Watershed Delineation Delineating Watersheds Watershed

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Watershed Delineation
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Delineating Watersheds

• Watershed (Catchment, Drainage Basin)
• A topographically delineated area drained by a
  stream system, or, the total land above some
  point, on a river/stream that drains past that
  point.
• Can range from a fraction of an acre to thousands
  of square miles.
• Why is this unit of area important to us as land
  and water managers?

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Delineating Watersheds Contd

• The watershed is a hydrologic unit of land used
  as a physical, biological, socioeconomic, and
  political unit for the management and planning of
  natural resources.
• Aids in describing and quantifying the variables
  that influence water quality
• Topography
• Soils
• Vegetation
• Land use, et.

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Watershed Delineation

• Identifies the boundaries of our hydrologic unit
  / area of study.

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Identifying Watershed Boundaries

• Steps to delineation
• Trace/outline outline the main stem of the stream
  that you want to examine
• Trace all perennial or influential tributaries
• Locate the lowest point/outlet of the main stem
  and work uphill

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Identifying Boundaries Contd

• Working uphill, Identify the ridges and hill tops
  that divide the water from flowing into separate
  watersheds
• When in doubt, consider,
• Where will the rain drops go

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Map Reading hints

• Water flows downhill (perpendicular to contours)
• Ridges make V shapes pointed downhill
• Draws make V shapes uphill

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Area / Size

• Important to watershed features because
• Estimating total annual water yield
• Flood potential
• Hydro characteristics drainage density, time of
  concentration, drainage shape, etc..
• Amount of ground that you must deal with

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Methods to Determine Size

• Planimeter
• GIS
• Historical Data (USGS etc.)
• Dot method using a grid

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GIS as a measuring tool

• Polygons have an area column

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Aspect and Orientation

• Different watersheds have unique differences and
  aspect has an affect
• Watersheds with south facing slopes are dryer due
  to evapotransperation and vegetation.

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Aspect Calculation

• Direction
• Vector perpendicular to the plane of the slope
• Values 0-360 (degrees)
• Southern aspect is 180

360
180
Aspect Map
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Aspect and Orientation

• Orientation- the General of the main stem of the
  watershed
• Obtain total stream length
• Determine 10 from the top and 15 from the
  bottom
• Connect these points and get the aspect of that
  line

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Watershed Orientation

• East/west orientation is likely to have slopes
  that are predominantly north/south

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Slope Calculation

• Rise over run
• Percent or angle
• Steepest point

If cells are 10 m2 then (7-3) / 10 .4100
40 slope
Slope map
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Slope

• The vertical difference between the upper and
  lower most points divided by the horizontal
  difference in the points.
• Why is slope important
• Runoff, stream transport power, sediment type and
  load, aggrading or degrading.
• Calculate for the entire reach and individual
  sites.

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Stream Profile

• Graph that provides longitudinal profile of the
  stream.
• X-axis is stream mileage
• Y-Axis is elevation
• Stream profiles help to stratify zones
• Alpine, foothills, basins
• Locate sample sites
• Zones of erosion etc.

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Profile Contd

• Using a 124,000 or better map
• Record interval at contours
• Using a graphing program, create a line graph
  with elevation as the y-axis and stream miles as
  the x-axis

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Sinuosity

• The repetition downstream of patterns of channel
  curves for a given length of stream. (channel
  length / valley length)
• Measure of
• Bends
• Curves
• Meanders
• Sinuosity is important for
• Stream Classification
• Variation in erosion and sediment concentration

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Calculating Sinuosity

• GIS
• Longitudinal Profiles in the field
• Standard Map and Map Wheels

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Stream Order

• Characterizes the drainage networks
• Broad reference for flow characteristics of a
  watershed
• Higher streams usually mean higher flow volume

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Calculating Stream Order

• Single tributaries are order 1 streams
• Two 1st order streams meeting order 2
• Two 2nd order streams meeting order 3
• 1st order meeting a 2nd order order 2

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Watershed Shape

• Watershed shape has an effect on flow
  characteristics
• Shape can be calculated by form factors and
  circulatory ratios
• Circular watersheds will concentrate water
  quickly and have a flashy discharge
• Long narrow watersheds tend to have steep slopes,
  high overland flow, high sediment yields, lower
  peak volumes over long periods of time.

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Relief Slope
Parent Material Soil Enhancement (3 layers)
Relief Relative Elevation
Organisms Fractional Vegetation
Relief Compound Topographic Index (acc. on
slopes perp. drain direction)
Climate/Relief Aspect
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