Flow%20Accumulation

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Flow Accumulation

• FE 423 - Lecture 5a

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Discussion Problems
Revisit Practice Midterm with Zonal
mean (value.ZonalStats(GRID_STATYPE_MEAN,
zone, Prj.MakeNull, zone.GetVTab.FindField("Va
lue"), FALSE))
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Discussion Problems
Overlay streamlines and the stream cover on a
hillshade of the HJ Andrews dem. Where does the
network look wrong? Does filling the dem fixes
the problem? Identify and quantify the changes
that FILL made.
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Discussion Problems
Road-side ditches can impact watershed hydrology
by intercepting surface and subsurface flow and
rerouting to the stream. Make a modified
flowdirection grid, in which roads intercept
downslope flow and route it along the ditch to
the streams. Use streamlines to verify that the
water now flows the way it should.
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OUTLINE

• The take-home midterm
• Flow Accumulation
• Stream Networks
• Stream Order
• Weighted flow accumulation
• Watersheds anti-flowaccumulation

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SCHEDULE
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FLOW ACCUMULATION

• Flow direction
• Water flows downhill
• Identifies upslope cells
• flowGrid.FlowAccumulation(weightGrid)
• dem.FlowDirection(ForceEdge).FlowAccumulation
  (weightGrid)

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Flow Accumulation
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STREAM NETWORKS

• enough water flow to stay open
• flow increase with upslope area
• Streams cells with a minimum value
• dem.FlowDirection(ForceEdge).FlowAccumulation(ni
  l) gt minCells

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STREAM ORDER
Like Contributing Area aStreamGrid.StreamOrder(dir
Grid, shreveMethod)
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WEIGHTED ACCUMULATION
The more there is, the more flows downhill
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0
0
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0
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18
0

• dirGrid.FlowAccumulation(weightGrid)

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WATERSHEDS

• anti-flowaccumulation
• From a given outflow, work upslope
• dirGrid.Watershed(srcGrid)

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Discussion Problems

• Revisiting the take-home midterm
• Zipping to move your project
• make directory c/temp/yourname/flowacc
• set working directory
• save project there
• do some work, then save close ArcView
• zip c/temp/yourname/
• move yourname.zip however you like
• extract yourname.zip to c/temp/yourname/
• double click projectname.apr
• This is your project, just as you left it.

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Discussion Problems

• Revisiting the take-home midterm
• Revisable themes
• look through the themes in the exam
• Which theme can be revised? Which cant?
• What do each group have in common?
• Rewrite them so that they can be revised
• Redo the exam assuming everything mature
• Redo the exam assuming that everything burns
• Map the mature/burned difference in WAR

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Discussion Problems
Map the stream network from the midterm dem.e00
assuming that all cells with a contributing area
of at least 100 cells is a stream cell. What is
the area of 100 cells? Make stream networks with
different contributing areas. Which one looks
best? Make a grid of all stream headwaters (the
first cell in each stream).
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Discussion Problems
Runoff from stands in the rain on snow zone is
more sensitive to harvest. Streams that have much
of their contributing area in the rain on snow
zone must be similarly sensitive. If the
rain-on-snow zone is 900-1200m, map the fraction
of the contributing area of each stream reach
that is in the rain on snow zone.
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Discussion Problems
We might want to put special restrictions on the
rain on snow slopes above the most sensitive
reaches. Identify the stream cells whose
contributing area is mostly in the rain-on-snow
zone Identify the rain-on-snow cells that
contribute to these most sensitive stream
reaches.
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Discussion Problems
Revisiting the road-side ditch problem By how
much is the contributing area of each stream cell
increased in shifting from natural downhill
routing to roadside ditch interception? Does the
resulting change look different if you use
relative vs. absolute change?