Transcript Relationship of Cover Type to Stream Flow

Objective: Have a working
knowledge of the relationship
between the vegetative cover
in a watershed and water
yield and water quality
Terms
• Hydrograph
– Graph of water discharge over specified time
period
• Base flow
– Rate of discharge in a stream where only the
throughflow and groundwater flow from
subsurface aquifers contribute to the overall
flow.
• Storm flow
– Rate of discharge during and after storm event
USGS Gauging Stations
• http://waterdata.usgs.gov/nwis/uv?cb_0006
0=on&cb_00065=on&format=gif_default&
period=31&site_no=03335500
Terms
• Transpiration
• Interception
• Stem flow
Terms
• Insolation
– Direct or diffused shortwave solar radiation that
is received in the Earth's atmosphere or at its
surface.
Effect of angle on the area that intercepts an incoming beam of radiation.
http://www.physicalgeography.net/fundamentals/6i.html
Perched water tables
http://pasture.ecn.purdue.edu/AGEN521/epadir/grndwtr/perched.html
Relationship of Cover Type to
Stream Flow
• Data for eastern U.S.
limited to Appalachian
Highlands
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Coweeta, NC
Fernow, WV
Leading Ridge, PA
Hubbard Brook, NH
Walker Branch, TN
Why would watershed research
be conducted in Appalachians?
• Small catchments
easily identified
• Can install weirs at
catchment outlets
• Precipitation adequate
to maintain continuous
vegetative cover
Measurements of Interest
• Changes in vegetative
cover
• Water volume yield
over time
– Before change
– After change
• Water temperature
• Nutrient level
Oregon, Umpqua National Forest, Joe Hall Creek/Elk Creek
confluence
http://www.singingfalls.com/stream/engineering/temperature.html
Treatments
• Clearcutting
– Logs not removed
– Logs removed
– Whole tree removed
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•
Harvest followed by deadening of all vegetation
Size and distance of roads
Partial cuts - strip cutting
Conversion to grass with succession
Conversion to Pinus, sp.
Use of Results
• Manage for water supply
• Make silvicultural
prescriptions based on
water and nutrient impacts
• Engineer drainage
structure
• Estimate impact of road
and skid trail construction
WLH, Northcentral PA
Impact of Forest Cutting
• Reduced transpiration
• Reduced interception by canopy
– Catches precipitation and increases evaporation
• Resulting in
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Increased storage
More water for remaining plants
Greater water movement within soil
Greater baseflow
Range of change in yield
• Increase yield by as much as 40 cm (15.7 in.)
• Reduce by as much as 20 cm (7.9 in.)
• Suppression of regenerating foliage
– Increases water yield and delays return to base level
• Replacement by evergreen forest
– Reduces yield below original base level with
deciduous forest cover
Return to base flow after cut
Stream Flow Increase, cm
40
35
30
25
HB
Fernow
Coweeta
20
15
10
5
0
1
2
3
4
Year
5
6
7
First Year Increase Following
Harvest
• Proportional to BA
removed
– 13% threshold value
• Also function of
energy available for
evapotranspiration,
(insolation index)
– Slope
– Aspect
– Latitude
Stream flow increase after 1 year
35
30
cm
25
20
15
10
5
0
0
20
40
60
Reduction in basal area
80
100
Affects relative to total removal
• Riparian forest buffers
– May increase yield proportionally less than same size
area left unharvested away from waterway
• Strip cutting
– May increase yield proportionally less than expected
because of increased water availability
Evergreen vs. Deciduous
• Rate of
evapotransporation
about same
• Evergreens have
longer period of
evapotranspiration
• For evergreens
interception lasts all
year
WLH, Central Idaho, June, 7,000 ft. elev.
Perched water tables
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No experimental data available
Increase water table, depending on topography
Seep areas would remain wet longer
Would be greater potential evapotranspiration
because of higher water table
• Partial cutting would affect stream flow less than
predicted by models
Other impacts
• Low flow rate
– Should increase low flow rates at end of growing
season
– Less affect with shallow soils
• Peak flow rate
– Less available storage in soil because of reduced
drainage
– Greater percentage of summer and fall storms appears
as stream flows
– Snow melt accelerated, increasing peak flow
– Roads and skid trail increase peak flow
Other Impacts
• Soil Moisture
– Issue for regeneration and wildlife cover
and food
– Change depends of insolation, soil
properties, etc.
– Potential for increase
• Stream temperature
– Assume removal of streamside
vegetation
• May increase maximum summer
temperature by up to 4-6 degrees
• Riparian forest buffers eliminate this
potential problem
Other Impacts
• Sedimentation
– Eastern deciduous forest
• Primary sources are roads, skid trails and landings
• Usually small impact on stream turbidity and bed loading, but
great enough for regulatory action
• Type of harvest has impact because of differences in road
system
– Western forests
• Additional sources are site preparation
– Slash piling and burning
– Solution
• Limit harvest in buffers
• Don’t drive or skid through streams
Affect on Nutrients
• Nutrient losses from harvesting difficult to measure
– Direct measures of soil nutrients difficult
– Use changes in nutrient levels in waterway as indicator
• Intensive whole tree harvest increase nutrient removals by
factor of 2 to 4.
– Affect on plant growth depends on
• overall nutrient level,
• amount of denitrification from decomposition
• Recharge from atmosphere and subsoil
• Increases not enough to cause algal bloom or other
negative impacts on aquatic ecosystems
– Issue is sedimentation which also carries nutrients into water
Synopsis of water related
silvicultural impacts
• See Figure 7, Stone,
Wank, and
Hornbeck,
Proceedings 5th
North Am Forest
Soils Conf.,
Colorado State
Univ,m Aug 1978
(handout in class)