GIS "Pro" Tips - Creating watersheds from a DEM

To create the watersheds for a specific area on a DEM, first you will want to create or use a set of streamlines (previous streamline creation is detailed HERE). Obviously, as with anything else, this method is not perfect. You may find that other methods work better for you. As I do with all my GIS stuff, take what works for you and change it to fit your needs.

Watershed Area:

With the creation of streamlines that are better than the DEM valued streams (as described previously)- do the following to obtain watersheds, and Longest Flow Paths.

• Simplify stream lines, especially if the vertex density is high on the streams. This will speed up the burning process. I think a simplified line with a Point Removal variance of 5 ft should be fine.
• Assign HydroID’s to the lines.
• Convert streams to ArcHydro Drainage Lines using “Create Drainage Line Structures”
• I often get an error running this tool, even though the “DrainageLine” file that is created is complete. Check the DrainageLine file to make sure the number of streams is the same as the original file. If it is, you should be fine.
• Use those Drainage Line Structures to burn into a DEM using “DEM Reconditioning”. 
• Leave values as default except set “Sharp drop/raise” = 100
• Run “Fill Sinks” tool on burned DEM
• Fill all
• Run “Flow Direction” tool
• Run “Flow Accumulation” tool

There may be better ways within Archydro to perform the following watershed tools. Perhaps by using the Catchment tool, which is essentially the Watershed tool under Spatial Analyst/Hydrology. Thinking about it, the Catchment tool may be the way to go but I haven't figured out a good way to do it, so here is my method.

• Using new point file, place points on Accumulation raster, just upstream of confluences for each stream segment.
• Give each point a unique ID (=FID+1)
• Run Snap Pour Point tool
• Run Watershed tool
• Convert Watershed Raster to polygon
• This will create a "catchment area" file. The watersheds that are delineated are really just the isolated sections that is being drained between confluences. 
• To create a file with the actual watersheds you are going to need to do some work. Starting with the furthest upstream catchments, this is what I do:
• Adjust watersheds so they actually go to the end of the streamlines to confluence and make sure they represent the actual watershed area.
• Sometimes roads, culverts, and other diversion structures don't show up on the DEM and LiDAR data so the watersheds form incorrectly. You may need to adjust the watersheds individually to match what actually should be the water drainage area. 
• If the watershed is the most upstream for that particular line segment, then the watershed should then be complete. Moving down to the next confluence you should have a three catchments that make up one watershed, the two watersheds at the end of the streamlines and the area between the first confluence and the next one. What I do to maintain the proper number of watersheds is I select the two end watersheds. Then I copy and paste them into the watershed file. The copy and paste would leave those two new ones selected. Then, while still holding down select I select the third watershed (the stream segment that runs into the confluence I am interested in) and I select "merge" and merge the three streams together into one large watershed.
• I continue through the entire watershed area until all the subwatersheds have been adjusted and built properly.

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There is sometimes other things that you may want to do with watersheds after they have been created. Here are the most common ones I have done:

Create Longest Flow Path for all watersheds:

• Take the Watershed shapefile and the Flow Direction file and run the “Longest Flow Path” tool.
• HydroID may need to be run on the watersheds first.

Create the Stream 10-85 Slope (the slope of the stream from 10% to 85% of the stream's length):

• ArcHydro: 
• Watershed Processing --> Flow Path Parameters --> Flow Path Parameters from 2D Line
• Input:
• Longest Flow Path
• Burned DEM (See above)
• Output will be in same location as mxd.
• In the output file:
• Verify lengths. If incorrect, recalculate.
• Verify Elevations, should be correct.
• Recalculate out Slope1085: “([ElevUP]-[ElevDS])/(0.75* [LengthMi])”
• Zero out other slopes.

Calculate Precipitation averages or centroid values:

• ArcHydro: Attribute Tools --> Compute Local Parameters
• Deselect all --> Select PRECIPIN (For average values) or PRECIP (for centroid values)
• Select the Watershed File
• Select Precipitation File (in same coordinate system as watershed file)

Other ArcHydro Tools of Note:

• Watershed Processing --> Basin Length Points
• This creates a point shapefile for the beginning and ending of each streamline. Good for determining total slope when the slope tool isn’t working right.