soil parameters

[Glenn Harwell]

I have some questions related to soil parameters.

Does it make more sense to adjust parameters like SOL_AWC, SOL_BD, SOL_K within all soil layers instead of just say the first layer? When I see in publications that  these parameters were adjusted often it is not specified if all layers or just certain layers were adjusted.

Secondly, why shouldn't we trust the SSURGO soil data and just leave these alone. I understand they can have a lot of influence on model results (sensitive) so including can improve the ability of a model to predict measured values.

Any thoughts?

Thanks!

[Jim Almendinger]

There are no subjective philosophies about which parameters to adjust and by how much. 

My general rule of thumb is to leave parameters based on measured or observed data alone as much as possible.  This includes things like slopes, and soils data.  In particular, the SSURGO data is pretty detailed and I'm generally impressed with the care soil scientists take in describing soil.  (The STATSGO data lumped many units together, some of which were quite different, so changing parameters could be easily justified.) 

So I tend to start with parameters that are not measured easily or at all -- like esco, and gw_delay. 

And some parameters can start with ArcSWAT-suggested default values, based on real observations, but can still be quantified only approximately -- I'm thinking of curve numbers in particular here.  Although they were originally conceived to be watershed-scale parameters, they have been quantified at plot scales in order to estimate their values for different crops (as I understand it), and this is how SWAT uses them.  But we apply curve numbers at HRU to subbasin scales that are much larger than plots, and so changing curve numbers as needed makes sense to me -- they are not rigid values and somewhat scale dependent (and rainstorm-duration dependent -- Paul McGinley had a paper on this). 

On the other hand -- even detailed data like SSURGO are not perfect and uniform, and if needed I'll modify those values as well.  It's just not my first choice.  I don't know if changing all layers, or just the top layer, is better / worse.  I tend to change all layers (at least the top 3) if it's a parent material property.  If it's an A-horizon property (organic matter, and perhaps AWC based on organic matter?), then I guess just the top.  You're right that the literature is a bit lazy in reporting what was actually done. 

Good question. 

-- Jim

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From: "Glenn Harwell" <harwel...@gmail.com>
To: "SWAT-user" <swat...@googlegroups.com>
Sent: Thursday, June 4, 2015 10:41:02 AM
Subject: [SWAT-user:5600] soil parameters

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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