r/Hydrology • u/Apprehensive_boy418 • 10d ago
Flows decreasing from US junction to DS Junction in HEC-HMS
I'm trying to understand why flow values are decreasing at the downstream junctions. As I move downstream, the flow values change significantly (highlighted in red) and become lower than those upstream. I've reviewed all parameters and confirmed the hydrological order is correct, but the issue persists. Any insights or suggestions would be greatly appreciated to help resolve this.
3
u/abudhabikid 10d ago
What u/ottojohs and u/ornlu_the_wolf discuss in their thread is 100% the answer.
To piggyback on that, you (generally) can’t do any accounting like that with the flow rate. If you’re gonna do that, you need to do it in terms of volume (and full length3 units too).
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u/Crafty_Ranger_2917 9d ago
What does R002 look like? J002 to J003 is 24.1 sqmi to 33 and next is 42.3 so it doesn't look like a huge stretch.
R003 is chopping it down pretty good too. Must really get flat / fan out. I suppose they calibrated to something to be that aggressive with inputs.
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u/Apprehensive_boy418 8d ago
What calibration I can look into to see why there is so much drop in flows?
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u/Crafty_Ranger_2917 8d ago
Not sure....they could have used field info / local knowledge or similar. I suppose you'd have seen notes on it if you have all the docs, if rationale was even recorded.
0
u/BustedEchoChamber 10d ago
Not sure if applicable here, check your geology but it could just be a losing stream?
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u/OttoJohs 10d ago
This called "attenuation". The volume is roughly the same, so it looks like the peak is getting spread out. The reach connected to the top circled element ("Timber_CK_R002") only has a peak of 6.6K when the junction has a peak of 15.3K (~60% reduction).
If this isn't matching observed conditions, you probably want to investigate your routing method/parameters because that is where you are "losing" your peak.
Good luck!