Watershed Impact Assessment

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Watershed Impact Assessment

• Watershed
• Delineation Procedures in
• N-SPECT

Chris McColl Center for Spatial
Information RGIS USDA-CSREES
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DEM Preprocessing

• Goal develop hydrologically correct DEM and
  develop layers to optimize performance
• Flow direction (D8 method)
• Flow accumulation
• Stream definition and segmentation
• Catchment delineation
• Iterative process
• Review results and fix terrain if necessary until
  correct flow patterns are established

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DEM Preprocessing (cont.)
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Raw DEM
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Filled DEM

• Sinks and other imperfections must be removed
  from Raw DEM so that water moves correctly
  through system

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Flow direction (D8 Method)

• Neighboring cell with the lowest elevation is
  identified as the outflow direction

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Flow Accumulation Grid

• Generated based on the flow direction grid
• Used to derive a stream network

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Streamlink Watershed commands

• Each streamlink is then associated with a basin
  that is given an unique identifier.

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Sub-basin delineation

• Delineation process can generate spurious
  results! need to perform visual QC

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Basin Hydrology

• Useful to overlay existing hydrology layer.

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DEM Reconditioning

• ArcHydros Terrain Preprocessing tool

Source Hellweger, 1997
http//www.ce.utexas.edu/prof/maidment/gishydro/fe
rdi/research/agree/agree.html
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Post reconditioning
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DEM Reconditioning Art Form?

• May take a few iterations to develop a reasonably
  accurate stream network and basin boundaries.
• The flow direction algorithm can get confused
  within flat topography higher resolution DEMs
  can sometimes provide better results (e.g. 100m
  30m 10m)

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Cautionary Notes

• The reconditioning DEM should be filled once
  again before using them.
• Topographic analyses should be conducted on the
  original "Raw DEM" (before building walls and
  reconditioning), to ensure that correct
  elevations are extracted (e.g. for slope
  computation).
• need to copy the original LS grid from the
  original delineation and place it in the new
  reconditioned DEM folder.
• In N-SPECTs case, if deriving soils data and
  associated K-factor grids use the original DEM.

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Practical Application

• Watershed delineation practice using the McLane
  watershed, Washington!