Working with Watershed Data
Managing extensions
First make sure there is a copy of hydro.avx and prjctr.avx from samples\ext\ in the ext32\
directory in the ArcView installation directory.
Click on File  Extensions and check Spatial Analyst, Projector!, and hydrologic modeling
extensions.
Import data
Open a new view and click on File  Import Data Source  “USGS DEM” and choose the
pine grove mills DEM. Name the output file “pgdem”.
Use the Import71 program to import the stream theme. Import71 should be located under Start
 Program Files  ESRI. Browse to the location of the rv-huntingdon.e00 file and then
browse to the location you want the output to be created. Name the output rv-hunt. Add both the
pgdem and the rv-hunt stream theme to a view. YOU MAY NOT SEE ALL THE DATA – SEE
NEXT SECTION.
Projections
If you turn both of these themes on, either you will only see one of them, or you will see neither
of them in your view (except possibly as pinpoints). This is because they use different
projections/coordinate systems to define the location of their features. The DEM uses units of
meters for X and Y coordinates, while the streams use decimal degrees.
If you look at the metadata that is downloaded with these files, you will discover that the DEM is
in UTM 1927 Zone 18 projection and the stream theme is unprojected.
Using the Projector! extension we can reproject the rv-hunt stream file to the same projection as
the DEM. Click on the stream file to make it active and then on the Projector! button.
The
output projection will be UTM 1927 Zone 18 with units of meters. Name the output file hunt27.
Add the newly projected shapefile to the view.
NOW BOTH DATA THEMES SHOULD OVERLAY IN THE DEM COORDINATE
SYSTEM.
Working with Grids
Make pgdem active
Click on Hydro  Fill and the process of filling the DEM will begin.
To save the calculation make it active and then click Theme  Convert to Grid and name it
pgfil.
This grid is similar to the original DEM, with all internally draining sinks filled.
Click on Analysis  Map Calculator
In the analysis window create this equation by selecting grids from the upper left menu and
clicking the subtraction button.
( [Pgfil] - [Pgdem])
This new grid will show you how much the original DEM was filled.
We can now calculate the flow direction and accumulation using the Analysis  Map
Calculator.
Flowdirection determines the direction of steepest descent based on the eight pour point model
with the eight neighboring grid cells.
Flowaccumulation sums the number of other grid cells that would flow through each grid cell
based on the flowdirection data.
Syntax
aGrid.FlowDirection (forceEdge)
Create the following equation.
( [Pgfil].flowdirection(false))
True means flow is forced off the edge of the DEM.
False means that a border surrounding the DEM is slightly lower than the edge grid cells,
therefore flow will only proceed off the DEM if there is no other lower neighboring grid cell.
Make the result active and click Theme  Convert to Grid and save as pgfdr
Syntax
aGrid.FlowAccumulation (weightGrid)
( [pgfdr].flowaccumulation(Nil))
Make the result active and click Theme  Convert to Grid and save as pgfac
Compare pgfac and hunt27
Burning streams into the DEM
We can now perform a process called burning streams into a DEM.
An existing delineation of streams is imposed on the DEM in order to improve the final
representation of streams and to preserve the relationship between the various calculations to be
performed.
Select the portion of hunt27 that overlaps the DEM (make the stream theme active and use the
select feature tool.)
Click Theme Convert to grid and name it hunt27grid
Use the extent and cell size “same as pgdem”
Use “fnode” to assign the value.
Click NO for join attributes.
This new theme will be used to lower the elevations in the DEM in the local areas that
correspond to the predetermined stream locations as defined by the stream theme.
Add the grid to the view
Click Analysis  Map calculator
([hunt27grid].merge({[pgdem]}))
Activate the result and click Theme  Convert to Grid and name it pgburn
Now you can repeat the stream delineation process above.
You can at this point prepare to delineate the watershed for any grid cell or group of cells on the
DEM.
Select any number of source points or lines and convert them to a grid named outletgrid.
Syntax
dirGrid.Watershed (srcGrid)
([pgfdr].Watershed(outletgrid))
This function will create a new grid of all grid cells that flow through the outlet grid cells based
on the flowdirection data.
Time permitting we may look at creating contours and hillshading and 3D analyst.