Introduction to
Geographic Information Systems in Forest Resources |
Exercise: Hydrologic Modeling
and Watershed Delineation
- Start ArcView and open a new project
- Create a grid subset
- Watershed Delineation
- Creating a depressionless DEM
- Flow direction
- Flow accumulation
- Watershed outlet points
- Delineating watersheds
- Automatically delineating watersheds
- Calculating watershed metrics
- Raster to vector conversion
- Converting a watershed grid theme to a polygon theme
- Stream network as line shape
- Centroid as point shape
- Watershed visualization
Start ArcView and open a new project
- Delete everything from your Zip disk.
- Download the project hydro.apr onto your
Zip drive..
- Using the startup project, open the hydro.apr project.
- Enable the Spatial Analyst Extension.
- Enable the Hydrologic Modeling v1.1 (sample) Extension. Note the
version of the extension!
Create a grid subset
- Add the grid theme Dem to the view.
- Add the Streams line feature theme.
- Perform a query on the Streams theme to select 27 Creek.
- With the Streams theme active, click the Zoom to Selected
button
.
- Zoom out one step
.
- Keep the relative proportion of your view (in X and Y) nearly square.
- The selection and zoom process is used only to set the display area
for grid processing.
- Your view should look something like this (assuming you alter the dem
theme's legend in the same way I did):
- Set the analysis properties to the current display extent and the cell size
of the Dem grid theme.
- Create the new grid theme subset with a Map Calculator operation:
This will create a grid subset.
- Do NOT save the project (yet). If you do, you will be making the new grid
permanent, and this will take up needed space on the Zip disk.
- Delete the Dem grid theme from the project. Zoom out one step.
Do you see how we used a selection, set the display area, and used the display
area for the output analysis extent? If it did not before, does the analysis
extent make sense now?
Now your view should look like this:
- Rename the Map Calculation 1 theme to Dem_clip (Theme >
Properties).
You have just created a new grid theme of elevation values, but limited to
a small spatial extent. Processing will be much faster with this small grid,
rather than using the entire forest area.
Watershed Delineation
- Creating a depressionless DEM
- Make the Dem_clip theme active.
- From the Hydro menu, select Fill Sinks.
While the grid is filling, you might want to entertain yourself with the
Onion.
- After a few minutes, a new theme, Filled Dem_clip, will be added
to the view.
This is the same data set as Dem_clip, but any areas of internal drainage
are filled in.
- Save the new theme:
- Make the new theme (Filled Dem_clip) active.
- Select Theme > Save Data Set from the menu. (Note that if
you have saved the project, the Save Data Set choice will be unavailable,
as the temporary grid will already have become permanent.
- Accept the default name of fill1.
Saving temporary grids before saving the project is important
so that you can have an opportunity to give the permanent grid a location
and name, rather than using ArcView's default choice. This is especially
important if you are using an AutoSave extension. If you are using AutoSave,
you should definitely save the data sets with a name you can recognize
later.
- Delete the Dem_clip theme from the view, since you will be working
on the filled grid from this point on. The grid data source for Dem_clip
(calc1) should automatically be deleted when the theme is deleted.
- If you want to save the project now, check that the calc1 directory
is gone from the Zip disk before you save.
It is important to have a depressionless DEM for all subsequent hydrological
analyses.
- Make the Filled Dem_clip theme active.
- Select Hydro > Flow Direction from the menu.
- When the new theme (Flow Direction) is added, save the theme (do
not simply rename it in the view) with the default name fdir1.
- Turn off the Filled Dem_clip theme.
- Turn the Flow Direction theme on. Note that the numbers refer to
coded direction of flow.
Direction of flow must be known for each cell, because it is direction of
flow that determines the ultimate destination of water flowing across the
surface.
- Make the Flow Direction theme active.
- From the Hydro menu, select Flow Accumulation.
- When the new theme is added, save it with the default name facc1.
- Turn off the Flow Direction theme and turn on the Flow Accumulation
theme.
- Alter the legend for this theme.
- Use only a single class.
- Make No Data cells transparent.
- Alter the value to show only high-flow cells (you should only need to
change the lower limit of the Value range, keeping the higher limit;
your higher limit may be different from mine).
Now those cells which have at least 5000 cells upstream are displayed
in red. All other cells are transparent. Note: you
will need to zoom in before you can see the details of the flow network.
Flow accumulations are important because they allow us to locate cells
with high cumulative flow. Pour points must be located in cells
of high cumulative flow, or the resultant watersheds will be very small.
- Create a new point theme by selecting View > New Theme from the
menu.
- Name the new theme pour_pts.
- Zoom in before adding any points. The boxes in this view show the general
location of the points which you will add. Refer to the images below for more
precise placement. These boxes do not appear on the
view, but are only displayed here to orient you.
Note the position of 27 Creek (the stream network in the center of this image
of the view). One watershed we are creating is just west of 27 Creek's confluence
with the Mashel River. At that location, there is no stream in the vector
data. The other watershed (Little Mashel) is east of 27 Creek .
- Zoom into the area defined by the westernmost box.
- Using the Add Point tool,
add a single point
within a high-flow cell, as shown here. Direction of flow is east to west,
so this outlet point will create a single watershed upstream on both flow
networks.
| You must zoom
in quite a way to do this, otherwise your pour point may not be located
within a high-flow pathway! |
Use the Identity tool to examine the values of the Flow Accumulation
theme. You will be able to tell which direction is upstream (lower accumulation
value) and which direction is downstream (higher accumulation value).
- Zoom back out to full view.
- Zoom into the area defined by the easternmost box .
- Add a single pour point within the high-flow network. Note that this point
is at the confluence of the two flow segments, so will create a watershed
that contains upstream areas of both streams.
Everything upstream from each one of these points will define a single watershed.
- Open the theme table for the Pour_pts theme. Alter the ID
field to represent unique values for each different record (e.g., 1
and 2). Once you have added the points and altered their IDs, stop
editing the theme, making sure to save the edits. If you do not alter the
ID values for the points, you will not generate unique watershed areas.
- Clear any selected set for the Pour_pts theme.
- Zoom to the extent of the Pour_pts theme.
- Set the Analysis Extent to Same as Flow Direction.
- Convert the Pour_pts theme to a grid theme, using ID as the
conversion field. Call the new theme pourg.
- Add the new theme to the view. Alter the drawing order so you can see all
the themes clearly.
If the Analysis Extent and cell size do not match an existing theme, there
may be problems of registration between the outlet grid and the other grids
necessary to delineate watersheds. Therefore, it is always a good idea to
set your cell size and analysis extent relative to an existing grid theme.
Note that the new Pourg theme cell covers the place where the last
pour point was added. You should pan and zoom to verify that the Pourg
cells are indeed located in a high-flow pathway. If the outlet grid cells
are not in high-flow pathways, the watersheds you create will be small. If
the outlet cells are not directly "over" the flow path, you will
not create watersheds that represent large drainage areas.
- Click the Watershed! button
. If you do not see
the button, maximize the ArcView application window. This is a custom button
that is associated with an Avenue script, and is not part of the sample extensions.
- Follow the instructions in the series of popup dialogs to select the Pour,
Flow Accumulation, and Flow Direction grids. Please take the time to think
about what you are doing, and specify the correct grid themes for each of
the dialogs! Otherwise, you will get incorrect results.
- A new theme will be added, called Gridn where n
is the sequential number of new watershed grids generated. Turn the theme
on.
These two large grid zones represent the areas upstream from the selected
pour points. Note that the larger watershed goes up to a ridge line on the
south and far east, but only up to the next sub-basin on the southeast. Also
note that both watersheds are artificially truncated by the analysis extent.
If you create "real" watersheds, it will take longer, but you should
not use an artificial analysis extent. Watersheds should be allowed to go
to the natural ridge line. We used an artificial truncation in order to run
through the process faster.
- Convert this to a polygon shapefile called watrshed.shp, and add
it to the view.
You have just delineated two watersheds, based on your elevation grid data
and the outlet points you chose. If your watersheds are very small, it is
because you located your outlet points outside of a high-flow pathway, or
you did not fill your original input elevation grid.
Automatically delineating watersheds
Compare your method with an automated method.
- From the Hydro menu, select Hydrologic Modeling.
- For the elevation surface, select Filled Dem_clip. Check all boxes.
- After a minute or two you will be asked to specify the minimum watershed
size. Enter 50000 (no commas). This means that watersheds will be automatically
generated from pour points with an upstream value of 50,000.
- Several minutes will pass, and the new watersheds will be displayed.
- If you are not satisfied with the number or sizes of these basins, click
the No button on the Hydro.dd Watershed dialog.
- You will be prompted to enter a different value for minimum watershed size.
You can either increase or decrease the value. Enter the value of 100000
(no commas).
- If you are satisfied with these, click the Yes button on the Hydro.dd
Watershed dialog.
In addition to the new watershed grid, you will be prompted for the name of
the output polygon shapefile. Accept the default watrshed.shp file
name. This will take several more minutes.
These are the watershed zones automatically generated with 100000 as
the minimum number of cells. Each watershed is at least 100,000 cells in area.
After the process completes, open the theme table for the new polygon theme.
It will contain several metrics for each watershed, including all the metrics
chosen in the check boxes.
- Open the theme table for the new polygon theme. Sort descending, based on
the Centroidx field. Notice that only 8 of the records have values. These
8 records correspond with the 8 watershed zones. Each field is a specific
watershed metric. (Your values may differ from mine).
You have just let ArcView automatically generate a series of watersheds. ArcView
also creates a polygon theme with metrics calculated for each watershed.
Calculating watershed metrics
Generate a few different grids representing various watershed metrics.
- Get a view ready for analysis
- Make the Watrshed grid theme active.
- From the Hydro menu, select Area. This will create a new grid
theme.
- When the new grid theme is added, make it active (but you do not need to
turn the theme on).
- Using the Identify tool, click on each of the watersheds, and the
area in sq. ft. will be shown in the Identify Results dialog.
- Add the fdir1 grid theme to the view
- Make the Watershed theme active.
- From the Hydro menu, select Flow Length by Watershed.
- Select the Fdir1 theme as the Flow Direction theme.
- Click No when asked to Calculate length to an outlet. This
will specify to calculate the maximum length of flow per watershed, rather
then length of flow per cell.
- When the Maximum Flow Length grid theme is added, turn it on, make
it active, and Identify a few cells. This is the calculated maximum
length along the flow path for each watershed.
- You can generate the full suite of watershed metrics for an existing watershed
grid by selecting Hydro > Hydrologic Modeling from the menu.
Select the filled DEM grid and the existing watershed grid in the dialog:
If you use this technique, make sure to look at the attribute table for the
watershed theme to see the fields that were created and their values.
Feel free to explore the other Hydrological analysis tools. Look
at the online documentation (F1)!!!!
Raster to vector conversion
- Converting a watershed grid theme to a polygon theme
- If you use the Hydrologic Modeling tool from the Hydro menu
and select Use Existing Watershed, a polygon shapefile will be automatically
created, with attributes calculated for the checked metrics.
- If you use manual methods of creating watersheds, you will need to use this
technique (which you have already done in previously in this exercise).
- Make the Watershed theme active.
- From the Theme menu, select Convert to Shapefile.
- Call the new shapefile wsheds.shp.
- Stream network as line shape
- Make sure you have a Flow Direction and Flow Accumulation
grid in your view..
- Make the Flow Accumulation theme active.
- From the Hydro menu, select Stream Network as Line Shape.
- Specify a minimum flow accumulation of 5000. This specifies that
stream vector data will be created only for cells with at least 5,000 upstream
cells.
- Specify Fdir1 as the Flow Direction theme
After a few moments the new theme, Stream Network Shape - (5000), will
be added to the view. The theme is automatically saved in the working directory
as rivn.shp, where n is incremented with the number
of stream network shapefiles created.
- Make the Watershed theme active.
- Select Hydro > Centroid as Point Shape.
- A point shapefile will be created that represents watershed centroids (sometimes
called centers of gravity).
Watershed visualization
- Create a new view.
- Add a copy of the grid theme Filled Dem_clip from the other view.
- Add the Pour_pts point feature theme
- Add the line feature themes Contour and Streams from the CD.
- Add a copy of the Watrshed.shp polygon feature theme.
- Alter its legend so that is is not filled and displayed with a red outline.
- Make the Filled Dem_clip theme active, and select Compute Hillshade
from the Surface menu.
Accept the default parameters for the hillshading.
- Alter the drawing order and legends so that features are discernible. Other
than the artificial truncation, does this look reasonable to you?
Syllabus