watershed delineation

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					Watershed Delineation
       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
   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?
  Delineating Watersheds Cont’d
► The  watershed is a hydrologic unit of land used as
  a physical, biological, socioeconomic, and political
  unit for the management and planning of natural
► Aids in describing and quantifying the variables
  that influence water quality:
     Topography
     Soils
     Vegetation
     Land use, et.
        Watershed Delineation
► Identifies the boundaries of our hydrologic
  unit / area of study.
  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
  Identifying Boundaries Cont’d
► 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
          Map Reading hints
► Water flows downhill (perpendicular to
► Ridges make “V” shapes pointed downhill
► Draws make “V” shapes uphill
                 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
     Methods to Determine Size
► Planimeter
          Data (USGS etc.)
► Historical
► Dot method using a grid
      GIS as a measuring tool
► Polygons   have an area column
       Aspect and Orientation
► Differentwatersheds have unique
 differences and aspect has an affect

   Watersheds with south facing slopes are dryer
    due to evapotransperation and vegetation.
         Aspect Calculation
► Direction
   Vector perpendicular to        7       7 7
    the plane of the slope
   Values 0-360 (degrees)         6       6 6
   Southern aspect is 180         5       5 5

                              Aspect Map
       Aspect and Orientation
             the General of the main stem
► Orientation-
 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
       Watershed Orientation
► East/west orientation is likely to have slopes
 that are predominantly north/south
               Slope Calculation
                                  6 5 6
 Rise over run                   5 7 6               40
 Percent or angle
                                  4 3 4
 Steepest point

 If cells are 10 m^2 then…

 (7-3) / 10 = .4*100= 40% slope

                                          Slope map
► 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
               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
► Locatesample sites
► Zones of erosion etc.
               Profile Cont’d
► Using   a 1:24,000 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
► 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
         Calculating Sinuosity
             Profiles in the field
► Longitudinal
► Standard Map and Map Wheels
              Stream Order
► Characterizesthe drainage networks
► Broad reference for flow characteristics of a
► Higher streams usually mean higher flow
      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
            Watershed Shape
► Watershed   shape has an effect on flow
► 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.
Slope           Parent
                (3 layers)


Index (acc.
on slopes
perp. drain   Climate/Relief:
direction)    Aspect

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