Have found the longest flow path in a watershed, but how to extract it as a polyline?

Question

I am doing some post-watershed analysis that requires me to 1) get the length of the longest flow pathway in a watershed from the upper reaches to the pour point (which I have thanks to Flow Length tool), and 2) have that exact (or simplified) flow pathway as a line, which is the step I cannot complete. This is an analysis involving dozens of input watersheds, so manual line selection from something like Stream Order results is not feasible. How can I get the longest flow pathway in a watershed as a line? TIA!

CandaceM · Answer

Hey Dan, thanks for your reply. Yes the issue with that is the Flow Length tool gives a raster that does not just give the one flow path - it gives all flow paths, including the maximum, and as such converts to many line segments, not just the maximum flow path, which would require manual selection of the segments that comprise the max flow path while visually comparing it against the flow length raster output. Not feasible for hundreds of watersheds, unfortunately.

CandaceM · Answer

Hi Robert - first, thanks for your reply! I have been working with this workflow for a few days, but like so:

1 - iterate watersheds file generated from previous analysis using same DEM input as in this tool

1a - Clip DEM to watershed

1b - Generate flow and accumulation rasters from clipped DEM

2 - run negative buffer on watershed polygon (-2m)

2a - convert buffer vertices to points, populate with watershed ID

3 - use Optimal Path as Line using input flow and accumulation rasters from 1b and input destination data as points from 2A. I use 'Each Zone' and select 'Create Network Path' options.

I get lines that are correct for MOST of the watersheds, but there are always a bunch that do not work - the flow line goes from 2 places along the watershed edge but doesn't come near the outlet. See my screenshot attached.

I have tried using the watershed polygons themselves (iterated for 1 at a time) as feature destination input but this did not work.

I am curious to how I would get your step #4 - Find the cell(s) with the maximum flow length inside the watershed and convert it to a point (that point is the “upper reach” end of the longest path)? When I run Flow Length, the only tool I know of to get any sort of flow length, it does give the max, but it also gives the lengths of all the other flow pathways in the input flow direction raster. My screenshot also shows that the max flow length for that watershed, according to Flow Length tool, is 935 m but the line produced by OPL tool is only 873 m.

Thanks for your input!!

Robert_LeClair · Answer

Try this approach in ArcGIS Pro (Spatial Analyst) to get the longest upstream flow path as an actual polyline, and do it in batch for many watersheds.

Steps (per watershed; can be scripted/iterated)

* Create hydrology rasters (if you don’t already):* Flow Direction (D8)
* Flow Accumulation

* Ensure analysis is constrained to each watershed (mask/clip) so “max upstream flow length” is within that basin.
* Run Flow Length (Upstream) to create the upstream distance raster.
* Find the cell(s) with the maximum flow length inside the watershed and convert it to a point (that point is the “upper reach” end of the longest path).
* Run Optimal Path As Line:* Input Back Direction or Flow Direction Raster = your Flow Direction raster
* Input Distance Accumulation Raster = your Flow Accumulation raster (commonly used with the flow-direction workflow)
* Input destination data = the max-flow-length point(s) from step 4

Output is a polyline for the traced flow path.

If you instead want the longest path along a derived stream network, then first generate stream polylines automatically from the DEM (threshold-based), then compute longest route on the network. Esri workflow for creating stream lines is here -  How To: Convert flow accumulation lines from a DEM to polylines in ArcGIS Pro