References
The i-Tree Stormwater Calculator is designed to help you estimate the long-term hydrological benefits of a tree planting project. This tool does not use the same values as other i-Tree hydrological estimates (Eco, Planting, MyTree) because it is powered by i-Tree Hydro, a semi-distributed, physical-based urban water balance model.
The Stormwater Calculator results are retrieved from nationwide simulations of i-Tree Hydro from 2010–2019. The following varieties of simulations were run for each area, with each variation independently changed:
- Different soil types
- Range of leaf area index values
- Range of impervious cover
- Replacement of trees with bare soil
With this matrix of simulations, we can extract runoff benefits from tree cover across soil texture, varying canopy thickness, and for a range of impervious cover underneath trees. The contiguous United States was split into quadrant simulation areas to provide an accurate modeling of varying topographic indexes and soil saturation. This varies from other i-Tree tools’ hydrological benefits, which are county-wide averages of i-Tree Hydro runs, providing less nuance for the adjustable parameters available in the Stormwater Calculator.
Leaf nutrient inputs are also assessed for select locations across IA, MN, MI, and WI based on incorporated results of phosphorus leaching experiments of leaves from select tree species by the University of Wisconsin–Madison (Klaubauf, 2024). Species identified in this project are not based on planting recommendations; they were selected as part of a list of most common species across the contiguous United States, with a large presence in Madison, WI. Based on the large variation of samples of a common species across the United States, locations eligible for this data were limited to an area of similar latitude and precipitation.
The Stormwater Calculator was created to be the first tree-based, as opposed to canopy-based, stormwater credit calculator to assist in species selection. Leaf nutrient inputs to the watershed can inform best management practices of priority street sweeping species and areas within a city.
Hydrology
- Avoided Runoff — Calculated by subtracting the difference in annual runoff from the simulation with trees, to the matching soil and impervious cover simulation without trees. Examples of how trees reduce runoff include the interception of rainfall by tree leaves and storage of water in roots for eventual transpiration. Monetary values of runoff are calculated at $0.008936/gallon, for more information on this figure see Understanding i-Tree (2023)
- Total Avoided TN/TP/TSS — White et al. (2015) provides event mean concentration (EMC) values for total nitrogen, phosphorus, and suspended solids, based on both land type and percentile of precipitation for the year. Based on the annual precipitation rate and selected land use by the user, mg/L EMC values are extracted based on the percentile of precipitation for the watershed that the project is located in. This loading is applied to the reduced runoff value, converted to liters, to derive the milligrams of nutrients that were not put into the watershed because of the tree.
Leaf Phosphorus
The leaf biomass of each species is used to derive milligrams of phosphorus, based on the leaf weight in grams. If total initial phosphorus is equal to TKP/DRP, all the P content from the species is organic and considered leachable by water, benefitting the most from priority for street sweeping in urban areas. Conversely, a low TKP/DRP as the percentage of total phosphorus identifies the species as mostly inorganic phosphorus, with lower impacts on runoff quality.
- Total (Initial) Phosphorus — In the leachable phosphorus study, total leaf P content was analyzed by inductively coupled plasma (ICP) mass spectrometry. This represents the maximum amount of phosphorus in leaf litter that could potentially be extracted by water. Another way to refer to this variable is the initial phosphorus content.
- Leaf TKP — Kjeldahl phosphorus (TKP) is measured as a representation of the total organic phosphorus. TKP is a reasonable estimation of total organic phosphorus in leaf litter, because of the low concentrations of DRP previously shown to exist in leaf litter leachate.
- Leaf DRP — Dissolved reactive phosphorus (DRP) is representative of the measured 48-hour leaching of phosphorus. Two days were chosen as a reasonable time frame for species to reach an asymptote of DRP concentration.