How to Use the i-Tree Stormwater Calculator

The i-Tree Stormwater Calculator estimates the stormwater benefits of tree-planting projects over the project lifetime. Benefits focus on avoided runoff, and the subsequent reduction in watershed pollutant loads from HUC8-specific event mean concentration (EMC) values for total nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS) in runoff.

Project Steps

Successfully creating a project in the Stormwater Calculator is simple. Work from left to right through the tabs, providing the required data as you go. To move forward tabs, select Next. To move to a previous tab, you can select the desired tab itself.

1. Location

On the Location tab, identify the geographic location of your study area, beginning with the state and continuing through the fields. Location information is used to determine available species, tree growth rates, precipitation, and the watershed (HUC8) for localized export coefficient values.

After selecting a location, you will need to select a representative precipitation year. This year will be iterated over for the lifetime of the project to assume benefits and can be used to curate tree calculation benefits for potential changes in future weather patterns. For example, by selecting a year with higher precipitation, tree benefits will be representative of a wetter future climate.

2. Parameters

The project Parameters tab allows you to enter data specific to the overall project.

Land Use

  • This represents the average land use character of your project area, and is used as part of determining the event mean concentration (EMC; mg/L) values for various pollutants, which will be higher in urban and agricultural environments and lower in forested areas. Urban/developed encompasses multiple land characteristics, including lawn grasses, single-family housing units, parks, golf courses, apartment complexes, and commercial/industrial use.

Soil Texture

    Soil texture determines the macropore size, infiltration rate, and other hydraulic properties of the soil in the project area. If you want to enter a custom soil composition (based on percentage of sand, silt, and clay), you can select Custom from the dropdown menu. This will open a column on the right side of the tab where you can enter soil percentages. The total must equal 100%.

Percent Impervious Cover

    Average impervious cover under the tree planting area. For estimates on impervious cover percentage, you can consult a pre-processed dataset such as i-Tree Landscape, or create a custom survey in i-Tree Canopy.

Years for the Project

    The i-Tree Stormwater Calculator will project the benefits for up to 99 years into the future. Enter the number of project years here.

Custom event mean concentration (EMC) values

    If you have known event mean concentration (EMC) values, in mg/L, for your project area, select this option and enter your custom values in the center column. This will override the HUC8-specific values, which are based on both land use and the percentile of annual precipitation for the selected project year.

3. Trees

The stormwater benefits of a project depend on a variety of factors, including tree species, size, condition, and lighting. Under the Trees tab, enter each unique configuration of these elements and the number of trees that feature those particular elements.

Tree Information

  • Species

    • Using the drop-down menu, indicate the species of one of the configurations of trees to be planted. You can toggle between scientific and common names with the buttons at the top. If your species is not available, choose the most closely related species (consider family relationships, growing form, and mature size). If that is not feasible, you can choose the most appropriate of the "Other" categories of trees.

  • DBH

    • Enter the DBH at the time of the project start. In most cases, this is the DBH at planting.

    Tree Details

  • Condition

    • The condition of the trees will affect how well they grow and thus future benefits. Enter the appropriate condition of the trees here. Given that this tool does not calculate percent mortality similar to other i-Tree Tools, it is recommended to adjust this condition to best fit your moratlity. If high mortality is expected, put "critical" or "dying" for tree plantings; if maintenance is planned for trees, "Fair" or "Good" will account for slower growth. "Excellent" conditions will account for no dieback or slowed growth, and best represent small-scale, optimal planting scenarios.

  • Exposure to Sunlight

    • The exposure to sunlight affects both how the trees grow and the degree to which a new tree adds shade to a building. Enter the appropriate exposure to sunlight here. For new trees, this will likely be "Full sun."

  • Number of Trees

    • Enter the number of trees with this configuration, meaning this combination of species, DBH, distance and direction to the nearest building, and other details.

    Additional Configurations

    To add a new configuration, select the "+" button on the left. A new row will be added where the above data can be entered for the new group of trees.

    Report

    The results are presented under the Report tab. The project summary is shown at the top. Each configuration that you entered on the Trees tab is shown as its own row, with a summary of the characteristics to identify the group. Benefits summed for the first year, last year, and lifetime of the project are summarized in four green tabs above the table:

    Summary

    Displays final growth DBH and canopy cover for the tree group, in addition to the cumulative hydrological benefits.

    Growth

    Displays information on the input tree conditions, final DBH, final canopy cover area, and the maximum leaf area index (LAI) at the final year of benefits. Leaf area index plays a large factor in how much rainfall is intercepted by trees, affecting final runoff values.

    Hydrology

    Hydrological benefits show the first, last, and cumulative runoff benefits. Reduction in runoff comes from a number of processes modeled by i-Tree Hydro, including the interception by leaves and the effect of growing tree roots on soil macropore size.

    Leaf Phosphorus

    Leaf phosphorus loading, which represents the amount of phosphorus returned to the watershed from unswept leaf litter, is available in select locations. Based on data from Madison, Wisconsin (see references), these values are available for select locations across Iowa, Minnesota, Michigan, and Wisconsin. This typically represents a net reduction in overall benefits and can be a helpful tool in planning where to prioritize street sweepings for high-leaching species concentrated in parts of a city. Species selected 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, Wisconsin.

    Units and Export Options

    Results can be viewed in English or metric units by toggling between the buttons above the table. The blue buttons immediately above the table allow you to copy the rows into your clipboard, download them as a .csv file, or save the project.