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Objective: Have a working knowledge of the relationship between the vegetative cover in a watershed and water yield and - PowerPoint PPT Presentation

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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

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Objective:Have a working knowledge of the relationship between the vegetative cover in a watershed and water yield and water quality


  • 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
USGS Gauging Stations

  • http://waterdata.usgs.gov/nwis/uv?cb_00060=on&cb_00065=on&format=gif_default&period=31&site_no=03335500


  • Transpiration

  • Interception

  • Stem flow


  • 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.


Perched water tables
Perched water tables


Relationship of cover type to stream flow
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
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
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



  • 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
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
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
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

First year increase following harvest
First Year Increase Following Harvest

  • Proportional to BA removed

    • 13% threshold value

  • Also function of energy available for evapotranspiration, (insolation index)

    • Slope

    • Aspect

    • Latitude

Affects relative to total removal
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
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 tables1
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
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 impacts1
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 impacts2
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
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
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)