318 Donovan et al.: Analysis of Vegetation and Stormwater Runoff in an Urban Watershed Arboriculture & Urban Forestry 2016. 42(5): 318–328 Statistical Analysis of Vegetation and Stormwater Runoff in an Urban Watershed During Summer and Winter Storms in Portland, Oregon, U.S. Geoffrey H. Donovan, David T. Butry, and Megan Y. Mao Abstract. Past research has examined the effect of urban trees, and other vegetation, on stormwater runoff using hydrologi- cal models or small-scale experiments. However, there has been no statistical analysis of the influence of vegetation on runoff in an intact urban watershed, and it is not clear how results from small-scale studies scale up to the city level. Researchers address this gap in the literature by estimating random-effects regression models of the effect of trees and other vegetation on total runoff and peak runoff for a summer (15–16 June 2010) and a winter (18–19 December 2010) storm in Portland, Oregon, U.S. Researchers found that additional tree canopy cover was associated with lower runoff in the summer storm, but the significance of the tree coef- ficient was sensitive to model structure. Researchers found that additional groundcover (grass and shrubs) associated with lower peak flow in the summer, and this result was robust to model structure. Neither trees nor groundcover were significantly associ- ated with winter stormwater runoff. Results suggest that trees and other vegetation can be effective at moderating stormwater runoff. However, vegetation is not as effective in the winter, which is consistent with past modeling and experimental studies. Key Words. Economics; Hydrology; Oregon; Portland; Runoff; Stormwater; Trees; Urban Forestry Vegetation. In urban areas, trees, and other types of vegetation, are increasingly being used in stormwater manage- ment to supplement traditional gray infrastructure (Soltis 1997; Keating 2002; Villarreal et al. 2004; Day et al. 2008). Although there is a wealth of research in wildland settings showing trees can re- duce and slow runoff (Heal et al. 2004; Link et al. 2004; Keim et al. 2005; Boegh et al. 2009), there has been less research in urban environments (Sanders 1986; Soltis 1997; Xiao et al. 1998; Xiao et al. 2000; Wang et al. 2008; Asadian and Weiler 2009). Fur- thermore, the research that has been done in urban environments has been based on small-scale ex- periments or hydrological models. There has been no statistical analysis of the influence of vegetation on runoff in an intact urban watershed. This is a significant gap in the literature, as wildland studies and small-scale experiments do not consider the built component of an urban watershed (e.g., im- pervious surfaces) and how this built component interacts with vegetation. Therefore, it’s not clear ©2016 International Society of Arboriculture how results from small-scale studies scale up to the city level, and it is important to understand the relationship between trees and stormwater runoff at the same scale as potential policy rem- edies. Researchers address this gap by quantifying the effect of vegetation on stormwater runoff in a combined-sewer system in Portland, Oregon, U.S. Literature Review Vegetation can affect stormwater runoff in three ways: interception, transpiration, and infiltration. Vegetation intercepts precipitation, which allows it to evaporate rather than landing on the ground and contributing to runoff. Transpiration occurs when vegetation draws water from the soil and releases it as water vapor from its leaves and stem. Finally, roots increase the infiltration of water through the soil. In wildland settings, several studies have shown that trees intercept significant amounts of rain (Heal et al. 2004; Link et al. 2004), and that forest structure and tree age are important determinants of intercep-
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