Minnesota's Evapotranspiration Credit System for Trees
Last year, The Kestrel Design Group developed a formula for the Minnesota Pollution Control Agency to estimate evapotranspiration benefits from trees to include in their stormwater crediting system. This formula is one of the first – or possibly the first – in the country to formally quantify this benefit. I spoke to Nathalie Shanstrom, a landscape architect with The Kestrel Design Group who participated in developing the formula, about what the process was like, the true value of evapotranspiration benefits, and what implications this credit system has for tree benefits going forward.
Did Kestrel Design Group develop this evapotranspiration calculator? What was that process like?
Yes, we developed it for Minnesota’s MIDS (Minimal Impact Design Standards) calculator as part of a contract to develop technical content for a new chapter on Tree Stormwater SCM’s (Stormwater Control Measures) for the Minnesota Stormwater Manual, which is published by the Minnesota Pollution Control Agency (MPCA). I want to clarify, though, that it’s not a calculator, it’s actually a formula that is based on a huge literature search that we conducted to review all the approaches for estimating evapotanspiration for Minnesota.
Based on that search, we selected the criteria and methods of measurement that we feel are the best way to estimate evapotranspiration for the purposes of “back of the envelope” stormwater benefit calculations like those in the MIDS calculator at this time. There are a lot of complicated ways to estimate evapotranspiration based on field measurements and other factors, but few people would have that data, and expecting anyone to calculate it using that data is therefore not realistic. We tried to use a formula that was within reach for anyone who wants to get credit for trees for stormwater – planners, developers, landscape architects, builders, and more. What we settled on is derived from a formula developed by Lindsey and Bassuk in 1991 that we adjusted factors on to apply to Minnesota.
Where and how will the formula be used, and on what kinds of projects?
Minnesota’s new Minimal Impact Design Standards (MIDS) focus on treating rain where it falls to minimize negative impacts from stormwater runoff and to preserve natural resources. As summarized on the MPCA website, MIDS contains four main elements:
- A stormwater volume performance goal for new development, redevelopment and linear that will provide enhanced protection for Minnesota’s water resources.
- New credit calculations that will standardize the use of a range of innovative structural stormwater techniques.
- Design specifications for a variety of green infrastructure best management practices (BMPs).
- A model MIDS ordinance package that will help developers and communitiesimplement MIDS
Has Minnesota officially accepted this formula? Where is it incorporated in to their policies?
The formula hasn’t been incorporated in to their official calculator yet, but the methodology is accepted and is on their website). Anyone who wants to can read more about it there and do the manual calculations. We’re expecting it to be added to the official calculator soon. Our whole literature review is on the website too, in case anyone wants to read it.
What is evapotranspiration, and how is water that is evapotranspired different from water that is held in the soil?
Evapotranspiration is the combination of water evaporating from the soil and transpiration from the plants growing in the soil. In other words it is the transfer of water from the soil to the air by evaporation of water directly from the soil, and water taken up by plants and released to the air as vapor through stomata in the plants’ leaves. Once a tree takes up and transpires water from the soil, it makes room in the soil for more runoff to be stored. Evapotranspiration capacity for trees is closely tied to the canopy size.
If evapotranspiration capacity is related to canopy size, how does the formula take that into account? Plenty of street trees have very small canopies.
Tree size is very important to evapotranspiration benefits, and we know a large tree has way more capacity than a small one.
That presents a challenge with allocating stormwater credits for ET, because the credit amount is determined during the design phase of a project, before the trees are even planted, and typically doesn’t change throughout the lifespan of the tree – when it will presumably be growing and providing increasing benefits. To maximize incentive to plant trees for stormwater, we recommended giving credits based on the projected mature canopy size for trees that are planted and maintained correctly and provided with adequate rootable soil volume. This is to incentivize planning for a mature tree, which means taking the available soil in to account. If you plant a tree with less soil than it needs to meet a mature size, the credit received is affected.
For example, if you planted a tree that can have an 800 square foot canopy at maturity, but you’re only giving it enough soil to reach a 400 square foot canopy, you will only get credit for the smaller tree. We based the soil requirements on the assumption that a tree needs two cubic feet of soil for a square foot of canopy area.
What are the implications of this formula for landscape architecture designs going forward?
To incentivize planting and maintaining trees correctly so they can provide the level of stormwater benefits they are capable of providing at their mature size.
You did an exercise to show us how this formula might be applied to a streetscape in Toronto. It shows that eight trees, five years after planting, can evapotranspire all the runoff from a 5mm (0.2in) rain event over three days and that eight trees, at maturity, can evapotranspire all the runoff from a 13mm (0.5in) rain event over three days. That’s a significant amount of water.
Evapotranspiration can be a very significant benefit of trees in the urban environment, especially during the growing season. When it’s really hot out, sometimes all the rain that falls can be evapotranspired. In some cases, researchers have found that if you water a tree, the evapotranspiration can actually exceed the amount of rainfall. Of course, evapotranspiration is seasonal, and drops significantly in winter, when the tree has no leaves. The formula takes this in to account by giving a yearly average – so it’s just a model. The actual numbers will depend a lot on the weather, the amount of rain, the irrigation, etc., and will vary from year to year and month to month.
Is Minnesota the first state to accept evapotranspiration in their stormwater calculations?
I don’t know of any other states that have attempted to quantify evapotranspiration in their stormwater calculations for tree SCM’s, There may be some continuous models that incorporate it but I don’t know of any event based calculations that do. For any of the more technical people out there, we assumed three days in between rain events.
To our knowledge, This is the first one that gives evapotranspiration credit on a tree by tree basis and factors in tree size. In fact, the formula also gives credit for tree interception and infiltration, which is uncommon.
One of the U.S. Forest Service foresters who was on our advisory team is always telling us that other states look at what Minnesota is doing in regards to crediting evapotranspiration and trees and stormwater generally. So we’re hopeful that other places will consider doing something similar.
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