University of British Columbia Undertakes Tree Canopy Research Project to Support Water Balance Model
Four-Way Collaboration
The Water Balance Model includes a tree canopy module so that the rainfall interception benefits of trees in the urban environment can be quantified. To populate the module with local data, a four-way collaboration has been established under the umbrella of the Inter-Governmental Partnership (IGP) that developed the Water Balance Model. The Greater Vancouver Regional District and Ministry of Community Services are providing funding, and the University of British Columbia and District of North Vancouver are making in-kind contributions in carrying out the applied research project. The District of North Vancouver is acting on behalf of the IGP in leading this on-the-ground initiative.
Tree canopy interception is the process of storing precipitation temporally in the canopy and releasing it slowly to the ground and back to the atmosphere. It is an important component of the water balance, easily accounting for up to 35% of gross annual precipitation. Removing trees will in general decrease interception and thus increase annual runoff and rainwater runoff. Vegetation also reduces rainfall intensity due to the temporal storage effect.
“It is exciting how quickly this project has come together,” observes Richard Boase of the District of North Vancouver. “It was only 12 months ago that we had our first 
meeting with Dr. Hans Schreier and Dr. Markus Weiler of UBC to initiate development of the Tree Canopy Module. At the time we discussed the strategic importance of a long-term relationship between the University and the IGP.”
“Well, we have made it happen thanks to Hans Schreier who has been our champion at UBC. We also cannot say enough about the support that we have had from SILG, the technical committee of the GVRD that has obtained the funding over the past five years that has enabled the IGP to bring a vision to fruition.”
The UBC Connection
Hans Schreier is an internationally renowned professor at the Institute for Resources and Environment. In 2004 he received the “Science in Action” Award from the United Nations International Year of Fresh Water, Science, Education and Conservation Program, for outstanding work in making watershed management knowledge and innovative, cost-effective applications possible in Canada and in Developing Countries.
Markus Weiler is the Forest Renewal British Columbia Chair of Forest Hydrology, and is an Associate Professor in the Departments of Forest Resources Management and Geography at the University of British Columbia. He previously taught at Oregon State University and in Zurich, Switzerland.
GVRD Stormwater Interagency Liaison Committee
SILG stands for Stormwater Interagency Liaison Group of the GVRD. The IGP began as a subgroup of SILG in July 2002, and then quickly expanded to become a provincial partnership with municipal representation from four regions: Greater Vancouver, the Fraser Valley, Vancouver Island, and the Okanagan Valley.
Commencing in 2002, SILG has provided annual funding towards the continuing development of the Water Balance Model. In 2005, SILG funded the research and development of the Tree Canopy Module because it recognized the need to develop a science-based understanding regarding the benefits of maintaining a tree canopy in the urban environment.
“It was in 2001 that SILG recognized he value of the water balance approach and funded the development of a working model to assess the affordability and feasibility of site design solutions for achieving performance targets”, adds Mark Wellman, P.Eng., SILG Coordinator.
“The results of this applied research were incorporated in Stormwater Planning: A Guidebook for British Columbia, published in June 2002. This guidance document, founded on BC case study experience, formalized a science-based understanding to set performance targets for reducing rainwater runoff volumes.”
At the time of module initiation in 2005, Dr. Markus Weiler noted that: “While considerable research has been undertaken in forest stands in the natural environment, very little has been done in an urban setting anywhere in North America. This initial collaboration between UBC and the IGP therefore opens the door to a long-term partnership to bring science into the community. Because of the urban context for the proposed research, our focus will be on quantifying the interception effectiveness of a single tree versus that for a cluster of trees.”
“The limited research that has been done in North America was undertaken by the Center for Urban Forest Research at the University of California, Davis,” adds Mark Wellman. “For this reason the Center was part of the project team that was led by Lanarc Consultants Ltd for development of the Tree Canopy Module.”
Objectives and Methods for Applied Research
“Under the current project, we will explore the variables influencing the interception process and hence quantify interception of trees and bushes within an urban environment. In particular, we will focus on the effects of tree density, tree structure and tree species. This research will directly inform urban planning and will be used to populate the Water Balance Model with real data,” reports Richard Boase.
“That’s correct,” explains Markus Weiler, “The experimental set-up will focus on directly measuring throughfall under different tree species – Douglas-fir, Western, red cedar, and Western hemlock. When measuring throughfall in combination with gross precipitation in an opening with short high temporal resolution, total interception and throughfall delay can be calculated. Since we have to account for different density and structure of trees in an urban setting, we plan to do these measurements under 4-5 different setting. In addition, we plan to replicate each measurement three times. This will result in around 40 to 50 sites. In order to account for changes in available energy, we will measure air temperature, wind speed and direction, and solar radiation.”
Except for some smaller studied based on some specific tree species, most interception research has focused on forested environments. Data and parameters from these experiments have been mostly used to parameterize hydrological models for urban areas. Usually tree canopy interception is not considered in conventional stormwater models. However, the structure and maintenance methods for urban forests are significantly different than natural forests.
Notes Hans Schreier: “There is a greater heterogeneity of tree species and spatial dimensions (height and structure) and trees are generally more isolated, with large distances between them. Hence, we can expect different interception processes, whereby evaporation changes, microclimate gradients change due to variations in leaf surfaces and stem surface areas, and wind associated rainfall can change the throughfall distribution and ratio between stemflow and interception.”
In theory, it is highly probable that interception losses of a tree that stands in an urban setting is larger compared to a tree within a forest stand. There may be even an optimum of tree density and structure whereby the interception is largest for a certain tree density. If this density can be determined, or even the relation of tree density to interception loss, local governments could provide urban developers with guidance as to how many trees need to be maintained within a residential lot to maintain a certain interception effect.
In reflecting on this need for information, Markus Weiler observes that: “This observation underscores the importance of maintaining some tree cover on residential lots. Also, it leads one to articulate the counter-intuitive argument that fewer trees is actually better from a rainwater management perspective – provided there is still a reasonable canopy. Hence, the importance of undertaking experimental research in urban areas to obtain data that will be directly used to develop such guidelines.”
Project Funders
SILG has provided funding for purchase of equipment in 2006, and has requested that the GVRD budget include additional monies in 2007 and 2008 so that a graduate student can complete the tree canopy interception data collection and analysis under the direction of Hans Schreier and Markus Weiler. On August 3rd 2006 the team of Weiler, Schreier and Boase installed the first prototype of the tree canopy rainfall interception equipment on a fir tree in Richard’s back yard.
The Ministry of Community Services is also providing financial support. “Provincial grant programs no longer support the traditional ‘pipes and pavement’ approach to 
drainage planning. The Ministry focus is on programs that reduce rainwater runoff volume at the site level, by capturing rain where it falls. Because the Tree Canopy Research Project will advance the rainwater management state-of-the-art in British Columbia, the Ministry is pleased to play its part as a project funder,” reports Glen Brown, Director of Infrastructure and Engineering.
Equipment Installation
Academics in action: Dr. Hans Schreier provides support while Dr. Markus Weiler does the drilling for the system that will monitor the effectiveness of the Douglas Fir in intercepting rainfall. The objective is to measure the percentage of rainfall that makes it through the tree canopy.
Applied research: Closeup of Dr. Markus Weiler in action