Overview

Minnesota was the first State to develop a robust, 

science-based approach for crediting engineered tree Best 

Management Practices (BMPs) within State stormwater 

regulations. With funding allocated in 2009 from the 

State legislature, the Minnesota Pollution Control Agency 

convened the Minimal Impact Design Standards (MIDS) 

Working Group to develop new standards that would 

ultimately be adopted into the Minnesota Stormwater 

Manual (Minnesota Pollution Control Agency 2013). 

Sub-committees were formed to develop stormwater 

credits and design specifications for a suite of green infrastructure BMPs, including one focused on trees. The tree 

BMP sub-committee was interested in credits for retaining 

existing trees but ultimately adopted the Tree Trench/Box 

credit, which was easiest to quantify and justify in stormwater standards. One valuable feature of Minnesota’s 

crediting approach is that it encourages well-designed tree 

BMPs with optimal uncompacted soil volume to maximize 

tree growth and function in processing stormwater runoff. 

Key elements of the Minimal Impact Design Standards 

include the following:

» Stormwater volume performance goal for new development and redevelopment projects with greater than 

1 acre of new impervious surface. 

» Requires post-construction runoff volume to be retained onsite for 1.1 inches of runoff from impervious 

surfaces. 

» Standardized credit calculations and design specifications for a variety of green stormwater infrastructure 

(GSI) BMPs, including: green roofs, bioretention 

basins, infiltration basins, permeable pavement, 

infiltration trench/tree box, swales, filter strips, and 

sand filters.

» A model ordinance package that helps developers and 

communities implement the new standards.

The MIDS approach has received widespread national 

attention for its innovative and robust crediting approaches. The unique manual was designed as an online 

Wiki format so that it could be easily adapted over time 

with new science, technical, and stakeholder input. It has 

been revisited and updated each year.

The science behind it

The Tree Trench credit methodology was developed by 

Kestrel Design Group and contract team, with oversight 

from the tree BMP sub-committee and multiple rounds of 

stakeholder input (Kestrel Design Group Team 2013). It is 

based on an extensive literature review of tree interception, evapotranspiration, and infiltration functions. Based 

on mean values found in Breuer and others (2003), the 

interception capacity is assumed to be 0.043 inches for 

a deciduous tree and 0.087 for a coniferous tree, and the 

canopy projection area is based on the diameter of the 

canopy at maturity, dependent on the tree species. The 

Stormwater Credits for Trees: 

Minnesota Case Study

U.S. Department of Agriculture

Where: Minnesota Stormwater Program 

When: Adopted in 2013 in the online Minnesota 

Stormwater Manual

What:

» Volume reduction credit for engineered Tree 

Trench/Box practices based on interception, 

evapotranspiration, and infiltration.

» Annual pollutant removal credits for total 

suspended solids (TSS) and total phosphorus 

(TP) are calculated based on volume reduction.

» Requires that users enter soil volume, treatment area, tree size, and other inputs into the 

Minimal Impact Design Standards Calculator.

Quick facts

Forest Service FS–1146b JUNE 2020
MIDS calculator provides default tree size options (small/

medium/large) that can be used in place of tree species. 

The team’s report reviews the pros and cons of a 

variety of methods for quantifying evapotranspiration, 

recommending use of the Lindsey-Bassuk (1991) single 

whole tree water use equation. This method relates the 

total water use of a tree to four measurements: (1) canopy 

diameter, (2) leaf area index, (3) the evaporation rate per 

unit time, and (4) the evaporation ratio. 

Pollutant removal for infiltrated and evapotranspired 

water is assumed to be 100 percent and is calculated by 

multiplying the volume of water reduced by event mean 

concentrations for Total Suspended Solids (TSS) and Total 

Phosphorus (TP) from the International Stormwater 

Database, version 3.

How the credit works

Minnesota provides a total runoff volume reduction credit 

for Tree Trench BMPs, by adding together the reductions 

provided by tree canopy interception, soil storage (infiltration), and evapotranspiration. The interception credit is a 

function of tree type and projected leaf area at maturity. 

The storage credit is a direct function of soil volume. The 

evapotranspiration credit is a function of plant available 

water and is indirectly related to soil volume (e.g., available pore space). The total runoff volume achieved for a 

particular storm is calculated as the lower value of the 

total runoff volume directed to the tree trench and the 

total storage provided by that trench through interception, 

infiltration, and evapotranspiration. The total volume 

reduction is also translated into annual pollutant removal 

values for TSS and TP. A Tree Trench BMP without 

an underdrain is assumed to remove 100 percent of 

pollutants, while a Tree Trench with an underdrain 

provides lower volume reduction and pollutant 

removal credits 

(Figure 1).

To calculate the credits, users must enter into 

the MIDS Calculator a suite of inputs based on the 

design of the particular Tree Trench BMP such as: 

» Site Characteristics 

• watershed area/land cover draining to the 

Tree Trench BMP 

• downstream/routing BMP

» Soil/Media Characteristics 

• soil volume of the tree box

• hydrologic characteristics of 

the soil

» Tree Characteristics 

• number of trees 

• most common tree type (deciduous or coniferous) 

• average tree size at maturity (small/medium/large)

Figure 2 shows one of the input screens for the MIDS 

calculator, demonstrating how the volume reduction 

credits are calculated based on the Tree Trench BMP 

characteristics provided. The figure illustrates how, in this 

crediting approach, the volume reduction based on soil 

storage (1201 cubic feet) far exceeds the volume reductions 

for evapotranspiration (72 cubic feet) and interception (5 

cubic feet). Thus, the credit incentivizes providing ample 

soil volume and high quality, uncompacted soil media that 

will promote infiltration and storage in the short term 

and enable trees to grow to their optimal size. A helpful 

summary and example of Tree Trench credits using the 

MIDS calculator is included in the Center for Watershed 

Protection’s Accounting for Trees in Stormwater Models

(Center for Watershed Protection 2018a). Detailed technical information on the credit equations, input definitions, 

and other guidance can be found in the online Stormwater 

Manual section Calculating Credits for Tree Trenches 

and Tree Boxes.

In developing the credit calculations, it is assumed the 

tree practice is properly designed, constructed, and maintained in accordance with guidance in the tree section of 

the Minnesota Stormwater Manual. The manual website 

notes that if any of these assumptions is not valid, the BMP 

may not qualify for full credit. 

Figure 1. The Minimal Impact Design Standards (MIDS) total volume reduction is 

translated into annual pollutant removal values for Total Suspended Solids (TSS), 

particulate phosphorus (PP), and dissolved phosphorus (DP). A Tree Trench Best 

Management Practice (BMP) without an underdrain is assumed to remove 100 

percent of pollutants, while a Tree Trench with an underdrain provides lower 

volume reduction and pollutant removal credits based on the type of media used.

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Figure 2. One of the Minimal Impact Design Standards (MIDS) Calculator Tree Trench Best Management Practice (BMP) 

input screens showing tree and soil inputs (white boxes) and model outputs (gray).

For more information on crediting trees in the context of stormwater management, refer to the 

suite of resources developed by the Center for Watershed Protection on Making Urban Trees Count. 

Adapted from: U.S. Department of Agriculture, Forest Service. 2020. Urban forest systems 

and green stormwater infrastructure. FS-1146. Washington, DC. 23 p.

https://www.fs.usda.gov/sites/default/files/fs_media/fs_document/Urban-Forest-Systems-GSI-FS-1146.pdf

A PRODUCT OF THE FOREST SERVICE NATIONAL URBAN FORESTRY TECHNOLOGY AND SCIENCE DELIVERY TEAM

USDA is an equal opportunity provider, employer, and lender.

Some of the model inputs used in the MIDS calculator 

for Tree Trench practices are only applicable to Minnesota 

and similar climates, so it is not recommended to use the 

calculator itself beyond those geographic zones. However, 

the equations and calculations behind the credit could 

readily be adapted for other climate zones.

References

Breuer, L., Eckhardt, K.; Frede, H.G. 2003. Plant parameter 

values for models in temperate climates. Ecological 

Modelling. 169: 237-293.

Center for Watershed Protection. 2018a. Accounting for 

trees in stormwater models. Ellicott City, MD: Center for 

Watershed Protection. 17 p. https://owl.cwp.org/mdocsposts/accounting-for-trees-in-stormwater-models/. (15 

May 2019).

Kestrel Design Group Team. 2013. The roles and effects 

of tree evapotranspiration and canopy interception in 

stormwater management systems and strategies: A review 

of current literature and proposed methodology for 

quantification in the MIDS Calculator application. MPCA 

Stormwater Manual Revisions. Prepared by the Kestrel 

Design Group Team for the Minnesota Pollution Control 

Agency. 21 p. https://stormwater.pca.state.mn.us/index.

php/File:Trees_Task_13_ET_interception_credits.docx.

(15 May 2019).

Lindsey, P.; Bassuk, N. 1991. Specifying soil volumes to 

meet the water needs of mature urban street trees and 

trees in containers. Journal of Arboriculture. 17(6): 141-149.

Minnesota Pollution Control Agency. 2013. Minnesota 

Stormwater Manual. Published in web-based format 

only. https://stormwater.pca.state.mn.us/index.

php?title=Main_Page. (15 May 2019).

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