How can Green Infrastructure be assessed in i-Tree Hydro v5?

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How can Green Infrastructure be assessed in i-Tree Hydro v5?

Post by rcoville » Wed Jul 29, 2015 3:04 pm

For now, we are limited in how we can model Low Impact Development (LID) and Green Infrastructure (GI) strategies in i-Tree Hydro v5.0. We can do so with basic land cover conversions or fairly technical changes to hydrological parameters. In addition to the challenges inherent in modeling green infrastructure in i-Tree Hydro, these types of Hydro projects tend to be on small, site-scale, non-watershed areas. Modeling such areas is a relatively new feature of this tool in itself. Green Infrastructure assessment is an innovative use of the tool and is not what Hydro has been designed for, but user feedback has guided our development team to work more on the subject. Here is some information about where we're at with this now:
Report on Site-Scale GI for Gateway Center in i-Tree Hydro v5-0-9_Draft.pdf
How can different LID strategies be translated into i-Tree Hydro now (v5.0)?
- Rain gardens, bioretention basins, bioswales, and filter-strips can be translated as the land cover they're mainly composed of when viewed from above. For example, a swale covered in grass would be herbaceous cover, a filter strip covered mostly with shrubs and trees would be tree cover and shrub cover).
A main limitation on this method, put simply, is that the model does not currently consider spatial relations between cover types, and thus cannot appropriately represent the benefits a bioswale (and other kinds of GI) provide due to those benefits coming mostly from spatial qualities of direction, topography, etc. This spatial limitation will be overcome when the model runs spatially-distributed, which is something already coded and in-store for a version 6 or other update down the line.
- Rain barrels, dry wells, and cisterns can be simulated (requiring some technical knowledge) by modifying hydrological parameters, specifically 'Impervious Depression Storage', to reflect the additional water storage these installations will provide. For example, the increased water storage and diversion rain barrels provide could be simulated by using the rain barrel's catchment area to derive an Impervious Depression Storage (IDS) amount that would be added to the existing IDS for the site.

There are other ways hydrological parameters could be manipulated to more accurately represent bioretention basins, tree pits, and other rain gardens, but they become very technical and are also very specific to certain types of project, so we can't provide a standard method of how to do so in version 5 of Hydro.
An example of this more complicated hydrological parameter manipulation: an infiltration trench could be simulated by representing it as soil, then changing the hydrological parameters to represent all soils as having super-soil properties. This effectively represents soils as having great capacity for receiving and storing water while slowly evaporating and infiltrating water into the surrounding ground or sewer system - basically the function of infiltration trenches. However, based on the current version of i-Tree Hydro, this technique only works if those super-soil properties can be applied to all non-impervious surface on site. So if your entire project is a parking lot that you add LID strategies too, all of which translating into the same super-soil characteristics, you can use this technique. If some of your soil cover and tree cover should not have super-soil characteristics, this technique is not appropriate as the hydrological parameters are applied project-wide and affect how water interacts with trees and other non-impervious land cover types.

We're working to standardize and simplify procedures for modeling GI in Hydro. As of now model developers from SUNY College of Environmental Science and Forestry are working to incorporate the following GI strategies as Step 2) Land Cover Parameter options: Tree Pits, Rain Gardens, Green Roofs, Rain Barrels, and Porous Pavement. Other features are also in the works that will greatly improve the process of assessing how a strategically planted and specially maintained tree has different effects than simply increasing % tree cover in an area.

If you have ideas or suggestions about this please send them to - it is with your feedback that i-Tree continues to be updated and improved upon!
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