Reach routing - Units of So and ChnSlp

[Rod Lammers]

I want to confirm the values that the GIS Pre-processor are providing for So (bed slope) and ChnSlp (channel side slope) are correct (in Route_link.nc). In the documentation, these are both given units of % (drop/length). However, the WRF-Hydro documentation says both should have units of m/m (Appendix A9 in the technical description).

To add to the confusion, in the WRF-Hydro channel routing code (https://github.com/NCAR/wrf_hydro_nwm_public/blob/main/src/Routing/module_channel_routing.F90), the units of So are specified as % (line 154). The calculations below this, however, appear to use both So and ChnSlp with units of m/m.

The GIS Pre-processor python code actually appears to calculate So with units of m/m (elevation drop / channel length), but the default values of ChnSlp don't make sense if they should be in m/m also. A ChnSlp of 0.03 would be a H:V ratio of 33, which is not reasonable, but a value of 0.3 would be a H:V ratio of 3.33, which makes more sense.

Could someone help clarify this issue? Thank you!

[aubrey]

Hi Rod:

On quick skim of the WRF-Hydro code it looks like both are in units of m/m (rise over run). I'm guessing the % listed in the units is just meant to indicate a fraction (e.g., 0.05 is 5% or 5/100 rise over run), but I do think it would be better listed as m/m.

I'm not sure where the default parameter values came from, so I can't speak to those. I just checked the NOAA National Water Model values for the contiguous U.S., and the average (by count) reach channel side slope is 0.58. These parameters are definitely meant to be derived/estimated for your local setup where possible. Here is one recent example: https://hess.copernicus.org/articles/26/6121/2022/

Hope that helps.

Thanks!

Aubrey