegetated plot to be 22% of annual runoff inflow. Soil moisture content data demonstrated localized reduction in soil conductivity in the upland garden throughout the summer, potentially indicating water stress on the plants imposed by the bioretention system design used for this study. It was shown that regionally native plant communities used in bioretention systems can help improve site hydrology while remaining resilient to seasonal runoff inflow fluctuations without requiring supplemental irrigation. This study examined upland shrubs and grasses spaced at native density. However, future research that explores higher plant density and alternative plant communities may yield improvements to bioretention designs that achieve site specific goals for nutrient removal, ET volume reductions, and infiltration. |
