Plant performance considerations for semi-arid bioretention & bioinfiltration system design

Bioretention and bioinfiltration are stormwater best management practices that have the potential to enhance the hydrologic performance of urban areas throughout the semi-arid American West. The University of Utah Urban Water Group is conducting a study to begin to characterize the hydrologic performance of these BMP’s in a xeric climate. A bioretention system composed of a native upland plant community has been established on campus and is designed to require no supplemental irrigation. Water tanks are used to simulate storm event volume and frequency corresponding to monthly averages. The research team has documented leaf level transpiration response to these simulated storm events throughout the summer growing season using a Licor 6400 portable photosynthesis system. The leaf area and gas-exchange measurements demonstrate plant responses to water availability, an understanding that is critical for the design and operation of engineered ecological systems in arid climates. Notably, shallow-rooted grasses demonstrate dramatic increases in transpiration rate following storm events, especially during times of drought, that contrast the modest change in transpiration rate observed for the deep-rooting shrubs with access to more persistent moisture. Following July, Utah’s hottest and driest month, total daily volume transpired is observed to increase by 10-50% percent for individual shrubs in response to summer storm events, but increases of over 100% are observed for the grass species. In the course of a week, the total volume transpired from this system may represent over 20% of the volume delivered during a single simulated storm event. Most importantly, these characterizations provide engineers with the information necessary to use natural plant behavior to meet engineering goals such as promoting infiltration, increasing topsoil residence time to encourage nutrient removal, tuning plant composition to accommodate drainage and/or system size constraints, and meeting the aesthetic needs of a site