Help me understand -- Catchments/stream definition/stream segmentation

Discussion created by eric.yunginger on Apr 15, 2014
Latest reply on Apr 23, 2014 by malnunn
I've been using ArcHydro for 6-8 years now, doing watershed modeling for drainage projects.  While I understand the majority of what the software is doing as I work through the process, I've never been able to fully wrap my head around a couple of the steps - mainly being the stream definition/stream segmentation process, and how that relates to final catchments.

I understand that in the stream definition process, the smaller number you use for "number of cells to define stream" will result in a more defined stream network, tracing further up 'feeder' streams; while a larger number used will create a stream network of only the larger flow areas.  I think where I ultimately get confused is when I get to generating catchments.

For the work I do, I am typically working with watersheds in the 5,000-20,000 acre range, and because of the type of work we are doing, when I get to the stream definition step, I use a definition area of 0.008 km^2 (~2 acres) to get the stream network to generate to the detail we want.  Now, the way I look at it, I would expect with a ~2 acre definition used in the stream definition, when catchments are generated, in a given 40 acre tract of ground, I would find somewhere in the neighborhood of 20 catchments.  Instead, I am finding a half dozen or so catchments, varying in size, but some as large as 15-18 acres.  I can't figure out how to reconcile this in my head - or worse, how to explain it to my boss.

I think I understand correctly that the stream definition threshold sets the minimum area flowing to a given point to start a stream -- but how does the stream segmentation take off after that?  How are catchments along a long flow path broken up?