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Clarification of algorithms in NSS computation process


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I've been a heavy WMS user for over 3 years and recently started using the NSS model, and was wanting to know if someone could provide some clarification regarding how WMS is computing the watershed geometric characteristics for input into the regression equations. The regression equations I'm using require an area, a channel slope (at 10/85%), and a main-channel length. 

I've created a watershed model that is performing satisfactorily, but some of the geometric output numbers are a bit puzzling, specifically the channel length parameter. I've compared WMS output to my own measurements to verify, and sometimes it's a struggle to match any of the WMS numbers at all. I'm referring to both basin-specific channel lengths, obtained by clicking on basin icons, and cumulative lengths, obtained by clicking on outlets. 

My question is, what exactly is it outputting for that parameter and how is it computing that value? Any information would be very helpful. 

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  • 3 weeks later...

Sorry it's taken so long to answer this.  The main channel length depends on the length of your stream in the watershed or sub-basin where the calculation is made.  So if your stream arc is longer in this basin, your main channel length will be longer.  It's the same with the 10/85 channel slope calculation.  This calculation depends on your stream location and length.  Since the 10/85 calculation depends on 2 points along your stream, this calculation may not be as accurate as something like the average stream slope.

Hopefully that's helpful, let me know if I can help with anything else,


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Thanks for trying to help. Perhaps an example would help clarify things?


I have a basin called 14B and the program gives an area, a channel slope, and a main channel length of 27.01 miles (verified as correct). Nearby is basin 16B, which shows a main channel length of 13.1 miles.


They both flow into a junction called 27C, which shows appropriately the combined area of the two basins. However, the main channel length is given as 25.1 miles?? It should be the cumulative longest flow path, but I am having a hard time figuring out where 25.1 miles even comes from. It seems like the calculation is working fine on individual basins, but when it comes to junctions, something is different and I can't figure out what. And it's like that throughout the model. 

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I'm afraid I'm still scratching my head. I'm not sure that we're talking about the same thing. 

I have attached an image showing the value I'm talking about, circled in red.  The channel length upstream of this point is about 30 miles and the channel length downstream is 2 miles to the next junction, and about 11 miles total. 


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