A bioretention system, or rain garden, in Portland, Oregon, US.

Trees have the potential to provide significant stormwater benefits. Trees are also valued for many other benefits, and are already part of virtually all development. They can be integrated even into the densest urban areas – and many cities have tree requirement ordinances, so trees are not only desired, but also mandatory. It is therefore not surprising that the use of Tree Stormwater Control Measures (SCMs) is rapidly growing.With the increasing use of Tree SCMs, there is also a growing need to quantify tree stormwater benefits.

Tree best management practices (BMP) stormwater benefits have been quantified in a number of different ways. Factors that typically influence which method is most applicable include project goals (i.e., what will tree BMP stormwater quantification be used for), ease of use, model cost, and accuracy.This blog series will describe uses and pros and cons of a number of representative models and calculators.

For the purposes of this discussion, I will divide the types of models and calculators to quantify tree SCM stormwater benefits into 4 categories:

• Tree benefit calculators
• Stormwater credit systems
• Single event stormwater models
• Continuous stormwater models

I will briefly summarize what I mean by each, describe the intended uses, and give an example.

Tree Benefit Calculators

Tree benefit calculators quantify many tree benefits, not just stormwater benefits. These are useful for advocacy projects (for example), but are not generally suitable to model stormwater benefits for engineering purposes. In addition to quantifying tree stormwater benefits (e.g. gallons of stormwater intercepted per year), they also translate the benefits into dollars. They can therefore also be useful for lifecycle cost benefit analysis. Examples include i-Tree, Tree Asset Manager, and the National Tree Benefit Calculator.

i-tree is probably the best known tree benefit calculator. It is a suite of tools designed to quantify the environmental services provided by a forest or a group of trees, as well as the structure of that forest. It is available at no charge from the USDA Forest Service. The four i-Tree analysis tools that quantify stormwater benefits include i -Tree Eco, i-Tree Streets, i -Tree Design, and i -Tree Hydro. Table 1 summarizes the stormwater benefits provided by each.

Table 1: summary of stormwater benefits provided by i-Tree tools

i-tree and Tree Asset Manager can be used for many other purposes in addition to estimating tree benefits and lifecycle cost analysis. For more information you can read an earlier series about using i-Tree to analyze the structure, function, and value of community forests, a case study in using i-Tree at a regional scale, and a case study in using i-Tree at a city scale.

Stormwater Credit Systems

Another common reason to quantify tree SCM stormwater benefits is to determine stormwater credits.

Definitions of stormwater credits vary. For the purposes of this blog, I will use the Minnesota Stormwater Manual's definition:

"the quantity of stormwater or pollutant reduction achieved either by an individual BMP or cumulatively with multiple BMPs…"

The manual further explains:

"Credits apply to a single pollutant or to runoff reduction. A BMP may thus generate credits for more than one pollutant. Total credit for a specific pollutant or for runoff reduction must therefore be computed individually for each pollutant or volume of runoff reduced…Ideally, stormwater credits are simple to calculate, easy to review and delineate on site plans and quickly verified in the field [italics added]."

Methods for quantifying Tree SCM stormwater benefits are therefore generally much simpler than stormwater models. I discussed examples of how various cities quantify tree stormwater credits in a previous blog.

Single Event and Continuous Stormwater Models

Stormwater models generally provide much more information on Tree SCM stormwater benefits than tree benefit calculators or stormwater credit methodologies, but they are also generally more complex.

As the 2015 Minnesota Stormwater Manual explains: "Some kind of stormwater model is needed whenever an estimate of the expected volume, rate, or quality of stormwater is desired. Modeling is also often necessary for the design of BMPs and hydraulic structures and for evaluation of the effectiveness of water quality treatment by BMPs… In practice, stormwater models are most commonly used either as planning and decision making aids for water management authorities, or as tools for developers who wish to design for and demonstrate compliance with regulations and principles governing protection of water and waterways."

Single event stormwater models only model one storm at a time, whereas continuous stormwater models can quantify SCM benefits over extended periods of time, including many storms, as well as the time in between storms (some models can do both single event and continuous modeling). Since evapotranspiration (ET) only happens between storm events (not during storm events), continuous models are better for quantifying ET.

Parts 2 and 3 of this  series will address how single event and continuous stormwater models, respectively, apply to quantifying Tree SCM benefits. Stay tuned.

Nathalie Shanstrom is a sustainable landscape architect with the Kestrel Design Group.

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