Phosphorus subbasin - reach

[Nikolay Yasinskiy]

Hello!
I haven't found an appropriate answer among posts. My question is
following.

When I study SWAT code in detail, I can't found any variable, that is
transferring phosphorus from subbasin to the reach (to be used in QUAL
2 E as concentration). I cannot see any link between them. Could you
help me to find this variable or any link between subbasin and route
blocks of the model?
Would appreciate any answer.
Nick Yasinskiy, hydrologist. Russia.

[Jim Almendinger]

Nikolay -

I haven't seen a response to your question, so I'll try from memory.  The three forms of phosphorus delivered from the land (HRUs and subbasins) are, basically, (1) soluble P, (2) particulate mineral P, and (3) particulate organic P -- here I'll call them sol_P, min_P, and org_P.  The particulate forms include (I think) both "active" (= easily desorbed) and "stable" forms of P.  In addition, groundwater can deliver P to the channel -- I'll call that gw_P.  

When these 3 forms of phosphorus are delivered to the reach, they are converted into the two categories (which is what I presume Qual-2e requires) called mineral and organic P.  Here I'll call them reach_min_P and reach_org_P.  The output from the subbasin (sol_P + min_P + org_P + gw_P) equals the input to the reach (reach_min_P + reach_org_P).  This conversion is confusing -- but I believe it's close to the following:

reach_min_P = sol_P + gw_P + active_min_P + active_org_P

reach_org_P = stable_min_P + stable_org_P
(You really should double-check that this is what the code is doing -- I'm operating from memory.)  

This conversion results in the convention that reach_min_P fraction essentially equals the dissolved P content in the reach, and the reach_org_P fraction essentially equals the particulate P content in the reach (including P adsorbed to mineral particles).  I've always been somewhat uncomfortable with a model such as Qual-2e that doesn't explicitly include a mineral-adsorbed particulate P fraction, since this fraction is commonly large in rivers in agricultural regions.  

In addition to other landscape sources of P, the subbasin adds chlorophyll to its reach.  If you don't turn on Qual-2e, this chlorophyll remains simply as chlorophyll and doesn't impact the P budget of your model.  If you turn on Qual-2e, it commonly assumes this chlorophyll is from algae, and when it decomposes this presumed algae, it adds extra phosphorus to the reach, based on the assumed P content of algae.  This is all in addition to the sol_P, min_P, org_P, and gw_P delivered from the subbasin to the reach.  

Cheers,

-- Jim

---

From: "Nikolay Yasinskiy" <nikolay....@gmail.com>
To: "SWAT-user" <swat...@googlegroups.com>
Sent: Monday, September 13, 2010 2:43:34 PM
Subject: [SWAT-user:2431] Phosphorus subbasin - reach

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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

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