Channel width and depth from DEM

[SWAT user]

Dear all,

I am wondering how SWAT model calculates average channel width and depth for each sub-basin from DEM data. I did not find any information in the theoretical guide.

Best Regards

[Jim Almendinger]

I don't know either. I believe Srini told me once it was based on a USGS (or USACE?) algorithm based on catchment area and perhaps average slopes. 

Perhaps it's right on for some parts of the USA, but my experience in Minnesota has been that ArcSWAT channel widths are about 3 times larger than actual, and that when I reduce channel widths (by dividing by 3), flow velocities increase (by 3x) up to about measured values.  Channel depths were "about right."  I have only a few data points and it would be a worthwhile project to document this more carefully, because it greatly affects channel erosion and sedimentation (and hence the effect of setting the SEDCON and SEDEXP parameters). 

My advice would be to use Google Earth to measure actual channel widths at a dozen reaches in your watershed (from downstream up to headwater reaches), and get a dozen actual / ArcSWAT width ratios, and simply use the average ratio to correct the ArcSWAT widths to actual.  Do this before sediment calibration (if you allow channel erosion and deposition) for sure, and one would think it would affect the hydrologic calibration as well, since velocities should affect propagation of flood peaks.  This might be more significant in affecting the timing of flood peaks in larger watersheds, although I have never heard this discussed in the literature and I'm only speculating here. 

Best,

-- Jim

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From: "SWAT user" <slim....@gmail.com>
To: "SWAT-user" <swat...@googlegroups.com>
Sent: Sunday, May 8, 2016 4:04:03 AM
Subject: [SWAT-user:6196] Channel width and depth from DEM

Dear all,

I am wondering how SWAT model calculates average channel width and depth for each sub-basin from DEM data. I did not find any information in the theoretical guide.

Best Regards

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Dr. James E. Almendinger
St. Croix Watershed Research Station
Science Museum of Minnesota
16910 152nd St N
Marine on St. Croix, MN  55047
tel: 651-433-5953 ext 19

[SWAT user]

Dear Jim,

Thanks for your explanation. I divided the channel width by 3, modified the following parameters by the new one (CH_W1, CH_W2 and CH_WDR) and re-ran the model.

There is no change of stream flow (FLOW_IN and FLOW_OUT) in the different sub-basin (almost values are same).

It seems that the model is not sensitive to channel width. Is it possible?

Best regards

[Jim Almendinger]

For me, flow volume did not change substantially, but velocity did.  The same flow (cms) can have very different velocities depending on the channel cross section.

-- Jim

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From: "SWAT user" <slim....@gmail.com>
To: "SWAT-user" <swat...@googlegroups.com>

Cc: "slim mti87" <slim....@gmail.com>
Sent: Monday, May 9, 2016 6:09:21 AM
Subject: Re: [SWAT-user:6199] Channel width and depth from DEM