Simplified Method for Creating a Watershed Layer from a DEM Using ArcMap and Spatial Analyst

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Simplified Method for Creating a Watershed Layer from a DEM  Using ArcMap and Spatial Analyst

Simplified Method for Creating a Watershed Layer from a DEM

Using ArcMap and Spatial Analyst

Preparation:

a)      In ArcMap, be sure the Spatial Analyst Extension is loaded and the Spatial Analyst toolbar is visible.

b)      Add in your DEM layer.

c)      Add in vector layers for rivers and shoreline. These will be used for visually locating a general area to place the pour point.

d)     Set your geoprocessing environment to use the same spatial reference system and cell size as your DEM layer.

e)      You now need to set your processing extents in the geoprocessng environment. Here is simple (and simplistic) way for doing this:

·         Using the river layer overlaying your DEM, zoom in to an area somewhat larger than the watershed of interest.

·         Set your raster processing extents to “same as display” setting.

Step 1: Create the Flow Direction and Flow Accumulation hydrology layers from your DEM

a)      Run the Fill tool

·         Input: (your DEM layer)

·         Output: DEMFill1[*]

b)      Run the Flow Direction Tool

·         Input: DEMFill1

·         Output: FlowDir1

c)      Run the Flow Accumulation Tool

·         Input: FlowDir1

·         Output: FlowAcc1

d)     Using the Spatial Analyst raster calculator, determine the actual river lines from the flow accumulation layer:

·         Make sure that FlowAcc1 is showing in the Spatial Analyst toolbar “layer” field.

·         Use raster calculator to evaluate “FlowAcc1 > 5000”. (Note the value of 5,000 is an arbitrarily high number representing large flows in the river bed. Depending on the size of your watershed, you may have to adjust this number.

·         In the resulting “Calculation” layer, resymbolize the “0” values to be “No Color” (i.e., transparent). You now have a raster layer showing the river course.

You now have a flow direction and a flow accumulation raster for your watershed area, as well a raster layer showing the river course.

Step 2: Create a Pour Point Layer (shapefile)

(These instructions are based on the document http://courses.washington.edu/geog460/readings/ArcMAP/Make%20a%20shapefile/ )

a)      Open ArcCatalog and navigate to your own working directory.

b)      Create a new shape file using the File-New menu.

c)      Choose a file name (e.g., PourPoint1), and choose “point” as the Feature Type.

d)     Sets shapefile’s coordinate system to be the same as the DEM’s by pressing the Edit... button, then Import, then navigating to your DEM.

e)      Click OK. You have now created an empty shapefile (i.e., it has no features in it yet).

Step 3: Specify a pour point in the PourPoint1 shapefile

The most critical stage in generating a watershed from your DEM is specifying the correct pour point(s). This technique is a simplified way of choosing a single pout point at a river mouth using visual inspection. Other, more automated and more rigorous methods exist, but are beyond the scope of this document.

a)      Adjust your map:

·         Place the new shapefile, PourPoint1, “on top” in the map’s table of contents.

·         The river and coastline vector layers must be visible just under PourPoint1.

·         The “Calculate” layer (the results of the raster calculator operation that shows the river course) should be visible under the river vector layer.

·         The DEM should be under all.

·         All other layers should be “unchecked” (not visible)

·         Change the symbology of your DEM to turn the “display background value” option on (with the value set to 0) and change the background to some contrasting color (e.g., red).

b)      With your DEM layer in the background and using the river and coastline vector layers as visual guides, zoom in to the river mouth (at the coastline) until the individual pixels (cells) in the DEM raster layer are large (1/4 inch or so) on the screen.

c)      Locate the cell location in the “Calculate” layer that seems to best represent the actual river mouth:

·         This may be the first cell where the “Calculate” layer meets the a zero-value (red) cell in the DEM; or

·         This may be a cell in the “Calculate” layer that is closer to the shoreline.

·         Your choice of cell will determine the “end” of your watershed.

·         NOTE: Use the raster river course layer and the DEM to determine your pour point, NOT the river and shoreline vector layers (these vector layers are for reference, but they are not consistent with the raster-based layers).

         

d)     Now that you have chosen the “river mouth” cell, you will place your pour point over that cell:

·         Turn on the “Editor” toolbar.

·         In the Editor menu, choose “Start Editing.”

·         At the “Start Editing” dialog, choose the folder in which you have saved PourPoint1. On the Editor toolbar, make sure PourPoint1 is visible in the “Target” field.

·         The “Task” field should be set to “Create New Feature.” Choose the “Sketch” tool (looks like a pencil).

·         Place the crosshairs in the center of the cell that you have identified as the river mouth. Click the mouse once.

·         In the Editor toolbar, choose “Save Edits”, the “Stop Editing”

You have just put a point feature in the PourPoint1 shapefile that corresponds with the mouth of the river. This point will be the “pour point” used in the Watershed tool in the next (and final) step.

Step 4: Run the Watershed Tool

·         Input raster: FlowDir1

·         Input Pour Point: PourPoint1

·         Output: Watershed1

Note that all of the cells inside the watershed have the value of 0 (zero). Those outside the watershed have the value of NoData.