Hydrological Modeling

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Hydrological Modeling
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Overview

• Introduction
• Watershed delineation
• Automatic delineation
• Flow length

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Introduction
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Watershed management
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Definition of watershed

• The region draining into a river, river system,
  or body of water
• American Heritage Dictionary
• The upstream area of any given point on the
  landscape
• Physically defined by drainage point and upstream
  area
• Also known as basin, sub-basin, catchment, and
  contributing area

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Watershed delineation
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How it works

• Water always flows downhill
• For any point on a grid representing a
  landscape,a drop of water can be traced downhill
  
• ? direction of flow is known for every DEM
  cell
• For any point on a grid representing a
  landscape,a flow pathway can be traced back
  uphill
• ? flow accumulation is known for every
  DEM cell
• Uphill back-tracing proceeds to a ridgeline or to
  the edge of the grid
• Termination of uphill back-tracing defines
  watershed boundary

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Watershed delineation

• Steps (with ArcToolbox)
• Create a depressionless DEM
• Calculate flow direction
• Calculate flow accumulation
• Create watershed Pour points
• Delineate watersheds

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1. Creating a depressionless DEM

• DEM must eventually drain off edge of grid
• Areas of internal drainage will result in
  unprocessed areas
• FILL routine fills in sinks or cuts off peaks
  creating a new grid with no drainage errors

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2. Flow direction

• Every cell flows into another cell or off the
  grid edge
• Flow direction is calculated as the direction of
  steepest downward descent
• Flow direction is calculated for each cell,
  resulting in a new grid theme

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2. Flow direction
direction of flow is saved as a code number
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2. Flow direction
north-flowing cells
coded as 64
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2. Flow direction

• Flow direction grid

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3. Flow accumulation

• Each cell has been coded for direction of flow
• Cumulative flow is calculated from flow direction
• Output grid is created where values are the
  number of upstream cells
• Lower accumulation values are ridge tops
• Higher accumulation values are valleys stream
  channels

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3. Flow accumulation
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3. Flow accumulation
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3. Flow accumulation
single class legend shows high flow cells
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3. Flow accumulation

• Fit depends on accuracy of the DEM and stream
  layers

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4. Watershed Pour points

• Watersheds are defined by outlets (pour points)
• Pour points should be placed in high-flow
  pathways
• Basins will be generated from pour point to
  ridgeline or to upstream sub-basin
• Pour points should be numerically coded per
  sub-basin
• Pour points must be converted to a grid layer

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4. Watershed Pour points

• Create as many pour points as necessary

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5. Delineating watersheds

• Preliminary steps are completed
• Filled DEM
• Flow direction
• Flow accumulation
• Pour points created converted to grid
• Run tool to create watersheds

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5. Delineating watersheds

• Watersheds represent area upstream from Pour
  points and terminate at ridgelines, uphill
  sub-basin boundary, or edge of the grid

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5. Delineating watersheds
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Automatic delineation
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Automatic delineation

• Pour points automatically selected by
  intersection of highest-flow pathways and grid
  edge

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Automatic delineation
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Flow length
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Flow length

• Flow distance for every cell to outlet

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Flow length
Flow distance for every cell to closest stream
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Flow length
Euclidean distance vs. flow distance
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Homework

• Read Hydrological Modeling Watershed
  Delineation, Map Layouts
• Study for the exam
• Presentations
• 3/5 Luke, Jennifer, Josh
• 3/7 Chris, Prati, Carolyn