DEM and Arcgis Hydrology Tools Exercise
January 28, 2013
Using Arcgis Hydrology tools
Daniel Sheehan (dsheehan@mit.edu, gishelp@mit.edu)
Anne Graham (grahama@mit.edu)
Introduction to the Hydrology Tools in Arcgis
Several hydrology tools are part of the basic ArcGIS Spatial Analyst Extension.
To see all of the hydrology functionality in ArcGIS, find it in the on-line help. Search for
“hydrology”. The first entry should be “An overview of the Hydrology Toolset”.
For this exercise, you should use the DEM that you downloaded earlier.
You will walk through three tools:
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
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Fill
Flow Direction
Flow Accumulation
You will also use the Raster Calculator to find cells in the Flow Accumulation grid that meet a
threshold for developing a stream channel and convert the output to vector (feature) format.
Write all output files to your T:\ drive folder.
The Fill command
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. Find the Fill command, as well as the
FlowDirection and FlowAccumulation commands in the Hydrology Section of the Spatial
Analyst Toolbox. You should see this form:
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Fill in and be sure to call your new DEM “filleddem” and click OK. The new DEM should not
be visibly different from the previous version.
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.
32
16
8
64
CELL
4
128
1
2
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.
Find the Flow Direction tool and start it. You should see this form:
Be sure to use the filled DEM (with no sinks) as the input and “flowdir” as the output.
You map should look something like this:
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While the numbers are not intuitive, the map should be somewhat easy to read. The areas of
each value represent areas of similar aspect. The light blue in the map represents land that
slopes to the west (value 16 from our chart above). Other colors have similar meanings. You
don’t analyze this layer directly but instead use other layers, such as the Flow Accumulation
layer.
Flow accumulation gives you the total area (number of cells times the area of each cell) that is
upslope and/or upstream of any given cell. Cells that lie on drainage networks have
substantially higher values than cells on hill slopes. The Flow Accumulation tools form looks like
this:
The resulting map should be a little difficult to interpret:
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The streams are highlighted because the default symbols are from black (small area
contributing to runoff) to white (large area contributing to runoff) but the number of cells that
are represented are large while your screen has limited pixels. This causes screen moirés which
make the lines look disconnected. If you zoom in, you should see the stream channels intact
and hopefully make the data easier to interpret.
Stream Threshold
Stream threshold values are used to determine the actual stream channels. This value is highly
dependent surface vegetation, rainfall amounts, and soil and/or rock type. A value of 0.3
kilometers, or approximately 3330 cells in an approximately 10 meter DEM, is a good
approximation for this area. Use this value or run various test values in terrain that you know
well to empirically test this value. To find the streams using this threshold value, find the Raster
Calculator in the Spatial Analyst toolbox under Map Algebra. You should see this form (fill in as
below):
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The output of this operation should be called streams. Now convert this cell based data to
vector format to make it easier to symbolize you data. Find the Conversion Tools in the Toolbox
and find Raster to the Polyline conversion tool:
One you have this new layer, change the symbology as below by double clicking on its line color
in the table of contents:
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Contour lines
Now draw 1 meter contour lines. You can find the Contour tool under the Spatial Analyst
section of the toolbox under Surface:
Be sure to run this command on the Filled Dem. Do your streams look reasonable? Do your
contour lines tell you that there may be more stream channels than you found use the 3330
stream threshold value?
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