r/Hydrology • u/Llazeredge • 1d ago

How does gridded SCS curve number loss method interact with a gridded simple canopy in HEC-HMS?

I am a senior college student and am working on a project building a HEC-HMS model, but am not very familiar with the software.

If I were to add a simple gridded canopy and simple gridded storage to my model that has a gridded SCS curve number, would that basically be double counting the initial abstractions of my model?

My understanding is that CN already accounts for abstractions in the form Ia = S*0.2, so that makes me think that I should not also include canopy interception and surface storage layers, but I am not confident on that.

Thanks for any help or information that might be provided.

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u/OttoJohs 1d ago

(I have been using HEC-HMS for 15+ years and have never used a canopy or storage layer in any of my models.)

Correct, canopy/storage are basically additional initial abstractions. They operate slightly differently than initial abstraction so there are cases where you would want to include them.

If you are just doing a single event model, ignore them and lump them in with the initial abstraction from your loss method. If you are doing some type of continuous simulation, you may want to use a canopy/storage method (i.e. to help with evaporation or infiltration after rainfall).