Trees: A Green, Cost Effective
Stormwater Management Practice
Vincent Cotrone
Extension Urban Forester
Penn State University
Trees & Stormwater
• Interception by
Canopy
• Increase Infiltration
into Soils
• Evapotranspiration
• Pollutant Removal &
Phytoremediation
• Soil Stabilization
• Streamside Buffers
Rainfall Interception by Tree
Canopies
• Average interception of
rainfall by tree canopy cover
ranges from 12-40% (varies
by rainfall event).
• Interception:
mature deciduous
500 to 2000 gallons per year
mature evergreen
4,000 gallons per year.
• In one study, existing
canopy in Dayton, Ohio
reduced runoff by 7% and
could be increased to 12%
by planting more trees.
1 inch of rainfall per acre is equal
to 27,000 gallons of stormwater
Rainfall Interception by a Tree
• Callery Pear – 9 yrs old, 28 ft tall, 19 ft crown
diameter, 276 sq ft crown
projection area, 3,846 sq ft leaf
area, 446 sq ft stem area.
• 0.5 inches of rain
• Total precipitation in
crown projection area =
86.1 gallons
• Total tree interception =
58.1 gal gallons or 67% of
the rain falling within the
canopy
USDA Forest Service Research at the Western Center for Urban Forestry, Davis, CA
Xiao, Q.; McPherson, E.G.; Ustin, S.L.; Grismer, M.E. 2000. A new approach to modeling tree rainfall interception. Journal of
Geographical Research Atmospheres 105: 29173-29188.
Interception By Species
20 Year Old Trees
•
•
•
•
•
•
•
Crabapple
22ft tall x 21ft spread
Kwanzan Cherry
17ft tall x 17ft spread
Red Oak
40ft tall x 27ft spread
Red Maple
29ft tall x 24ft spread
Zelkova
38ft tall x 34ft spread
White Pine
32ft tall x 20ft spread
Hackberry
144 gal/yr
312 gal/yr
767 gal/yr
1,014 gal/yr
1,624 gal/yr
786 gal/yr
1,394 gal/yr
47ft tall x 37ft spread
Source: USDA FS PSW-GTR-202 August 2007 & PSW-GTR-199
Gallons of Stormwater
Intercepted per year
Interception by
Hackberry
Tree Age in Years
Source: Adapted from McPherson et al 2006
NYC Street Tree Interception
Average Interception by Species
London Plane – 2,875 gal/yr
Silver Maple – 2,948 gal/yr
• New York City’s street
trees intercept rain,
reducing stormwater runoff
by 890.6 million gallons
annually, with an
estimated value of $35.6
million.
• Citywide, the average tree
intercepts 1432 gallons of
stormwater each year,
valued at $61 per tree.
Source: NEW YORK CITY, MUNICIPAL FOREST
RESOURCE ANALYSIS April 2007 , Peper,
McPherson, Simpson, et.al.
Pittsburgh Stratum Project
$334,600 in stormwater reduction savings
(42 million gallons or 1,400 gallons per tree) annually.
Interception
Avg. 500 – 2000 gal/tree/yr
Increase Urban Tree Canopy
Urban Tree Interception rates
exceeded 40% for small storm
events, but were less than 4%
for large storm events (Wang,
Nowak, Endreny 2006).
Increasing canopy cover
over impervious surface
had the greatest effect on
reducing runoff.
UFORE Hydro Study Dead Run,
Baltimore, MD
Interception
Depends on Rain Event
Xiao, Q.; McPherson, E.G.; Ustin, S.L.; Grismer, M.E.; Simpson, J.R. 2000. Winter
rainfall interception by two mature open‐grown trees in Davis, California.
Hydrological Processes 14:763‐784.
Infiltration
• Urban/Suburban Soils
become highly
compacted
(1.8 – 2.0 Bulk Density)
•
•
•
•
Loss of Organic Layer
Loss of Root Channels
Infiltration decreased
Virginia Tech Study –
Rooting in to Subsoil
below is increased
infiltration rates by
153%
Forest Infiltration
• Forests filter and regulate
the flow of water.
• The forest floor acts as an
enormous sponge,
typically absorbing up to
18 inches of precipitation
before gradually releasing
it into natural channels
and watercourse
(sub-surface flows)
Source: Your Water- A Forest Product, Green America
series, 1986
Rainfall Infiltration
Natural Areas are Giant Sponges!
When forest understory/ leaf litter was converted to
turf in a North Carolina watershed, the mean
infiltration rate went from 12.4 in/hr to 4.4 in/hr. (Kays,
1980)
Rooting of Native Trees and
Prairie Vegetation
Native perennials & trees have deeper root systems than turf grass.
Hourly Infiltration Rates
Infiltration Rates
Importance of Evapotranspiration
“the single largest component of the natural hydrologic
regime, evapotranspiration (ET).” PA DEP Stormwater BMP
manual
Villanova Urban Stormwater
Research
Bioinfiltration Site
– 2010: 62% ET capture
The remaining portion of
rain goes to infiltration
Annual Evapotranspiration and Streamflow
For Different Land Uses in PA
100
Evapotranspiration
Stream Flow
Percent
80
60
40
20
0
Forest
Meadow
Cropland
Bare Soil
Pavement
Effect of Vegetation on Water Budget
Precipitation = 40 inches
This image cannot currently be displayed.
Deciduous Forest
Coniferous Forest
Evapotranspiration
23 inches
17 inches
29 inches
Streamflow (Runoff)
11 inches
ET & Vegetation Removal
Annual Precipitation for PA = 40 inches
Uncut
100% Cut
Evap. = 24 inches
Evap. = 14 inches
Stream flow = 16 inches
Stream flow = 26 inches
Streamflow (Runoff)
Evapotranpiration
• Mature Tree – 100 gallons per day
(Akbari, 1992)
• A large oak tree can transpire 40,000
gallons (151,000 liters) per summer or 79
gallons per day. (Thomas 2000)
• A mature maple tree can transpire 65140 liters per summer day
(Cermak et.al., 2000)
• The uptake of soil water by tree roots
increases soil water storage potential,
effectively lengthening the amount of
time before rainfall becomes runoff.
• Factors influence transpiration rates,
including leaf shape, size, number of
pores (stomata), and waxiness of the
leaf surface (Metro, 2002)
Transpiration Studies
• Study in Suburban
Minnesota –
• Conifers transpired
2 times more than
deciduous trees
– More Leaf Area
– Longer Growing
Season
Peters, E.B.; McFadden, J.P.; Montgomery, R.A. 2010. Biological and
environmental controls on tree transpiration in a suburban landscape. Journal of
Geophysical Research. 115: G04006.
Evapotranspiration
• Bald Cypress – large
wetland species
• Evapotranspiration =
880 gallons per day,
depending on soil type
and saturation (Keating,
2002)
Evapotranspiration
Forest Grown Trees
Open Grown Tree
Phytoremediation
Pollution Removal
• Plants remove contaminants
from soil and water, including
metals, pesticides, solvents,
oils, hydrocarbons, etc.
• In one study, a single roadside
sugar maple removed
60mg of cadmium,
140mg of chromium,
820mg of nickel, and
5200mg of lead
in a single growing season
(Coder, 1996)
Traditional Lawnscapes
• Turfgrass may be the largest
single crop in the
Chesapeake Bay covering
up to 3.8 million acres (9.5%)
• Over 100 million tons of
fertilizer are applied to
residential lawns and
gardens annually. (Audubon stat)
• 70 million pounds of
synthetic pesticides are
used on lawns each year –
10 times the rate/acre used
by farmers. (Redesigning the American
Lawn by F. Herbert Bormann, Diana Balmori, Gordon
T. Geballe, Yale University Press, 1993)
Turf areas are becoming a
major source of nutrient
loading for waterways
Engineered Soils are Gap
Graded Soils containing 80%
angular 1-1.5” stone and 20%
clay loam soil. They were
designed to create rooting
space along with load bearing
for sidewalks or parking lot
paving
Experiments at Cornell and Virginia
Tech using engineered soil mixes and
Trees showed an increase of infiltration
rate by 27 times compared with
unplanted control soils.
Pollutant Removal from
Structural Soils
Typical Parking Lot Trees
Limited Soil Volumes for Rooting, Compacted Soils, Constant Moisture Stress
Larger Canopy Trees Need
Larger Soil Volumes
“ A 30 inch diameter tree provides 70
times the ecological services of a 3”
diameter tree”
Greg McPherson, et. al. USDA Forest
Service Research
Cornell University-Structural Soil
Rooting in to Subsoil below is
increasing infiltration rates by 153%
CU Soil Under Porous Paving
Installed to a
24” depth
Parking Lot in Ithaca, NY with
Porous and Non-Porous Asphalt
Accommodates the 100 year storm for Ithaca
( 6" of rain in 24 hours).
Infiltration Rate was > 25”/hr
Bareroot Accolade Elms Planted
Three Years After Installation
Five Years After Installation
Silva Cell
Projects
www.DeepRoot.com
Silva Cell Installation
www.DeepRoot.com
Set Urban Tree Canopy Goals
• Perform UTC
Assessment
• Set Goals for
Increasing Tree
Canopy to 40% or
greater.
• Incorporate Tree
Planting into Green
Infrastructure Plans
and MS4 Permits
40% canopy = 0.8”/24hr storm intercepted by the canopy +
1.21”/24hr storm in soil
UTC overview - http://nrs.fs.fed.us/urban/utc &
http://www.forestsforwatersheds.org/urban-tree-canopy
Examples of UTC reports - http://nrs.fs.fed.us/urban/utc/pubs/
• City of Lancaster’s
Green
Infrastructure Plan
• Need to capture
over 1 Billion
Gallons of
Stormwater to
reduce the
Combined Sewer
OverFlows
THE GREEN INFRASTRUCTURE BENEFIT CALCULATOR
PROJECTS FUTURE BENEFITS FOR CSO AND MS4
AREAS
Manage over 1,200 Acres of Impervious Area
Capture over 1 Billion Gallons of Stormwater Runoff over the long term
Preserving Existing Forests
During Development
Green Infrastructure
Urban Vegetation Benefits
Cooler air temperature
Building energy conservation
Air quality improvement
Water quality improvement
UV radiation reduction
Greenhouse gas reduction
Aesthetics
Noise reduction
Wildlife habitat
Social / physiological benefits
Human health
i-Tree
www.itreetools.org
National Tree Benefits Calculator
http://www.treebenefits.com/calculator/
Kingston – 25”diameter Pin Oak
annually provides:
$247 in benefits each year
Intercepts 3,600 gallons of stormwater
Saves 163 KW/hours each year and 53 therms
Removes 1,100 lbs of atmospheric carbon
Increases Property Value by $95
Summary
• Annual Interception
– 500-2000 gal/tree/yr
– 15-25% Evergreens
– 10-20% Deciduous
• Transpiration
– 5-15%
– Depends on species & weather
• Plant Large, Broad Spreading Canopy Trees Over
Pavement (Conifers or Deciduous)
• Utilize Engineered or Structural Soils to Increase
Infiltration, Root Growth, and Tree Health.
• Set Community Wide Canopy Goals – 40%+
Penn State
Cooperative
Extension
Penn State is committed to affirmative
action, equal opportunity, and the
diversity of its workforce.
Vincent Cotrone
570-825-1701
[email protected]
www.PATrees.org