Question: How can reduced stormwater flow and pollution be quantified into money saved by a municipality in the northeast USA?
Answer: Reduced stormwater flow and pollution is valued in i-Tree as 'net avoided runoff', meaning the amount of surface runoff (and associated pollutants) that no longer need to be managed due to the effects of trees. i-Tree Eco valuates avoided runoff at a national average of $0.008936/gallon based on the USFS' Community Tree Guide series, which estimates that value regionally nationwide based on stormwater treatment and management costs or fees. In the Northeast Community Tree Guide, the value is higher than the national average at "$0.01 per gallon of treated and controlled runoff" based on the following document:
McPherson, E.G.; Simpson, J.R.; Peper, P.J.; Gardner, S.L.; Vargas (Ravdin), K.E.; Xiao, Q. 2007. Northeast community tree guide: benefits, costs, and strategic planting. - View Document [4.24 MB]
'Avoided Runoff' in i-Tree Eco means the amount of surface runoff without any trees minus the amount of surface runoff with current tree cover. i-Tree Eco uses simplified Hydro simulations to estimate net avoided runoff (Hirabayashi, 2013).
In many Hydro projects, there will be more variables at play between land cover scenarios than what is modeled in Eco for its hydrology estimates, so avoided runoff in Hydro can't always be attributed to tree cover alone. You could still apply the Community Tree Guides value at your own discretion, with $0.008936/gallon being the national average used in Eco and $0.01/gallon being the Northeast regional reference value.
To meet the criteria of "per gallon of treated and controlled runoff" for that Northeast valuation, I recommend only applying it to impervious surface runoff with the assumption that all of that flow, and only that flow, will need treatment and control. How appropriate that is may differ from place to place. To meet the criteria of "benefit provided by trees", I recommend setting up your Hydro project to compare your Base Case (actual conditions of interest) with a '0-Tree Alternative Case' that has all its tree cover removed and replaced with whatever type of cover was beneath it (Herbaceous % taking place of Previous Cover under Tree Canopy, Impervious % taking place of Impervious Cover under Tree Canopy). The difference between Impervious Surface Runoff in the 0-Tree Alternative Case vs. in the Base Case is 'Net Avoided Runoff by Trees'.
Section for FAQs regarding i-Tree Hydro
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