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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 144 gal/yr 22ft tall x 21ft spread • Kwanzan Cherry 312 gal/yr 17ft tall x 17ft spread • Red Oak 767 gal/yr 40ft tall x 27ft spread • Red Maple 1,014 gal/yr 29ft tall x 24ft spread • Zelkova 1,624 gal/yr 38ft tall x 34ft spread • White Pine 786 gal/yr 32ft tall x 20ft spread • Hackberry 1,394 gal/yr 47ft tall x 37ft spread Source: USDA FS PSW-GTR-202 August 2007 & PSW-GTR-199
Interception by Hackberry
Tree Age in Years
Gal
lons
of S
torm
wat
er
Inte
rcep
ted
per y
ear
Source: Adapted from McPherson et al 2006
NYC Street Tree Interception • 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.
Average Interception by Species London Plane – 2,875 gal/yr Silver Maple – 2,948 gal/yr
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
Villanova Urban Stormwater Research
Bioinfiltration Site – 2010: 62% ET capture The remaining portion of
rain goes to infiltration
“the single largest component of the natural hydrologic regime, evapotranspiration (ET).” PA DEP Stormwater BMP manual
Annual Evapotranspiration and Streamflow For Different Land Uses in PA
0
20
40
60
80
100
Forest Meadow Cropland Bare Soil Pavement
Perc
ent
EvapotranspirationStream Flow
Effect of Vegetation on Water Budget
Streamflow (Runoff)
Evapotranspiration
Precipitation = 40 inches This image cannot currently be displayed.
Deciduous Forest Coniferous Forest
23 inches 29 inches
17 inches 11 inches
ET & Vegetation Removal
Stream flow = 16 inches Stream flow = 26 inches
Evap. = 24 inches Evap. = 14 inches
Annual Precipitation for PA = 40 inches
Uncut 100% Cut
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 65-140 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
Open Grown Tree Forest Grown Trees
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
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.
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
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
Projects
Silva Cell
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
www.itreetools.org
i-Tree
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 vjc1@psu.edu
www.PATrees.org
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