Avoided Runoff and Urban Forests

Eco-Values of the Urban Forest: i-Tree Analysis
Surface runoff is water that the land cannot absorb. Surface runoff, particularly from storms, can be a cause for concern in many urban areas because the large amounts of paved surfaces will increase the amount of water that cannot soak into the ground. These large volumes of stormwater runoff can carry surface impurities into streams, wetlands, rivers, lakes, and oceans, contributing pollution, garbage, and excessive nutrients into aquatic ecosystems.

Urban forests, however, are beneficial in reducing surface runoff. Trees and shrubs intercept precipitation, while their root systems promote infiltration and storage in the soil.

Avoided Runoff by NCA Urban Trees

Approximately 2.9 million cubic meters of surface runoff are avoided because of these urban forests. Manassas National Battlefield Park was not included in this analysis because reference precipitation data was not available for the reference dates.

Annual avoided surface runoff is calculated based on rainfall interception by vegetation, specifically the difference between annual runoff with and without vegetation. Although tree leaves, branches, and bark may intercept precipitation too, only the precipitation intercepted by leaves is accounted for in this analysis. Precipitation estimates are based on local weather estimates from the user-designated NOAA weather station.
Total Avoided Runoff by Parks. Prince William has the highest (1 million cubic meters/year) followed by Chesapeake and NCA East. Wolf Trap has the lowest (4,000 cubic meters/year).

Avoided Runoff Rates by Parks. Rock Creek and Prince William have the highest rates (240 cubic meters/ha/year) followed by Chesapeake. Antietam has the lowest (36 cubic meters/ha/year).

The Value of Avoided Runoff

Tree canopy captures stormwater on leaves and branches, increasing the time before rainfall reaches the ground, which is particularly helpful in reducing stormwater runoff from paved surfaces. Tree roots reduce erosion and create soil conditions that better allow rainwater to infiltrate into the ground and recharge the groundwater supply. The root systems of trees and shrubs further act as a filter system for rainwater, catching pollutants and helping purify the water.

Next page: Structural Values

Previous: Carbon Storage

Footer: silhouette of a forest in many layered greens

NPS Graphic


Environmental Protection Agency. (n.d.). Urbanization and Storm Water Runoff. Retrieved from
Ferguson, K. (2017). UERLA Resource Brief: Ecobenefits of National Capital Region Park Trees. Retrieved from

Garner, J. (2018). UERLA Resource Brief: Quantifying Ecological Benefits of Trees in the National Capital Region. Retrieved from
Urban Ecology Research Learning Alliance. (2018). i-Tree Ecosystem Analysis: Urban Forest Effects and Values. Retrieved from

Antietam National Battlefield, Chesapeake & Ohio Canal National Historical Park, George Washington Memorial Parkway, Harpers Ferry National Historical Park, National Capital Parks-East, Prince William Forest Park, Wolf Trap National Park for the Performing Arts more »

Last updated: January 7, 2022