Arboriculture & Urban Forestry 42(5): September 2016 tion rates. Specifically, Pypker et al. (2005) found that a 25-year-old Douglas-fir plantation intercepted more rain than an old-growth stand of Douglas- fir, and Nadkarni and Sumera (2004) found that trees with a denser canopy intercepted more rain. Interception rates are also influenced by weather. Rates are higher following a period of dry weather (McJannet et al. 2007) and decline as a storm pro- gresses (Jetten 1996). Weather variation can also make it harder to draw general conclusions about intercep- tion by forest type (Crockford and Richardson 2000). The findings of rain-interception studies in urban areas are generally consistent with those con- ducted in wildland settings. In Davis, California, U.S., Xiao et al. (2000) found that an open-grown deciduous tree intercepted less winter rain than an open-grown conifer. They also found that inter- ception rates varied from 100% at the beginning of a storm to 3% at the end. Asadian and Weiler (2009) studied the interception rates of six trees (Douglas-fir and red cedar) in British Columbia, Canada, and they found that red cedar intercepted more rain than Douglas-fir, and interception was influenced by canopy structure and storm inten- sity. Guervara-Escobar et al. (2007) found that the mean interception rate of an open-grown evergreen was 60% across 19 summer storms. Several studies have used models to estimate the interception rate of urban trees. Wang et al. (2008) used a hydrology model to estimate the interception rate of trees in Baltimore, Maryland, U.S. Their model showed that trees can signifi- cantly reduce runoff; however, results only held for low-intensity, short-duration storms. Sanders (1986) modeled the effect of urban develop- ment and vegetation on stormwater runoff in Dayton, Ohio, U.S. His model showed that development increased runoff, whereas vegeta- tion reduced both total runoff and runoff rate. This analysis explores the effect of vegetation on stormwater runoff in Portland, Oregon. Researchers analyzed two storms in 2010—a summertime (leaf on) and wintertime (leaf off) event—across 34 sewer monitoring sites. The objective is to quan- tify the effect of trees and other vegetation on total runoff and peak flow. The current study is the first to analyze this relation holistically in an intact urban watershed: researchers didn’t rely on hydro- logical models and measured runoff in sewers as 319 opposed to within or under trees. Therefore, the study is a useful complement to existing hydro- logical models and small-scale experiments, which have been used to justify significant investments in green infrastructure. In addition, the study is at the city scale, which matches the scale of likely policy interventions (e.g., tree planting programs). MATERIALS AND METHODS Study Area Portland is a city in northwest Oregon with a population of 619,360 in 2014 (U.S. Cen- sus 2014). It has a maritime climate with a mean annual rainfall of 109 cm (National Oce- anic and Atmospheric Administration 2011), which falls mainly in the winter and spring. Approximately 70% of homes in Portland are connected to a combined-sewer system, in which sanitary flow and stormwater runoff share the same system of pipes. Approximately 772 com- munities in the U.S., serving 40 million people, have combined-sewer systems (U.S. Environ- mental Protection Agency 2008). When most combined-sewer systems were built, sanitary flow was not treated, so a combined system was viewed as an economical way of disposing of sanitary flow and stormwater runoff. Now that sanitary flow is treated before release, the man- agement of combined-sewer flow presents chal- lenges, because stormwater runoff is far more variable than sanitary flow, which can lead to the release of untreated sanitary flow into rivers and backup of sewer flow for residential customers. In 1991, Northwest Environmental Advo- cates sued the City of Portland under the Clean Water Act because the City released untreated sanitary flow an average of 50 times a year into the Willamette River or the Columbia Slough. In response, the City built nels, which became known as three storage tun- the big-pipe project. These tunnels were designed so that untreated flow would be released into the Wil- lamette River an average of four times in the winter and once every three summers (based on 40-year development projections) and released into the Columbia Slough once every five winters and once every ten summers (Port- land Bureau of Environmental Services 2012). ©2016 International Society of Arboriculture
September 2016
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