What’s the Science Behind Green Infrastructure Tracking in OpenTreeMap?

When it launched in early 2014, TreeMapLA became our first OpenTreeMap site to support tracking green stormwater infrastructure (also known by the acronym “GSI”, not to be confused with “GIS”!). 26 GSI features – or as TreePeople decided to call them on their map, “Watershed Solutions” – have been mapped on TreeMapLA so far, including rain gardens, rain barrels (otherwise known as cisterns or tanks), and turf/concrete reductions. These 26 features offer over $5,000 a year in savings to Los Angeles from water conservation and reduced stormwater runoff. The capability to map and track GSI features isn’t automatically available in every OpenTreeMap, however, because the scientific calculations used to generate the environmental and economic benefits of the features often require customization for each map. So what’s involved in adding green infrastructure tracking to your OpenTreeMap site?

i-Tree Streets Climate Zones map

Thoroughly researched i-Tree Streets Climate Zones cover the entire US.

The value of trees in reducing stormwater runoff and encouraging water filtration and conservation is well documented and researched. Based on research by the US Forest Service and the publication of the resulting data and algorithms in the public domain i-Tree software suite, we built our own open source tree ecosystem benefit calculation system for use in OpenTreeMap. A user can start an OpenTreeMap site anywhere in the continental United States, add a tree with info on the species and diameter, and instantly have access to the same peer-reviewed ecosystem benefit calculations as they would if they conducted an analysis project with the i-Tree Streets desktop software. For maps located outside of the continental US, such as yegTreeMap and Ecology Ottawa, we can work with a map owner and enable benefit calculations based on an approximate i-Tree Climate Zone for their area.  We have always been proponents of open data at Azavea, and OpenTreeMap is an example of exactly the type of innovation supported by open data.

Unfortunately, there is no equivalent, comprehensive “i-Tree-like” software or data we can use to support GSI benefit calculations anywhere in the country. Calculating the environmental impacts of GSI can be just as complex as calculating the environmental impacts of trees. Green stormwater infrastructure can be built for many reasons, including capturing rain and stormwater for use in building and irrigation systems; infiltrating it back into the ground water supply; and preventing it from taxing traditional “gray” infrastructure like sewers and water treatment systems, where it can also be contaminated before reaching our rivers, lakes, and streams.

So to calculate the benefits and impacts of GSI at a basic level, one needs to know how much stormwater a particular location receives! There are numerous climate regions in the US, all with different levels of annual rainfall and precipitation. Different microclimates might even exist within the same city. At a minimum an OpenTreeMap site tracking GSI features would need to draw upon a GIS data layer of precipitation levels in the geographic area covered by the tree map as well as unique  parameters and values based on the type of GSI feature (rain garden, rain barrel, green roof, etc).

At a more complex level, different types of land use (commercial and residential zoning areas, for example) might have different water usage patterns and thus benefit more or less from GSI features like green roofs or rain barrels. Some jurisdictions, like Philadelphia, may offer billing credits for green stormwater infrastructure features. The existing amount of impervious surfaces (like concrete or asphalt) in a community and the value placed on removing them might also be a factor to be considered. Finally, the impact of different types of GSI features varies based on how large a catchment area they serve, the tank storage capacity, or other design factors.

In building TreeMapLA’s watershed solutions feature, we were fortunate to work with some very smart people – our partner Kelaine Vargas of Urban Ecos, and Edith de Guzman, TreePeople’s Director of Research – to design a calculation system that took into consideration these many factors.

TreeMapLA's calculated watershed solution benefits.

TreeMapLA’s calculated watershed solution benefits.

Together, we were able to develop and integrate into TreeMapLA calculations for stormwater runoff (gallons and dollar value) and water conservation (gallons and dollar value) metrics for three different types of “watershed solutions” (rain gardens; rain barrels, cisterns, and tanks; and turf/concrete reduction projects).

Process of adding a Watershed Solution on TreeMapLA.

Process of adding a Watershed Solution on TreeMapLA.

When a user visits TreeMapLA to add one of these watershed solutions, the software guides them through a several-step process to determine and collect:

  • the type of solution
  • the location of the solution (which is connected to info on annual precipitation)
  • for rain barrels, the capacity in gallons of the barrel
  • if applicable, the catchment area or area of replaced turf or concrete associated with the solution
  • if applicable, whether the area was improved with plants and other materials that consume less water
  • if applicable, whether an irrigation system for the area existed, exists now, or was improved for efficiency
  • if applicable, whether excess runoff flows to impervious or pervious surfaces

Based on this data, OpenTreeMap decides to apply one or more equations developed by TreePeople to calculate the amount of reduced stormwater runoff and/or reduced water usage. A flowchart of this process and equations is available here.

“We embarked on this project with the purpose of benefitting other OTM users,” says TreePeople’s Edith de Guzman. Because of this preliminary work with TreeMapLA, any OpenTreeMap.org map can support green stormwater infrastructure at significantly less cost than was required to implement it the first time.

While it requires a bit of extra thinking, it’s absolutely possible to track the impact of green stormwater infrastructure projects using OpenTreeMap.. You could choose to base your map’s calculations off of TreeMapLA’s, or we’re happy to discuss other calculations suitable to your community. We’re also very interested in supporting other types of green infrastructure features such as green roofs, bioswales, and more. Let us know if you have a GSI feature you would like to track.

Even if OpenTreeMap isn’t exactly right for your GSI project, Azavea also has a broad base of expertise in other types of GIS tools and spatial analysis projects related to stormwater and watersheds. We’d love to hear from you!

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