Using the Stock Hydrology Tools in ArcGIS with the Arc Hydro Tools Extension and in the Raster Calculator

Several hydrology tools are part of the basic ArcGIS Spatial Analyst Extension. The University of Texas at Austin Hydrology program under Professor David Mainment has produced an extension to ArcGIS which incorporates these tools into a GUI. The commands are also accessible through the Raster Calculator. The tools can be downloaded at from ESRI (http://support.esri.com/index.cfm?fa=downloads.dataModels.filteredGateway&dmid=15).

To see all of the hydrology functionality in ArcGIS, find it in the on-line help. Open up help (ArcMap's Help menu >> ArcGIS Desktop Help) and go to the Contents tab. Go to Extensions >> Spatial Analyst >> Spatial Analyst Functional Reference. There is a section called Hydrologic Surface Analysis. There is also tools for groundwater modeling.

For this exercise, you should use the DEM that you downloaded last week.

Adding the toolbar

Click on Tools and then Customize. You should see this form:

Click on Arc Hydro Tools then click close.

Once you have the Toolbar available, this is what it should look like:

Using the Tools

The following steps are performed using the tools in the Arc Hydro Toolbar

Identifying and filling sinks

Sinkholes occur naturally in certain types of landscapes, such as karst (limestone) where the rock is soluble. Outside of these areas you will find sinks in your DEM data which are simple errors in the data, due to a typo, a place where the scale of the data does not adequately represent an existing drainage channel, or some other source. Generally, hydrology tools is GIS do not deal easily with sinks, whether natural or an error in the data. You will need to remove the sinks before you look at flow direction and flow accumulation.

Using the toolbar's drop down menu bar, click on Terrain Processing then click on DEM Manipulation then click on Fill Sinks. You should see this form:

You will see this form. Be sure to select the original DEM. This process can be time consuming so be patient.

Flow Direction and Accumulation

Now create a flow direction grid (again, available from the toolbar's drop down menu). You should use the corrected (filled) DEM and save the resulting grid to the same folder as above. The flow direction grid is very similar to the aspect grid you created in the last lab except the values in the grid are much more limited.

This illustration shows how the values are set. If the flow of water would flow to the east, the value for the cell would be 1, to the southeast, the value would be 2, etc. This is the required format of data for input for creating flow accumulation grids. The Flow Direction grid is created from the command of the same name under the Terrain Processing menu. Be sure to use the filled DEM (with no sinks) as the input.

Now that you have the flow direction grid, find the flow accumulation grid, again using the Terrain Processing menu dropdown list. The cell values in this grid depend on how much area above the cell in the watershed flow into this cell. The cells at the edge of the watershed will have a value of 0. The water of these cells flow into down slope cells, which then would have a value of 1, or more if more than 1 cell flowed down slope into this cell. This process is compounded, using the flow direction grid as a guide. Use the new Flow Direction Grid as the input.

Stream Network

You can now find the stream network that are identified in your model. Use the drop down menu to find this feature. You will again need to identify your flow accumulation grid here:

You will also have to set a number of cells that are required to contribute to flow to create a stream using this form:

This is variable for different climates. Remember that a cell represents an area of 30 by 30 meters or 900 square meters. It is approximately 400 cells for the climate and vegetation of New England. It might be 300 cells for the Pacific Northwest. The values for desert regions might be higher since water may flow fast over land without vegetation. You may want to see a 1:24,000 contour map of the area you are working in to compare the value that you are using and the results that you are seeing to what the USGS uses. You may also want to visit the area with a GPS to check your values in person.

Stream Catchment areas

You should delineate the areas of each sub basin in your DEM. To do this you will need to do the following three steps:

• Stream definition (define the area that is required to produce sufficient flow to define a stream - already done [Stream Grid - STR])
• Stream segmentation (segments streams into reaches defined by intersections with stream branches)
• Catchment grid delineation (develop a grid of stream catchment areas based on stream segments)
• Convert the Cat grid (catchment grid) to Polygons using the Spatial Analyst menu (dropdown menu, finding Convert then Convert Raster to Features)

Flow Path Tracing

Now try the Flow Path Tracing tool on the Arc Hydro toolbar.

The Flow Path Tracing tool produces a graphic that shows which path the rain will flow. The rainfall tool does work well, although its output is a screen graphic, not a shapefile that you can save and use later.

Using the watershed tool in the Raster Calculator

For this section of the lab, you will need to create a point shapefile. To do this, open ArcCatalog (from the Start Menu, to All Programs, to Arcgis, finally ArcCatalog). Next, navigate to your H:\ drive. Click on the File menu and then select New then Shapefile to see this form:

Name the shapefile then "Edit" the map projection to see this form:

"Import" the projection from your DEM. This opens a file dialog which you should use to navigate to the DEM you created. Double click on the DEM and then click OK to see the "Details" of the projection, as shown in the example above. Click OK to complete the previous form (creating the new shapefile).

You will use the watershed tool in the Raster Calculator. To do this, you will need to use the point shapefile your created in ArcCatalog. The point shapefile should have the same projection as your DEM. Add the shapefile to ArcMAP. Find a point on a significant stream in your area. Add a point to the shapefile there. Zoom in far enough to ensure that your point is exactly on the stream itself, and not just nearby. Add an ID of 1 to the record in the layers table (which is editable while the shapes, or points, are editable). This point might be a dam location or a bridge crossing where you want to find out what the maximum flow is like above, or where you are interested in having a reservoir and need to know what land uses and land covers exists above this location in the watershed.

Be sure to set the Spatial Analyst options so that the extent and cell size are set to that of your DEM using the Options menu choice in the Spatial Analyst toolbar:

You should also set the Spatial Analyst temporary folder to a folder on your H:\ drive. Using the Spatial Analyst drop down menu, convert this feature shapefile (points) to a raster layer. You will have to pick the point shapefile from the drop down list. You should also make sure the cell size is the same as the cell size of your DEM. Save this to the same folder that the shapefile is in. Create the new raster dataset. This new grid will have a single cell, covering the area where your point is.

Next, export the Flow Direction grid to the same folder that the shapefile is in. Add it to the map.

Now, using the raster calculator (not the Hydrology watershed tool - find it in the Spatial Analyst dropdown menu), find the watershed above this. According to the help, watershed is a function that returns a grid that has cells only for the watershed above the point(s) in your source grid (your point shapefile that you created). The command is:

watershed([flow direction grid], [point grid])

And looks like this in the Raster Calculator:

This function requires that the grids are all in the same projection and in the same folder (on your H:\ drive). The instructions above indicated you should do this. If you have problems running this command, check the source tab under the properties form for the grids involved in the command, to be sure that they are all in the same directory.

Finally, create a grid with elevations only in the watershed. Use the raster calculator to do this, using this syntax (where the watershed grid is called Calculation and the Filled Dem is called FilledDEM):

[Calculation] * [FilledDem]

and display this on your map.

Created 9/30/03, by Daniel Sheehan
Last updated 10/9/07 by DS