Batch Polygon Watershed Delineation Tool - Not all Inputs have Outputs

Question

Hi,

I am new to using ArcHydro, trying to use Polygon Watershed Delineation Tool to delineate watersheds for forest plots.

The tool runs successfully, but it has not generated an output watershed for all of the input polygons. The input file has 957 polygons, but the output file has only 660.

I am assuming that some small plots at the edges of the area might be too small, but is there a reason it is not generating watersheds for the remainder?

Apologies if this is an obvious question, but as I say, I am new to ArcHydro..

Thanks,

Paul

Paul_Duffy_Waters_of_LIFE · Answer

Hi Dean,

Thanks for getting back to me about this. I was also having another issue with the tool generating very large watersheds. I think this was due to the fact that some of the plots were near the outlet of the Catchment and overlapped the streams that flowed out of the catchment.

I ended up buffering the streams by 5m and erasing this buffer from the forest plots then re-running the tool. This then generated an output watershed for all of the input polygons, and they were a little more realistic in terms of size!

Thanks,

Paul

DeanDjokic · Answer

Paul,

There can be several reasons, including the one you mentioned.  If you are new to Arc Hydro and have not done so yet, please review "Arc Hydro: Project Development Best Practices" (that and many other AH docs are linked at https://www.esri.com/en-us/industries/water-resources/arc-hydro). Few things to get out of the way.  1. Possible projection issues.  Make sure your polygons are in the same projection as your flow direction raster (best practice - have a feature dataset that has the same SR as your raster data and load your polygons into it and then use it as input to the tool).  2. Resolution of the DEM compared to the size of the polygon.  If your polys are small compared to the fdr/dem cell size, you can get strange results.  Solution is to use finer resolution DEM (and corresponding fdr derived from it).  3. "Strange" fdr.  Was fdr developed from hydro-conditioned DEM?  If not, your fdr can return "nothing" when doing watershed analysis (there is no contributing area).

One thing you can try is to isolate one of the polygons that did not succeed (copy it into its own FC), and run the tool just on that one poly.  If it does not work, then try to do the process by hand:

1. convert polygon to raster (make sure you correctly specify environment for cell size, raster snapping, and extent to the one of fdr).

2. run Spatial Analyst watershed function using polygon raster representation from #1 and fdr as inputs.

If that does not return expected results (it will be a raster), then "you have a problem", most likely with fdr of polygon size.

Hope this helps.

Dean