326 Donovan et al.: Analysis of Vegetation and Stormwater Runoff in an Urban Watershed shrubs lose their leaves in winter, and therefore interception and transpiration rates are lower. In addition, colder, wetter winter weather reduces water loss from transpiration and infiltration. DISCUSSION The results suggest that, for summer storms in Port- land, shrubs and grass reduce peak flow. The results for trees were more ambiguous. Using model-based estimators, the relationship between tree cover and total flow was significant. However, when sandwich estimators were used to correct for heteroskedas- ticity, the relationship between trees and total flow was no longer significant. In winter, neither total flow nor peak flow were affected by vegetation. The results provide support for the use of bio- swales and other ground-covering vegetation to augment traditional gray infrastructure. How- ever, increasing groundcover was associated with relatively modest reductions in peak flow. Plac- ing bioswales strategically, or combing them with other mitigation techniques, may have a signifi- cant effect on fixed infrastructure costs, but they shouldn’t be considered a panacea. Future research could fruitfully focus on the impact of bioswales and bioswale structure on stormwater runoff. The results do not provide definitive support for the use of trees in stormwater management. Some of this ambiguity may be a consequence of the study design—two-day storms were analyzed, and so tree cover was time invari- ant. However, the results suggest that wildland studies and single-tree experiments should not be blindly used to justify the use of trees in urban stormwater management. These studies do not consider the built component of urban watersheds, and it’s far from clear how trees interact with built infrastructure. More than anything, this study emphasizes the need for more research in intact urban watersheds. Determining how the hydrological effects of vegetation translate into changes in stormwater treatment costs is problematic because treat- ment costs are not simple linear functions of total flow. Rather, there are many threshold effects. For example, in some circumstances, a modest reduction in flow might stop a sewer pipe from backing up into people’s basement, which would avoid significant short-term damage. In the long ©2016 International Society of Arboriculture term, fewer backups may avoid the cost of replac- ing a small but otherwise functional pipe. How- ever, in other situations, the same reduction in flow may have little effect on treatment costs. Although it may be difficult to determine the effect of vegetation on stormwater treat- ment costs in a specific case, cost savings are likely to be higher in the following circumstances, all else equal: cities with combined-sewer sys- tems, cities with high summer rainfall, and cities with undersized but otherwise functional pipes. The study has several limitations. Researchers were only able to analyze two storms, so the results are, to some degree, an artifact of the idiosyncrasies of these storms. The coefficients of interest would, no doubt, have been dif- ferent if different storms had been analyzed. How different, researchers are unable to say. Therefore, the results should be interpreted cautiously. However, analyzing more storms would not have added additional variability in vegetative cover, as both tree cover and grass- and-shrub cover were time invariant in the analysis. Therefore, additional storms may not have provided more insight into the relation- ship between vegetation and stormwater runoff. The measures of vegetation cover are another source of uncertainty. Specifically, the imagery used is subject to classification error. In addition, tree canopy obscures underlying groundcover, so researchers couldn’t determine how much of the area under tree canopy was covered by impervi- ous surface or vegetation. Finally, imagery was used from 2007 to estimate canopy cover in 2010. Despite these limitations, researchers believe the unique nature of the study provides useful support and caveats to past studies that have identified a relation between vegetation and stormwater runoff. Although trees and other vegetation may be a useful complement to traditional stormwater infrastructure, it is important not to overstate their benefits. However, it is also important to consider the other benefits of trees, which include reduced energy consumption (Akbari et al. 1997; McPher- son and Simpson 2003), increased sale price of homes (Anderson and Cordell 1988; Donovan and Butry 2010), reduced crime (Kuo and Sullivan 2001), and improved public health (Lovasi et al. 2008; Donovan et al. 2011; Donovan et al. 2013).
September 2016
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