Hydrologic Analysis with GIS

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Hydrologic Analysis with GIS

• Eric Anderson
• 3/18/03

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Outline

• DTMs (Digital Terrain Models)
• Hydrologic Modeling Procedures
• Filling Sinks
• Determining Flow Direction/Accumulation
• Developing CN Numbers
• Potential Runoff Map

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DTMs

• Grid Overlay
• Each cell assigned a value (elevation, land
  cover, etc.)

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Color Classified Elevation Data
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Raster Tools in ArcMap

• Spatial Analyst Raster processing extension in
  ArcMap
• Very Powerful Tool
• Allows slope, aspect, azimuth, classifications,
  and other raster manipulations

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Deriving Attributes from Rasters

• Using a DEM
• Sloperate of elevation change expressed as
  percentage or degrees
• Aspectcompass direction of the steepest slope
• END OF THE REVIEW

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Preprocessing DEMs

• DEMs must be preprocessed to eliminate inherent
  errors
• Sinks
• Most common error is a sink
• Places where water might drain, but are
  disconnected with the entire drainage system

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Sink Example
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Flow Direction and Accumulation

• Basis of most all GIS hydrological analysis
• Once flow direction is known it is possible to
  determine how many cells flow into a given cell
• Basis of watershed delineation and stream
  networking

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Flow Direction and Accumulation

• SLOPE
• Slope of each cell based on its eight neighbors
• Largest slope will be assigned to the center cell

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Flow Direction and Accumulation

• FLOW DIRECTION GRID
• Determine flow direction with slope grid
• If one cells slope is lower than its neighbor,
  flow will be in the direction of that cell

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Flow Direction and Accumulation

• FLOW ACCUMULATION
• Trace backwards up the flow direction grid and
  determine the number of cells flowing into each
  cell
• Accumulated flow is the number of cells flowing
  into each cell
• generate a grid to represent the total number of
  cells flowing into each cell
• Cells that have high accumulation are designated
  as stream channels

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Flow Accumulation Surface
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Predictive Equations

• Runoff Availability Equation
• Can be used to predict the amount of water that
  will become runoff given a specific rainfall
  event
• For example, how much runoff can be expected if
  there is an inch of rain over a clay soil with
  dense vegetation
• Based on soil characteristics and land cover
• Q (sqrt(p 0.2 (1000/CN 10))) / (p 0.8
  (1000/CN 10))
• Where Q total water available for runoff
• p precipitation
• CN curve number

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Runoff Availability Equation

• Equation based on the CN number
• CN number based on soil and land cover
  characteristicsboth easily available in raster
  or vector format for GIS
• CN values range from 0-100 (like a percentage)
• Note concrete has a CN value of nearly 100

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Runoff Availability Flow Chart
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Combining Land Cover and Hydrologic Soil Group
LayersRaster Calculator
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Raster Calculator
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Combine Land Cover and Hydrologic Soil Group
Layers

• Enter in Raster Calculator
• Hydrologic Soil Group.Combine( Land Use and
  Cover )
• Raster calculator will create a new grid
• Values in the new grid represent zones of unique
  combinations of values in the input grids
• Each of these soil/landcover combinations is then
  assigned a value of Curve runoff based on a table
  you can get from the NRCS

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Plug in CN Grid and Precipitation Grid into
Available Runoff Equation

• Original Equation
• Q (sqrt(p 0.2 (1000/CN 10))) / (p 0.8
  (1000/CN 10))
• Raster Calculator Operation
• ( (Precipitation - 0.2.AsGrid) ((1000.AsGrid
  / CN) - 10.asgrid)).sqr / ( (Precipitation
  0.8.AsGrid) ((1000.AsGrid /CN) - 10.AsGrid))

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Potential Runoff Map
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Questions?