7w Watershed

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7w Watershed
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Watershed

• Idea in watershed is to
• find all the cells in a DEM which drain into a
  selected drain
• OR
• Find all the watersheds in the DEM image
• The additional functions are
• Flow calculates direction of water flow FROM a
  given pixel
• Runoff calculates the accumulation of flow

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Basins or ArcHydro

• Basins is a Hydrologic modeling system based on
  raster images
• Pit fill
• Watersheds
• Flow direction (Flow in IDRISI)
• Flow accumulation (Runoff in IDRISI)
• Rain drop tracing (Not in IDRISI)
• So these are pretty basic tools for the
  hydrologist and pollution modeler

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Watershed Automatic

• Does a pit fill automatically
• Finds all the watersheds in a DEM image that are
  enclosed by the image
• That is, their outlets are in the image
• You have to specify the minimum size of
  watersheds to be identified
• Each watershed is given a unique serial

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Automatic
Watersheds of minimum size 50 pixels
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Seedy (drain point)
What are these, watershed 1 or background?
100 pixel sheds
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The Exercise

• Us Pitremoval module to, guess what, remove any
  pits in the DEM
• Make the seed image with all 4 seeds with
  different values (see thumbnails). The watersheds
  will take the value of the seeds
• Run Watershed with Automatic seeding turned
  off.
• Use extract (tabular) to get required values of
  area etc
• What follows is more information re watersheds
  and hydrologic functions

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Flow Direction
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Runoff (flow accumulation)
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Runoff streams?
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How does watershed work?

• First find the cells which drain into the
  startingdrain
• Next find the cells which drain into the previous
  set of cells which drained into the drain
• And so on until there are no more cells that
  drain, eventually, into the drain!
• For example

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Problems with watershed

• Sometimes you get only ½ of the watershed
• If that happens pick two cells for drains OR pick
  the stream if one available
• The old way to get watersheds was to draw them on
  a Topo map and digitize the vector outlines
  based on height of land
• Now watershed delineation on a DEM is more common
• Despite the fact that both Topo maps and DEMs
  have unknown accuracy!

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Summary

• Both Watershed functions are in found in many GIS
  systems
• Best done in a raster system
• Both are based on the accuracy of the DEM and
  that is a problem
• LIDAR is probably the best but costs lots of