Soil Moisture Balance in Bioretention Systems

Stormwater control measures treat water quantity and quality in urban and suburban areas. Bioretention systems capture stormwater runoff and remove a volume of stormwater from the downstream receiving body. There is also some water quality treatment as pollutants of concern are trapped within the bioretention system. The amount of stormwater a bioretention system can capture is a function of the void space and soil moisture. Within the design of bioretention systems, the focus is often on a soil's ability to infiltrate. Evapotranspiration (ET) plays a substantial role in the water budget of bioretention systems, as shown by this research. Further, not only does a portion of the stormwater runoff retained go to infiltration and ET immediately after a rainfall event, but during dry time ET is further reducing the amount of moisture within the soil and thereby restoring the capacity of the bioretention system to store stormwater for the next rainfall event. This is a key point that has been neglected in continuous models of stormwater control measures and only grossly accounted for in bioretention standards. The present work quantifies ET from two different bioretention systems over two years using weighing lysimeters (i.e. bioretention with an underdrain and bioretention with an internal water storage layer). Both bioretention systems yield substantial ET annually with more ET observed in the system with an internal water storage layer, which creates higher soil moisture conditions.