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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 ... Main recharge occurs in spring and fall ... – PowerPoint PPT presentation

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Title: Trees and Their Role in Storm Water Management


1
Trees and Their Role in Storm Water Management
  • Mindy Habecker
  • Dane County UW-Extension

2
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

3
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

4
All water is part of this cycle
5
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

6
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7
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

8
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

9
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10
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

11
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

12
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

13
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

14
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

15
Complications?
16
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)

17
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

18
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

19
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

20
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

21
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

22
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

23
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

24
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

25
Groundwater Surface Water Flows
26
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

27
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

28
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

29
Contact Information Mindy Habecker Dane County
UW-Extension 224-3718 Habecker_at_co.dane.wi.us
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