I am using two online watershed delineation tools- eRAMs and Model My Watershed (MMW)- to auto delineate watersheds using water quality monitoring sondes as my outlet points. I have three pieces of equipment on different streams so I have three different outlet locations. Two out of three outlets/sondes have similar watershed delineations, but one sonde location has dramatically different watershed delineations. Here is pic of sonde location, the green marker is our coordinates and the red circles are different outlet locations I've manually added in eRAMs tool:
Even after trying multiple outlet locations between the bridge and the streams confluence with another stream, as well as even trying locations upstream of the bridge, the tools result in the same watershed delineations shown below:
Note: eRAMS result is green boundary, MMW result is orange boundary, Creek where sonde is located is highlighted in red.
I lean toward the MMW delineation, as the delineation encompasses the creek on which the sonde is located. Note that these tools move the manually placed outlet points to nearest downstream NHD cell, but even the moved outlets are above the confluence.
What might be the reason these tools have such different delineations?
I believe that both tools delineate based on 30 x 30 m DEMs and the NHDPlusV2 Medium Resolution stream network. eRAMs uses TauDEM tools/algorithms found here: https://erams.com/documentation/watershed-delineation-tools/ It's difficult for me to interpret the subprocesses of MMW's Rapid Watershed Delineation algorithms listed on GitHub (limited development background), but I think they are modified versions of TauDEM algorithms, similar to eRAMs.