Water Balance of Soil Mixes for Rain Gardens

Rain gardens have proven to be an effective means to control stormwater; thus, in Pennsylvania’s Stormwater Best Management Practice’s Manual, rain gardens are the preferred stormwater control measure (SCM). Currently, volume capture in Pennsylvania is calculated using a static bowl equation that does not include the dynamics of infiltration and evapotranspiration (ET). In addition, many states’ design guidelines for rain garden soil mixes are often restrictive and vague. The goal of the work described in this paper is to develop guidelines for soil mixes that consider the balance of infiltration and ET using bench-scale experiments at Villanova University. A total of twelve discrete weighing lysimeters were filled with soils that span the lower portion the USDA classification triangle. The lysimeters were divided evenly into two flow path configurations: vertical 46 cm deep columns and horizontal 20 cm deep boxes at a 2.5% slope. Both lysimeter configurations utilized loamy sand, sandy loam (typical rain garden mix in PA), loam, silt loam, and clay loam soils. Switchgrass was planted in all but two lysimeters to provide a non-vegetated control with sandy loam soil. Runoff was added into the lysimeters during rainfall events to simulate actual rain garden performance and the lysimeters were weighed daily for two weeks after the event. Preliminary results of five storm simulations provide an average ET rate of 3.3 mm/day for the vertical lysimeters and 2.6 mm/day for the horizontal lysimeters. In the vertical configuration the silt loam soil produces the most volume removal through ET at 44% of the total volume removal with an average ET rate of 4.2 mm/day. In contrast, the loamy sand soil produces the least volume removal through ET at 25% of the total volume removal.