[SWAT-user:2260] Nutrient transport calibration

[Raj]

I am using SWAT to simulate Nitrate and Phosphorus transport in a
snowmelt driven catchment in Canada. I first calibrated (daily)
catchment runoff, which was reasonable. But monthly calibration of
nutrient transport always seem to produce to lots of nutrients,
especially aftter snowmelt season in April and May. For example,
simulated nitrate-N concentration during these months are about 30 mg/
l, while observed values are less than 5mg/l. I am using the following
parameters and ranges for the model calibration and keeping everything
else as default.

CMN.bsn 0.0001 0.003
PSP.bsn 0.1 0.4
ERORGP.hru 0 5
ERORGN.hru 0 5
BC4.swq 0.1 0.7
BC2.swq 0.2 2
RS5.swq 0.05 0.1
RCN.bsn 0 0.5
N_UPDIS.bsn 1 30
P_UPDIS.bsn 1 30
NPERCO.bsn 0.01 0.3
PPERCO.bsn 10 17.5
SOL_NO3.chm 0 10
SOL_ORGN.chm 0 10
SOL_SOLP.chm 0 10
SOL_ORGP.chm 0 10

I have not used any fertilizer inputs. Does anybody have an idea why
SWAT is simulating excess nutrients?

Thanks

Rajesh

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[Jim Almendinger]

Are you simulating stream water quality processes -- i.e.., do you have Qual2E turned on?  If so, try turning it off and see if your nutrient values don't drop.  If this works, let me know and I'll explain what I think may be happening.  

-- Jim

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Dr. James E. Almendinger, Senior Scientist

St. Croix Watershed Research Station

Science Museum of Minnesota

16910  152nd St. N

Marine on St. Croix, MN  55047

tel: 651-433-5953 X 19

fax: 651-433-5924

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[Raj]

Thanks Jim, for your reply. I had the QUAL2E turned on (IWQ=1 in .bsn
file). When I turn off the QUAL2E, there was a very small reduction in
nutrient value. Please let me know, what is happening.

-Raj

[Naresh]

I think the denitrification levels are low in your model. Check this
in your output.std. Try varying the CDN and SDNCO variables in .bsn.
to set higher denitrification. Be careful that SDNCO even when set to
0 in the interface is set as 1.10 and CDN when set to 0 is reset as
1.4 within SWAT code. A value SDNCO=1.10 implies that denitrification
starts when moisture levels are at 110% of field capacity. Try setting
SDNCO to 0.6 - 0.9 and see if that helps.

Naresh

[Jim Almendinger]

Raj --

My understanding is that QUAL2E requires some amount of chlorophyll a in the stream to start with, or else its calculations somehow fail.  So SWAT always adds some "starting" chlorophyll a to each stream reach, in case QUAL2E is turned on.  

If QUAL2E is turned off, this chlorophyll just passes downstream without impacting nutrient loads.  

However, if QUAL2E is turned on, then it interprets this chlorophyll as algae.  When that algae decomposes it further presumes that the nutrients in that algae are released to the stream, according to the P and N content parameters (the fraction of algal biomass that is P and N -- you can set this).  The problem is, this is all "extra" P and N that are not related to the actual loads of P and N transported by the overland runoff (or any other flow) in SWAT.  I.e., this extra P and N are not "real."  

Depending on your watershed, the added amount may be significant.  In my last project, turning on QUAL2E resulted in a 20% increase of the P load.  I felt this was unacceptable, and since then I just haven't used QUAL2E at all.  

The error can be reduced by reducing the P and N content of algae to negligible levels (like, from 0.015 to 0.001 or something like that, if I remember the values more or less for P).  However, then your QUAL2E calculations of algal growth and decay per unit nutrient level will be pretty screwed up.  

That's the current state as I see it now; the code has not been corrected to my knowledge.  The fix would be to simply check to see if QUAL2E is turned on, and if it is, then the portion of the P load from the HRU corresponding to the amount of added algae (chlorophyll) should be subtracted from the other P-load components (ORG_P, SED_P, and SOL_P) before being delivered to the reach.  Reducing the size of the "seed" load of chlorophyll might also be appropriate.  

Cheers,

-- Jim

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From: "Raj" <rajesh.r...@googlemail.com>
To: "SWAT-user" <swat...@googlegroups.com>
Sent: Friday, May 21, 2010 6:57:27 PM
Subject: [SWAT-user:2265] Re: Nutrient transport calibration

Thanks Jim, for your reply. I had the QUAL2E turned on (IWQ=1 in .bsn
file). When I turn off the QUAL2E, there was a very small reduction in
nutrient value. Please let me know, what is happening.

-Raj

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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

Experience a once-in-a-lifetime opportunity: see authentic 2,000-year-old manuscripts, including the earliest biblical writings. The Dead Sea Scrolls. Once forgotten. Now unforgettable. Visit www.smm.org/scrolls.

[Raj]

Thanks Naresh for your response, Yes denitrification was very low in
my model and when I adjusted the parameters as you suggested, it
reduced nutrient outputs for most months. Now I need to figure out why
nutrient concentration go up in May after snowmelt runoff recedes. Any
thoughts on that?

Cheers
Rajesh

[Naresh]

I would run the model on a daily scale for this month, track few HRUs
that are producing high nutrients and try to do a mass-balance for
them (what went in, what came out)...