NOTE: if you haven't already, you will need to activate the Spatial Analyst extension to complete this lab.
ArcMap > Customize drop down > Extensions. Click in the Spatial Analyst box to toggle on the check mark > Close.
In order to do anything your boss asked, you first have to define a catchment boundary. There are many ways you could do this. You could manually delineate the watershed based off of the contours (i.e. create a polygon shape file, and carefully draw the catchment). Numerous algorithms exist for deriving watersheds from digital elevation models. We’ll use one such algorithm that is built into the Spatial Analyst extension in ArcGIS. Delineating a watershed in ArcGIS requires using a sequence of hydrologic tools that create new output rasters at each step. Generally, the series of tools builds on itself by using the output raster from the previous tool as the input raster for the next tool in the series. The process is relatively user friendly, as the tool dialog boxes prompt you to use specific rasters for the input rasters.
here for more information).
Navigate to the Spatial Analyst > Hydrology Tools > Fill in the Toolbox or search for the Fill tool.
In the Fill dialog (below), specify the original input DEM and the name of the output raster you wish to create (make sure the name designates that it is a filled DEM).
Make sure that all subsequent Hydrology Analyses in this lab are run on this filled DEM.
Specify the pit-filled DEM (NOT the original DEM) as the Input raster, and specify the new output raster as a flow direction raster. You will use this flow-direction grid for other hydrology analyses.
WARNING! If you enter your regular DEM instead of the pit-filled DEM raster, this will not work correctly!
Lab 6 that defines the boundary of Big Cottonwood Creek watershed, but your boss would like you to delineate the boundary of this watershed to ensure the boundary is defined correctly.
You can "eyeball" the outlet from the DEM and hillshade and then use a tool called "Snap Pour Point" to force your 'best guess' pour point to snap to the lowest point or drainage channel as determined by the DEM. An alternative to guessing or using the Snap Pour Point tool is to determine where the lowest point of your intended watershed is. You can do this using the Flow Accumulation tool. You will use this for Task 2.
The dialog box prompts you to input your flow direction raster. (See, it's fool proof!) The output should be named something that designates the grid as a flow accumulation raster.
WARNING! If you enter your DEM instead of the flow direction raster derived above as your Input Flow Direction Raster, this will not work!
Your output may look something like the image on the left (the green 'temporary' watershed polygon is just there for reference). Change the symbology of the flow accumulation raster to show 2 classes. Make the smaller values class "no color" and the larger values something that will show well on your map (double click on the symbol box to change symbol properties) (see below).
What to Submit
For this task you should present your delineated watershed in a way that illustrates a difference between it and the watershed shapefile acquired from the UT AGRC. See What to Submit section on main Lab 8a page and Lab 8a rubric (Canvas) for details.
 If you are not happy (i.e. watershed delineation is wrong), edit your pour point shape file and move the pour point, save it, and try the analysis again. Alternatively, in ArcGIS 10 a new feature called Snap Pour Point, can be used to do this automatically for you.
Back to Main Lab 8 Page Ahead to Task 2
Assignments > Labs (4930/6920/6915) > Lab 08 - GIS Modelling Choice > Lab 08A - Morphometric Analyses >