* Corresponding author: yeganeh@interchange.ubc.ca 16 A New Approach in Measuring Rainfall Interception by Urban Trees in Coastal British Columbia Yeganeh Asadian 1 * and Markus Weiler 2 1 Department of Forest Resources Management, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada 2 Institute of Hydrology, The University of Freiburg, Fahnenbergplatz 79098 Freiburg in Breisgau, Germany Interception loss plays an important role in controlling the water balance of a watershed, especially where urban development has taken place. The aim of this study was to illustrate the importance of urban trees as a form of ‘green infrastructure’ where they reduce stormwater runoff and rainwater intensity. In addition, trees cause a delay in precipitation reaching the ground. Interception loss was studied in the North Shore of British Columbia. We applied a unique methodology for measuring throughfall under six different urban trees using a system of long polyvinyl chloride pipes hung beneath the canopy capturing the throughfall and draining it to a rain gauge attached to a data logger. Different tree species (Douglas-fr [Pseudotsuga menziesii] and western red cedar [Thuja plicata]) in variable landscape sites (streets, parks, and natural forested areas) and elevations were selected to ensure that the system adequately captured the throughfall variability. Interception and throughfall were monitored over a one year cycle for which the results of seven discrete storm events for coniferous trees from the District of North Vancouver during 2007 to 2008 are presented. Cumulative gross precipitation for seven selected events was 377 mm. Average canopy interception during these events for Douglas-fr and western red cedar were 49.1 and 60.9%, where it corresponded to average net loss of 20.4 and 32.3 mm, respectively. The interception loss varied depending on canopy structure, climatic conditions, and rainfall characteristics. Key words: urban environment, throughfall, interception loss, stormwater runoff Introduction Urbanization has resulted in profound changes to natural watershed conditions by altering terrain, vegetation, soil characteristics, and surface conditions. Urban development impacts climatic conditions and alters the hydrological processes leading to more fashy runoff and increased pollution in urban watersheds (Sanders 1986; McPherson et al. 1997). The losses in vegetation cover and increases in impervious surfaces, such as paved roads, sidewalks, and concrete buildings, increase the total amount of runoff, the fashiness of runoff events, fooding, erosion, and the cost of stormwater management. Villarreal and Bengtsson (2004) noted that stormwater runoff prior to development was regulated by trees, vegetation, and natural soils. Trees and soil function together to reduce stormwater runoff. Trees reduce stormwater runoff by intercepting rainwater on leaves, branches, and trunks. Some of the intercepted water evaporates into the atmosphere and some infltrates into the ground, decreasing peak fows and the total amount of urban runoff. Trees also slow storm fow events by reducing the volume of water that must be managed at one time and the rainfall intensity. Trees are generally overlooked in urban planning, but they are an integral component of the urban infrastructure, capable of controlling the hydrological processes, regulating air and water quality, reducing Urban Heat Islands (UHI) and absorbing CO 2 (Sanders 1986; Taha 1997). Stormwater managers have started to use trees as a tool to help reduce stormwater generation and, in this way, reduce the cost of constructing traditional stormwater control infrastructure. The value of the tree for stormwater management has been calculated based on the avoided costs of handling stormwater runoff (McPherson et al. 1997; Zipperer et al. 1997; Villarreal and Bengtsson 2004). McPherson et al. (2005) reported that in some cities in the U.S.A., the urban tree investment can be between $13 to $65 per tree annually in planting and maintenance cost. In return, gains in stormwater services are between $1.37 to $3.09 per dollar that would have otherwise been invested in traditional stormwater management. Another study has estimated the worth of the U.S.A.’s urban forests as $400 billion in terms of stormwater management mitigation alone (American Forests 1996). These studies demonstrate the importance of trees as source controls capable of treating stormwater at the site level by reducing the runoff component within the hydrological cycle. Urban vegetative cover is arranged as individual or stands of trees that contribute to the sustainability of the environment. From an urban hydrological point of view, the most noticeable effect of vegetation is rainfall interception by the canopy (Xiao and McPherson 2002; Guevara-Escobar et al. 2007; McJannet et al. 2007a, 2007b). Canopy interception losses frequently modify the intensity and distribution of precipitation reaching Water Qual. Res. J. Can. 2009 · Volume 44, No. 1, 16-25 Copyright © 2009, CAWQ