This document describes how to use the ArcView GIS tool to delineate watershed boundaries from a DEM (digital elevation model). The spatial analyst ArcView extension is required. In this document, avenue code that creates input files for the HEC model will be used to generate watershed boundaries. The hydro extension can also be used to estimate watershed boundaries from a DEM. The approach described here offers several advantages over the approach in the hydro extensions though (e.g., we are allowed to select the desired watershed outlets).

Note this exercise can also be completed with the Watershed Delineation extension that can be downloaded from the ESRI WWW page. The version of the Watershed Delineation extension at the ESRI WWW page does not allow watershed outlet points to be easily input as a theme.

Start ArcView and load the desired project. To add the avenue routines for watershed delineation, make the document manager window active, select the Project pull down menu and select the Import option as shown below.

Copy the wshddelin.apr project from in ~engelb/public-web/abe526/wshddelin31.apr to your account (wshddelin.apr - save to a file. You may wish to place this in a subdirectory. Create a directory named tmp in the same directory you place the wshddelin.apr. The watershed delineation tools create temporary files and places them in the tmp directory. Choose the wshddelin.apr project as shown below from your account. This will load the avenue code for watershed delineation. Note that the ArcView interface will change to reflect the additional capabilities.

The analysis properties should be set before continuing the analysis. Select the Analysis pull down menu, select the Properties option, set the Analysis Extent to the DEM theme, and set the Analysis Cell Size to that of your DEM.

From the Hydro pull down menu, select the Fill Sinks option as shown below. You will be prompted to provide the name of the DEM theme and a name of a filled DEM that will be created. The Fill Sinks option identifies "sinks" or depressions and "fills" them so that flow can be routed to compute the watershed boundary. A DEM that has the sinks filled is created. This filled DEM is used in the next step to compute flow direction.

The dialog box that appears after selecting the Fill Sinks option is shown below. Provide the name of the DEM theme to be filled and the name of the filled DEM that is to be created. Note that it may take several minutes or more for the filled DEM to be computed. Reducing the extent of the analysis (Analysis Properties) can speed the computation.

The filled DEM is added to the view once it is created. An example is shown below. Next select the Flow Direction option from the Hydro pull down menu. The filled DEM should be the input to the computation of the flow direction theme.

The flow direction theme is shown below. Next compute the flow accumulation grid by selecting the Flow Accumulation option from the Hydro pull down menu. This will create a theme that shows how much area flows through each grid cell.

The flow accumulation results are shown below. Note that it may be desirable to edit the legend of the flow accumulation theme to obtain the level of detail of interest. By adjusting the legend, you can increase the detail of the estimated stream/channel locations. To edit the legend, double click with the mouse in the legend area of the theme's legend you wish to edit.

The legend of the flow accumulation theme has been edited as shown below to obtain more detail in approximating stream/channel location.

Next select the Stream Delineation option from the Hydro pull down menu. You will be prompted to select a threshold for an area to be called a stream. The threshold units are the number of grid cells that flow through an area. For instance in the graphic below, the 10000 indicates that 10000 grid cells or more would need to flow through an area before it is called a stream. For this analysis, 10000 was replaced with 500. Note that one can adjust the legend in the previous step to identify the desired threshold.

The results for a threshold of 500 are shown below.

The Hydro pull down menu contains other items that may be of interest. However, in this case we will not explore all options. We wish to delineate a watershed above a desired point. In this case the point is the stream gauging and sampling station at the Purdue University Animal Science Farm. The location of the stream gage is contained in a text file. The following will demonstrate how to add the text file and use the location of the gauging station to delineate the watershed boundary.

Make the document manager active and select the table document icon. Select the Add button as shown below. A popup dialog will appear. In this case the gauge information is in a txt file so txt files are selected and the name of the file is selected.

The table that is added is shown below.

Next select the view window to make it active. Select the View pull down menu and select the Add Event Theme option. A popup dialog will be shown as below. In this case the Table name (gages.txt) is correct and the Xfield and Yfield have been correctly assumed. If these are incorrect, use the menus to select the correct values. Select the OK button once values are correct.

The gages.txt theme is added to the view as shown below. The gages.txt theme has been turned on and the location of the gauge at the Animal Science Farm can be seen.

The watershed delineation algorithms require gages in the form of a grid. Convert the gage.txt theme to a grid equivalent by making the gages.txt active (click to the right of the theme's name), select the Theme pull down menu and select the Convert to Grid option.

We are now ready to delineate the watershed. From the Hydro pull down menu, select the Sub-watershed Delineation option. Provide the name of the flow direction grid, the name of the outlet grid, and the name of the watershed theme to be created. The watershed boundary generated is shown below.

You may have noticed that the estimated watershed differs from the watershed boundary for this area that we have used in class. This is largely due to differences in the watershed boundary that result because of the presence of roads and the ditches along these roads. The elevation theme used in estimating watershed boundaries in this document is not detailed enough to show the effect of roads and ditches along roads. For flat areas such as that used in this exercise, roads and their ditches are often important in delineating small watersheds.