Forms of P in SWAT outputs

[Jane Frankenberger]

SWAT Community -

The forms of phosphorus (organic vs. mineral, and particulate vs. soluble/dissolved) that SWAT outputs is difficult to understand.   I believe the following is correct (although please correct me if it is not):

• At the HRU level,  three forms of phosphorus are simulated: MinP, SedP, and OrgP.  OrgP is only particulate, while MinP is only dissolved.
• In the channel (.rch) there are only two forms: MinP and OrgP.
  

So this brings up two questions:

1. In the reach output, where is the inorganic, particulate P (simulated at the HRU level as SedP)? Does Min P include both dissolved and particulate (i.e., is it the sum of Min P (the dissolved form from the HRU) + Sed P)?
2. Is dissolved/soluble organic P simulated at all?

Thanks for your insights.

Jane Frankenberger

[Jim Almendinger]

This question has been lingering around for years, and I haven't seen a clear answer yet -- and I don't have one either.  A careful reading of the code should be able to answer this.  I'd think Mike White would know off the top of his head, since he's worked so much on re-vamping some of the P transport, if I recall. 

From the documentation, the hru and sub forms of P transported in overland runoff are the following:

hru SEDP = particulate mineral P, adsorbed to mineral particles

hru ORGP = particulate organic P

hru SOLP = dissolved mineral P

(no mention of dissolved organic P)

It seems that in the reaches, the common interpretation is that --

rch ORGP = particulate P, whether organic or mineral

rch MINP = dissolved mineral P

My working hypothesis is that, at the edge of the reach (conceptually), as the overland runoff enters the reach (stream) --

rch ORGP = hru ORGP + hru stable SEDP

rch MINP = hru SOLP + hru active SEDP

where active SEDP is the easily desorbed P from mineral grains, and stable SEDP is the less available P on the mineral grains. 

***this is an assumption that should be checked!!***

In the stream water-quality documentation, there isn't much if any discussion of particulate mineral P.  It seems that QUAL2E is considering mostly the uptake or release of nutrients by algal growth or decomposition, or sorption or desorption from bottom sediment (or perhaps they include suspended sediment), and settling of ORGP.  It's unclear to me if ORGP includes algae or not, or whether ORGC is the labile C produced when algae decomposes...

I've avoided the issue by just dealing exclusively with total P, which is about the same as sticking my head in the sand. 

It would be a significant contribution to this group if somebody would take a look at the code and come up with an answer, or if we could get Mike or Jeff to give us an answer on this one. 

Sorry for the equivocal answer, but hopefully someone will be able to improve upon it.  It's an important item to get cleared up.

Cheers,

-- Jim

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From: "Jane Frankenberger" <fran...@purdue.edu>
To: "SWAT-user" <swat...@googlegroups.com>
Sent: Sunday, February 8, 2015 6:55:11 AM
Subject: [SWAT-user:5314] Forms of P in SWAT outputs

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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

[Jim Almendinger]

I see in an earlier post (from 2010) I also considered that the active fraction of the hru ORGP may also contribute to the rch MINP component (there are active & stable forms of both mineral & organic P coming from the hru).  But I was guessing there as well, although I based my guess on comparing delivery of P from a subbasin to a headwater reach (with no upstream contributions of P), so the P into the reach = only the P delivered from the subbasin.  If I recall, the rch MINP entering the reach was larger than the SOLP from the land, so some parts of the hru SEDP and / or ORGP had to be contributing to the rch MINP input. 

-- Jim

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From: "Jim Almendinger" <jalmen...@smm.org>
To: "Jane Frankenberger" <fran...@purdue.edu>
Cc: "SWAT-user" <swat...@googlegroups.com>
Sent: Monday, February 9, 2015 6:17:43 PM
Subject: Re: [SWAT-user:5317] Forms of P in SWAT outputs

[Frankenberger, Jane]

The forms of phosphorus that SWAT outputs has confused many of us, and I don't think a definitive explanation has yet been posted to this group.   I believe the following is correct (although please correct me if it is not):

• At the HRU level,  three forms of phosphorus are simulated: MinP, SedP, and OrgP.  OrgP is only particulate, while MinP is only dissolved.
• In the channel (.rch) there are only two forms: MinP and OrgP.
  

So this brings up two questions:

1. Where does the inorganic, particulate P (simulated at the HRU level as SedP) go in the reach output? In the reach, is Min P the sum of Min P (the soluble form from the HRU) + Sed P?
2. Is soluble organic P simulated at all?

Many thanks for any clarifications.

Jane Frankenberger

[Jim Almendinger]

All --

After some email correspondence between Jane and Mike White, the basic relation between forms of phosphorus from the hru, versus forms in the reach (rch), is as follows:

rch ORGP = particulate P = hru ORGP + hru SEDP 

rch MINP = dissolved mineral P = hru SOLP + hru GW_P

Note that the names in the reach are confusing, because ORGP is not just "organic" -- it includes the mineral particulate SEDP.  This is just a "historical" hold-over earlier versions.  I suspect it may be from the QUAL-2E terminology, which may consider all particles as being organic (i.e., algae).  There does not appear to be any place where soluble organic P would be differentiated from particulate organic P. 

If you do not activate the stream water quality routine (QUAL-2E), then I think the rch ORGP and rch MINP remain unaltered as they are routed downstream.  (Except -- I'm not sure what happened when they enter reservoirs, whether one or both forms get settled according to PSETLR.) 

If you activate the stream water quality routines -- then things will start to shift between ORGP and MINP, and some can be lost or gained from the steam-bottom sediment as well. 

So much for now -- thanks for Jane and Mike for initiating the conversation.

-- Jim

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From: "Frankenberger, Jane" <fran...@purdue.edu>
To: "SWAT-user" <swat...@googlegroups.com>
Sent: Saturday, February 7, 2015 4:16:06 PM
Subject: [SWAT-user:5320] Forms of P in SWAT outputs

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[Florencia Hastings]

Hi all,

Thanks a lot for the explanation on phosphorus—this is very helpful.

I would also like to better understand the equivalent relationships for nitrogen. Specifically:

• What is the relationship between nitrogen species at the HRU level and those reported in the reach (output.rch)?
  For example, how do variables such as ORGNkg_ha, ORGPkg_ha, SEDPkg_ha, NSURQkg_ha, and NLATQkg_ha map into reach variables like ORGN, NO3, NO2, and NH4?
  Is there a similar grouping (e.g., dissolved vs. particulate) as described for phosphorus?

• Does anyone know which routines in the SWAT2012 code handle these transformations and partitions for both nitrogen and phosphorus? I’m trying to trace this through the Fortran code (from HRU export through routing to the reach), so any pointers to key subroutines would be very helpful.

• I also tested setting IWQ = 0 (i.e., in-stream water quality turned off), expecting that *_OUT ≈ *_IN. However, I still observe differences between IN and OUT in output.rch.
  Is this expected due to routing processes (e.g., storage, mixing, settling), or are there still active processes affecting nitrogen even when IWQ is turned off?

Any insights or references would be greatly appreciated.

Thanks!