Overview

In 2010, the U.S. Environmental Protection Agency (EPA) 

established the Chesapeake Bay Total Maximum Daily 

Load (TMDL)—or “pollution diet”—to reduce the amount 

of nitrogen, phosphorus, and sediment entering the Bay 

through the region’s waterways. The TMDL covers 64,000 

square miles that stretch across parts of six States and the 

District of Columbia. Each of these jurisdictions has committed to reaching ambitious pollutant load reductions by 

2025, as documented in phased watershed implementation 

plans. In order to track and credit progress towards these 

targets, the States and the District of Columbia must provide detailed reporting of the number and type of approved 

BMPs implemented on all agricultural and urban lands. 

While the Chesapeake Bay TMDL and modeling tools 

have always assigned low pollutant loading rates to 

forest land cover, they did not have a way to account for 

and credit the water quality value of urban tree canopy 

(individual and small patches of trees in developed areas 

not large enough to be classified as forest). Thanks to 

investments by the Chesapeake Bay Program partners in 

high-resolution land cover data, distinct mapping of forest, 

urban tree canopy over turf, and urban tree canopy over 

impervious cover became available in 2016. 

A BMP expert panel was convened in 2015 to provide 

recommendations on how urban tree canopy (including urban tree planting) should be credited in the TMDL context. 

All documentation of the literature, modeling approaches, 

and crediting decisions are provided in the report the 

panel developed (Law and Hanson 2016). Following review 

and revision with Federal, State, and other stakeholders, a 

new BMP credit for urban tree canopy expansion, as well 

as a higher credit for urban forest planting (i.e., reforestation of developed/turf areas) were officially adopted in 2016 

for use in the TMDL. Having tree BMP credits approved 

for use in the TMDL has helped incentivize the District 

of Columbia and other local jurisdictions to include tree 

planting targets as part of their Municipal Separate Storm 

Sewer System (MS4) permits.

The science behind it

The tree canopy BMP expert panel, with support from the 

Center for Watershed Protection, completed a thorough 

literature review on the water quality benefits of urban 

trees and existing tree crediting approaches. Hynicka and 

Divers (2016) constructed a water-balance modeling approach to estimate pollutant loading rates for tree canopy 

over turf grass, tree canopy over impervious cover relative 

to turf, and impervious cover without trees. To account 

for spatial and temporal variation in precipitation, 11 

years (2005 to 2015) of daily weather data were used from 

each of 8 regional locations spanning the Chesapeake Bay 

Watershed. The relative pollutant load reductions are summarized in Table 1. 

Stormwater Credits for Trees: 

Chesapeake Bay Case Study

U.S. Department of Agriculture

Where: Chesapeake Bay Watershed 

(DC, DE, MD, NY, PA, VA, WV)

When: Adopted in 2016 as approved Total Maximum 

Daily Load Best Management Practice (BMP) credits 

by Federal and State agencies

What:

» BMP credits are earned for urban tree canopy 

expansion for dispersed plantings over turf or 

impervious surface and urban forest planting 

for full reforestation.

» Tree canopy is mapped and credited as a land 

use class in the Chesapeake Bay model, with 

reduced pollutant loading relative to turf or 

impervious cover.

» States get credit for newly planted trees for 10 

years, after which the tree canopy is tracked 

directly through high-resolution imagery.

Quick facts

Forest Service FS–1146a JUNE 2020
The expert panel used a variety of tree species, growth, 

and mortality scenarios in i-Tree Forecast to establish an 

average canopy acreage credit per tree planted (144 square 

feet per tree, or approximately 300 trees per acre).

How the credit works

Under the Chesapeake Bay modeling and TMDL framework, every acre of land in the watershed has a designated 

land use class and associated pollutant loading rate, based 

on high-resolution land cover mapping, other datasets, 

and best available science. Like many BMPs in the TMDL 

framework, the urban tree canopy BMPs are credited 

based on a land use change or the conversion of a given 

acreage of land from a higher loading land use (e.g., turf 

grass or impervious cover) to a lower loading land use 

(urban tree canopy or forest). For these land use change 

BMPs, States, and local governments track and report 

the total acreage of each BMP implemented on an annual 

basis, and the Chesapeake Bay modeling tools calculate 

the resulting pollutant reductions. 

The urban tree canopy expansion BMP

includes tree planting projects on developed 

land that increase the tree canopy overlying 

turf or impervious surfaces but do not create 

forest-like conditions. Trees do not have to 

be planted in a single contiguous area. Trees 

planted in a riparian forest buffer or as part 

of a structural BMP, such as bioretention 

practices, are not included; these are tracked 

under separate BMP credits. Each tree planted 

is given credit for creating 144 square feet of 

urban tree canopy (equivalent to 300 trees 

per acre), which reflects average growth at 10 

years after planting. The credit is calculated within the 

Chesapeake Bay model based on the percentage reduction 

in nitrogen, phosphorous, and sediment pollutant loads 

relative to the underlying land use cover. 

The urban forest planting BMP includes projects that 

create forest-like conditions. Trees must be planted in a 

contiguous area specified in a documented planting and 

maintenance plan and conform to the State’s planting 

density and associated standards for forest conditions. 

Urban forest planting BMPs result in a change of land use 

from turf grass to forest land. The credit for this BMP is 

calculated based on the difference between the land use 

loading rate of turf grass and forest land across the acreage of the urban forest planting.

For both BMP credits, the credit expires after 10 years, 

at which point the canopy coverage is assumed to be 

tracked and directly credited as a land use through new 

high-resolution imagery/land use data.

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.

Table 1. Tree canopy relative land use loading rate reductions in total 

nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS) 

in relation to underlying land use cover 

Land use Total nitrogen 

reduction (%)

Total 

phosphorus 

reduction (%)

Total suspended 

solids (TSS) 

reduction (%)

Canopy over turf 23.8 23.8 5.8

Canopy over roads 8.5 11.0 7.0

Forest 85.0 90.7 81.6*

*

Percent reduction is based on an average Municipal Separate Storm Sewer System 

(MS4) land use loading rate for sediment. Source: Hynicka and Divers 2016.

References

Hynicka, J.; Divers, M. 2016. Relative reductions in non-point source pollution loads by urban trees. In: Recommendations 

of the expert panel to define BMP effectiveness for urban tree canopy expansion. Ellicott City, MD: Center for Watershed 

Protection and Chesapeake Stormwater Network: 1-26. https://owl.cwp.org/mdocs-posts/recommendations-of-theexpert-panel-to-define-bmp-effectiveness-for-urban-tree-canopy-expansion/. (8 October 2018).

Law, N.; Hanson, J. 2016. Recommendations of the expert panel to define BMP effectiveness for urban tree canopy expansion. Ellicott City, MD: Center for Watershed Protection and Chesapeake Stormwater Network. 237 p. https://owl.cwp.org/

mdocs-posts/recommendations-of-the-expert-panel-to-define-bmp-effectiveness-for-urban-tree-canopy-expansion/. 

(8 October 2018).

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