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– 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

• 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– 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

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7

Year

Str

eam

Flo

w In

crea

se,

cm

HB

Fernow

Coweeta

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

0

5

10

15

20

25

30

35

0 20 40 60 80 100

Reduction in basal area

cm

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 evapo-transporation 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

• 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.