Create a watershed model using the Hydrology toolset

Summary

A watershed is an upslope area that contributes water flow as concentrated drainage. This area can be delineated from a digital elevation model (DEM) using the Hydrology toolset from the Spatial Analyst toolbox.

Procedure

The following instructions provide a workflow to create a watershed model using the Hydrology toolset from the Spatial Analyst toolbox and convert the model to watershed bounding polygons.

1. Run the Fill tool.
   1. In ArcCatalog, navigate to Toolboxes > System Toolboxes > Spatial Analyst Tools > Hydrology > Fill.
   2. Use a digital elevation model (DEM) for Input surface raster.
   3. Verify the path name for Output surface raster.
   4. Input the Z-limit, if necessary.
   5. Click OK.

Note:
The Fill tool fills sinks to remove imperfections from the DEM. A Z-limit affects the result of the tool. It is recommended to state the Z-limit if the depths of the sinks are known. The Sink (Spatial Analyst) tool can be used to identify the sinks and their depths prior to using the Fill tool. When a Z-limit is not specified in the Fill tool window, all sinks, regardless of depth, are filled.

2. Run the Flow Direction tool.
   1. In ArcCatalog, navigate to Toolboxes > System Toolboxes > Spatial Analyst Tools > Hydrology > Flow Direction.
   2. Use the DEM output from Step 1 for Input surface raster.
   3. Verify the path name for Output flow direction raster.
   4. Click OK.

Note:
The Flow Direction tool determines the direction of flow from each cell to its steepest downslope neighbor.

3. Run the Flow Accumulation tool.
   1. In ArcCatalog, navigate to Toolboxes > System Toolboxes > Spatial Analyst Tools > Hydrology > Flow Accumulation.
   2. Use the output raster from Step 2 for Input flow direction raster.
   3. Verify the path name for Output accumulation raster.
   4. Click OK.

Note:
The Flow Accumulation tool calculates the accumulated flow to each cell, as determined by the accumulated weight of all cells that flow into each downslope cell.

4. Run the Snap Pour Point tool to locate the pour points to cells of high accumulated flow.
   1. In ArcCatalog, navigate to Toolboxes > System Toolboxes > Spatial Analyst Tools > Hydrology > Snap Pour Points.
   2. Either input a point feature class or a raster for Input raster or feature pour point data.

Note:
Cells that are not NoData (cells with values) are considered pour points and are snapped if a raster input is used, while a point feature input specifies the locations of cells to be snapped.

3. Use the output raster from Step 3 for Input accumulation raster.
4. Verify the path name for Output raster.
5. Click OK.

Note:
Pour points are points at which water flows out of an area, usually the outlet or re-entrant locations from the flow accumulation. The Snap Pour Point tool snaps these points to the cell of highest flow accumulation within a specified distance.

5. Run the Watershed tool.
   1. In ArcCatalog, navigate to Toolboxes > System Toolboxes > Spatial Analyst Tools > Hydrology > Watershed.
   2. Use the output raster from Step 2 for Input flow direction raster.
   3. Use the output from Step 4 for Input raster or feature pour point data.
   4. Verify the path name for Output raster.
   5. Click OK.
6. Run the Raster to Polygon tool to create polygon features from the watershed raster.
   1. In ArcCatalog, navigate to Toolboxes > System Toolboxes > Conversion Tools > From Raster > Raster to Polygon.
   2. Use the output from Step 5 for Input raster.
   3. Verify the path name for the Output polygon features.
   4. Click OK.