Quantifying Tree Canopy Contributions to Stormwater Runoff Reductions in Urban Watersheds of the Southeastern United States

Urban stormwater is a major contributor to surface water degradation in the United States, prompting cities to invest in ways to naturally capture, store, and slowly release runoff through “Green Infrastructure” (GI). One often overlooked potential contributors to stormwater management are urban trees, an integral part of a given city’s GI. Interception is of particular interest and describes a tree’s ability to capture and store rainfall, reducing the volume of stormwater that can degrade urban streams during storm events. While rainfall interception for full canopy, forested environments is well studied, limited research is available that characterizes the interception of open-grown trees, which are commonly found in urban areas. Nine trees from three common, urban, native species planted in three Knoxville-area parks were studied to quantify interception. Continuous measurements of were made using several automatic rain gauges positioned underneath tree canopies. When compared to gross precipitation, the measurements quantify interception, throughfall (rainfall that passes through the canopy) and stemflow (rainfall that travels down the trunk) of each tree. Data was collected from January 2018 to May 2019 to account for seasonal variations in canopy cover and precipitation patterns. Annually, red maples (Acer rubrum), white pines (Pinus strobus), and willow oaks (Quercus phellos) intercepted 24.4%, 52.4%, and 33.2% of gross throughfall, respectively. Further results demonstrate the effect of event duration, rainfall intensity, and seasonal variations on the interception potential of each species. Overall, these observations are a step toward allowing the storage capacity of urban trees to be properly credited as part of urban watershed restoration efforts