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
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 loss
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
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
• Interception lasts all
year
Perched water tables
http://pasture.ecn.purdue.edu/AGEN521/epadir/grndwtr/perched.html
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 streamflows
– 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 Stone, et. al.,
Figure 7.