Module 3 - Watershed Modeling and Delineation
Modeling flow is an invaluable GIS analysis application that essentially shows where things accumulate, and/or what path things may take. These "things" I am referring to can range anywhere between rainfall runoff in a stormwater system, sediment tracks along a channel, an entire stream network, a river system, water-sewer pipes, and/or even electrical lines--basically, anything with a "flow". As our textbook states, "such analyses are useful in environmental planning, forestry, wildlife biology, and hydrology" (Mitchell).
For our lab assignment, we focused in creating a flow accumulation model to determine where water flows and gathers over a terrain surface. Specifically, we learned how to delineate an entire stream network and watershed from data solely obtained through a Digital Elevation Model (or DEM), at the island of Kauai, Hawaii. After creating my model, I created a cartographic output (as shown above) comparing against an existing model delineated by the U.S. Geological Survey National Hydrography Dataset team.
There is a very specific procedure and sequence of tools to use to accurately delineate a stream network and model a watershed analysis. Because of its cumbersomeness, ModelBuilder and/or Python scripting are highly encouraged to be used. However, as a fellow graduate student, I learned how to do it the hard way first :). I used the majority of Hydrology tools under the Spatial Analyst Extension Toolbox within ArcGIS (as shown in the screenshot to the right).
2. Then I used the Flow Direction tool to create an output determining the direction of flow from the steepest path of every cell in the raster dataset (using the eight-direction D8 calculation method).
3. Then I used the Flow Accumulation tool to calculate the weight of all raster cells flowing downslope.
4. Then I used the Con tool under Spatial Analyst > Conditional to define in more detail the stream network.
5. Then I converted this file to a vector format using the Stream to Feature tool.
6. Then I used the Stream Link tool to create (and be able to identify) individual stream segments.
7. Then I used the Stream Order tool to characterize hierarchy of these stream segments (there are 2 methods to choose from: Strahler or Shreve).
8. Then I used the Watershed and Basin tools to create their respective boundaries.
9. And finally, I manually modeled a "pour point" (and later on used the Snap Pour Point tool) to compare how the watershed outlets shifted.
For our lab assignment, we focused in creating a flow accumulation model to determine where water flows and gathers over a terrain surface. Specifically, we learned how to delineate an entire stream network and watershed from data solely obtained through a Digital Elevation Model (or DEM), at the island of Kauai, Hawaii. After creating my model, I created a cartographic output (as shown above) comparing against an existing model delineated by the U.S. Geological Survey National Hydrography Dataset team.
There is a very specific procedure and sequence of tools to use to accurately delineate a stream network and model a watershed analysis. Because of its cumbersomeness, ModelBuilder and/or Python scripting are highly encouraged to be used. However, as a fellow graduate student, I learned how to do it the hard way first :). I used the majority of Hydrology tools under the Spatial Analyst Extension Toolbox within ArcGIS (as shown in the screenshot to the right).Here is a quick (notice the emphasis) synopsis of the major processes undertaken (in order):
1. The very first step in our stream network and watershed modeling analysis was to make the DEM hydrologically correct using the Fill tool to fill-in the sinks.2. Then I used the Flow Direction tool to create an output determining the direction of flow from the steepest path of every cell in the raster dataset (using the eight-direction D8 calculation method).
3. Then I used the Flow Accumulation tool to calculate the weight of all raster cells flowing downslope.
4. Then I used the Con tool under Spatial Analyst > Conditional to define in more detail the stream network.
5. Then I converted this file to a vector format using the Stream to Feature tool.
6. Then I used the Stream Link tool to create (and be able to identify) individual stream segments.
7. Then I used the Stream Order tool to characterize hierarchy of these stream segments (there are 2 methods to choose from: Strahler or Shreve).
8. Then I used the Watershed and Basin tools to create their respective boundaries.
9. And finally, I manually modeled a "pour point" (and later on used the Snap Pour Point tool) to compare how the watershed outlets shifted.
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