Avoided runoff - ECO vs Streets results

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Avoided runoff - ECO vs Streets results

Post by picknb » Thu Sep 10, 2015 12:58 pm

I am modeling a large shopping mall with a 11 buildings and 416 trees. When I run ECO I get a value for avoided runoff of 34,396 gallons per year. When I do a simple check in Streets (just Species, DBH and Condition), I get 292,195 gallons per year of interception (avoided runoff).

I understand that the energy savings will be different because ECO is considers tree shading on building (distances and angles to nearest building), but why would the avoided runoff be much different?

Where can I find the details on the algorithms used by these two site-based modules?

Any suggestions on how to check what's going on?
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Re: Avoided runoff - ECO vs Streets results

Post by rcoville » Fri Sep 11, 2015 9:46 am

Hi picknb,

Thanks for your interest in using i-Tree Tools. The key to the results you're seeing is that interception in i-Tree Streets is different, and more inclusive, than avoided runoff in i-Tree Eco and the Hydro model. The following paper details the differences in methodologies between the rainfall interception model, used by i-Tree Streets and Design, and the avoided runoff model, used by i-Tree Eco: Basically, interception (in i-Tree Streets) is how much rain hits the trees, whereas avoided runoff (in i-Tree Eco) is how much rain is not becoming runoff solely due to the trees' presence.
Without any trees, not all rain would become runoff as some would infiltrate into soil or be captured in depressions; i-Tree Eco is modeling how much water would not become runoff specifically due to the trees present, and therefore i-Tree Eco has more exclusive results. i-Tree Streets is modeling how much rain is hitting the trees and is not subtracting how much intercepted rain passes through the tree due to throughfall, stem flow, and leaf drip, and therefore i-Tree Streets has more inclusive results.
Interception is still useful, even with water becoming runoff after intercepted, because interception slows stormwater which helps water integrate into subsurface flows and thereby does reduce runoff and water pollution.

For more information, you may be interested in the following papers in addition to the above.
On Eco: On Streets:
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