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How To: Create a threshold raster to be used as an input for the Spatial Analyst Hydrology tools

Summary

Creating a watershed requires computing the flow direction across a Digital Elevation Model (DEM) as well as defining the locations used to determine the catchment area. A point feature class is often used as the ‘feature pour point data;’ however, the Watershed geoprocessing (GP) tool also accepts a flow accumulation threshold raster. Using a threshold raster is particularly useful when pour points do not exist and watersheds need to be generated for the entire DEM based on the junctions of a stream network that has been derived from flow accumulation.

Procedure

The threshold raster defines the cell values that are determined to have enough accumulation to be classified as a stream, and the threshold raster is generated from a raster created by the Flow Accumulation GP tool.

Note:
This workflow requires a license for the Spatial Analyst extension.


Note:
This workflow assumes that all preparatory work on the input elevation surface, the DEM, has already been performed. For more information, refer to the following links:
Creating a depressionless DEM
Understanding drainage systems

  1. Create the flow direction and flow accumulation rasters.
    A. Open ArcMap and add the DEM.
    B. Run the Flow Direction GP tool with the DEM as the input.
    C. Run the Flow Accumulation GP tools with the flow direction raster as the input.

    Note:
    For more information about these tools, refer to the following links:
    Flow Direction (Spatial Analyst)
    Flow Accumulation (Spatial Analyst)

  2. In the Properties of the output flow accumulation raster, change the symbology of the raster to show classified breaks.

    Note:
    Manual manipulation of the symbology on the flow accumulation raster shows the cells where water is prone to accumulate, which correlates with large cell values.

    Class Breaks prior to manual manipulation.
    [O-Image]
    Defined Class Breaks based on a minimum threshold value of 1700.
    [O-Image]
  3. When the threshold is determined, create a classified raster that only contains the cells with flow accumulation.
    A. Open Raster Calculator.
    B. Enter the following statement in the Map Algebra expression field and run the tool:

    Code:
    SetNull("RASTER_NAME" < 1800, 1)


    Note:
    This statement contains a hypothetical threshold value.


    Note:
    For more information, refer to the following links:
    Overview of the rules for Map Algebra
    Set Null (Spatial Analyst)

    This creates a raster that defines the stream network by assigning NoData values to those cells that do not meet the threshold. The remaining cells, representing flow accumulation, are contained in a single row in the raster attribute table.

  4. Use the Stream Link GP tool to assign unique values to the sections between junctions of the stream network contained in the raster.

    Note:
    For more information, refer to the following link:
    Stream Link (Spatial Analyst)

  5. Use the Watershed GP tool to determine the contributing area above a set of cells in a raster.
    A. Open the Watershed GP tool.
    B. Specify the flow direction raster created in Step 1 as the Input flow direction raster.
    C. Specify the threshold accumulation raster created in Step 4 as the feature pour point raster.

    Note:
    For more information, refer to the following links:
    How Watershed works
    Watershed (Spatial Analyst)