Trees and Their Role in Storm Water Management

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Trees and Their Role in Storm Water Management

• Mindy Habecker
• Dane County UW-Extension

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Trees The Original Multi-taskers

• Provide social, ecological, and economic benefits
• Their beauty inspires writers and other artists.
• Their leaves and roots clean the air we breathe
  and the water we drink

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Benefits of Trees in Urban Areas

• Save Energy
• Improve air quality
• Extend life of paved surfaces
• Increase traffic safety
• Increase real estate values
• Increase sociological benefits
• Protect our water resources

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All water is part of this cycle
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Storm Water and the Hydrologic Cycle

• Urbanization dramatically alters the hydrologic
  cycle
• Increases runoff
• Increases flooding frequency
• Decreases infiltration and groundwater recharge
• Nationwide impervious surfaces have increased by
  20 in the past 20 years

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More Trees Means Less RunoffSome Statistics

• Fayetteville, Arkansas increasing tree canopy
  from 27-40 reduced their storm water runoff by
  31
• South Miami residential study found that a 21
  existing tree canopy reduces the storm water
  runoff by 15
• For every 5 of tree cover added to a community,
  storm water is reduced by approximately 2

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How Do Trees Effect Stormwater?

• Above ground effects
• Interception, evaporation and absorption of
  precipitation
• Ground surface effects
• Temporary storage
• Below ground effects
• Infiltration, permeation and filtration

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Above Ground Effects

• Intercept rainwater on leaves, branches and
  trunks slowing its movement
• Evaporation of some of this intercepted
  precipitation of the tree surfaces
• Absorption of a small portion into leaves or stems

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Ground Surface Effects

• Leaf litter and other organic matter can hold
  precipitation and stemflow on a site, reducing
  the amount and peak rates of runoff
• Roots and trunk bases of mature trees tend to
  create hollows and hummocks on the ground

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Below Ground Effects

• Organic material from leaf litter and other tree
  detritus tends to increase infiltration rates by
  increasing pore spaces in soil
• Organic material also increases the
  moisture-holding capacity of these sites
• Root mats of trees also tend to break up most
  soils further improving infiltration and
  moisture-holding capacity

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Below Ground Effects cont

• Deep roots tend to improve the rates of
  percolation of water from upper soil horizons
  into lower substrates
• Trees take up water through their roots that is
  eventually transpired onto leaf surfaces and
  evaporated
• Tree roots act as natural pollution filters
  (biofilters) using nitrogen, phosphorus and
  potassium

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EPAs Tree Canopy Target Goals

• Set to protect a communitys green infrastructure
  and maximize the environmental benefits
• For metropolitan areas east of the Mississippi
• Average tree cover for all land use 40
• Suburban residential
  50
• Urban residential
  25
• Central business districts 15

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Complications?
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Complicating Factors

• Presence of soil compaction
• Presence of soil textural discontinuity
• Has the site been disturbed in the past?
• Management of the ground surface
• Is litter layer removed?
• Is soil surface exposed in winter?
• How much of the surface is like a natural forest?
  (number and size of trees)

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Water Movement in Soils

• Forces affecting the energy of soil water
• Matric force (absorption and capillary)
• Gravity
• Osmotic forces
• Field Capacity is the amount of water held in the
  soil after gravitational water had drained away
• Movement of water is the soil is controlled
• Gravitational forces if saturated
• Matric forces if unsaturated

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Soil Factors Influencing Infiltration

• Infiltration is the mode of entry of all water
  into the soil
• Rate of infiltration determined
• Initial water content
• Surface permeability
• Internal characteristics of the soil
• Intensity and duration of rainfall
• Temperature of soil and water

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Soil Factors Influencing Infiltration cont.

• Soil compaction reduces the infiltration rate
• Microrelief under trees provides catchment basins
  during heavy rains
• Removal of litter layer reduces the infiltration
  rate
• Forest soils have a high percentage of macropores
• The frost type found in forest soils promotes
  infiltration year-long

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Importance of the Litter Layer

• Absorbs several times its own weight
• Breaks the impact of raindrops
• Prevents agitation of the mineral soil
• Discourages formation of surface crusts
• Increases soil biotic activity
• Increases incorporation of organics
• Slows down lateral movement of water

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Affect of Micropores in the Soil

• Develop in old root channels or from burrows and
  tunnels made by insects, worms or other animals
• Lead to better soil structure
• Increases organic matter incorporation
• Increases percolation rates and root penetration

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Soil Frost Types

• Granular
• Small frost crystals intermingled with soil
  particles
• Found in woodland soils with litter
• May be more permeable than unfrozen soil
• Honeycomb
• Has loose porous structure
• Found in highly aggregated soils and also formed
  in organic layers and litter layers

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Soil Frost Types cont

• Stalagtite
• Forms partially fused, columnar ice crystals
• Connects a heaved soil surface to the soil below
• Concrete
• Forms intricate multiple thin ice lenses
• Common in soils going through freeze-thaw
  sequences
• Common in exposed soil areas (agriculture)
• Much less permeable that other frost types

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Implications of Frost Types

• Forests and prairies rarely yield runoff
  regardless of steepness, even when frozen
• Forested areas provide storm water protection and
  protect the quantity and quality of groundwater

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Groundwater Surface Water Flows
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Black Earth Creek Study

• Black Earth Creek receives 80 of its water from
  groundwater
• Main recharge occurs in spring and fall
• Recharge from the agricultural uplands is highly
  variable
• Wooded hill slopes generate no significant runoff
• Forested slopes are significant recharge areas

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Trees and Storm Water Conclusions

• The impact of urban trees on hydrology is
  extremely variable and complex, in general
  increases in tree cover and tree size over a site
  will result in reduced total runoff amounts and
  peak runoff rates.
• Effects are greatest during the growing season
• Effects are greatest on sites whose soils are
  relatively impermeable

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Trees and Storm Water Conclusions cont

• Trees have a relatively greater effect on smaller
  storm runoff amounts than on large storm events
• Surface and below-ground effects on runoff are
  much more significant than the above-ground
  effects
• All of the effects on runoff are greatest when
  urban trees are large and well-established on
  undisturbed sites

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Contact Information Mindy Habecker Dane County
UW-Extension 224-3718 Habecker_at_co.dane.wi.us