CNOP or CN2

[Stalled Mower]

All,

I am worried that by using CNOP that I am misrepresenting curve number for my winter wheat and I need to clarify how SWAT uses the CNOP and CN2 parameters:

1. If I use CNOP instead of CN2 will it completely override CN2 for all months and all crop growth stages?
2. If I set CNOP does the curve number remain constant throughout the year or is it adjusted according to the crop growth?

Any advice would be appreciated.

Regards,

Phil

[Jamil]

Hello Phil,

I am not 100% sure but I think I should be right:

Using the "Edit subbasin input" in the ArcSWAT interface allows you to select the CN value under the "general parameter" section. 

Then, in the "operation" section you can use CNOP values that will override CN2 value from the general parameter section. CNOP values are related to an operation (planting, tillage operation....) and will be effective until you specify a different CNOP value in the following operations.

Once you have defined your management schedule, you can ask the interface to save these settings for one HRU, or generalize to several HRUs or even the whole subbasins of your catchment. All othr HRUs in which you do not specify management operations will keep the default CN value.

Again I am not an expert but this way of doing worked for me.

Regards,

Jamil

[Jim Almendinger]

 I think Jamil has it right.  CNOP will override the original CN2 or any previous CNOPs, and will remain effective until CNOP is specified again, e.g., in a succeeding rotation.  It is adjusted for soil moisture, but not plant growth during the growing season.  I've generally changed CNOP at the time of planting.  Using CNOPs is particularly important for rotations with very different crops -- like a corn-alfalfal rotation.  Colleagues have argued -- probably correctly -- that CNOP really should be changed several times during the growing season of each crop, rather than being set only at the time of planting.  I tend to lean toward the rule that simpler is better, except when it's not.  

But, note that there is (I think) an alternative way of adjusting CNs, based on ET rather than soil moisture.  That may account for crop growth over the season, assuming ET is related to LAI.  I should learn more about this.  It might be simpler and better (unless it's not).  

Cheers,

-- Jim

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
email: din...@smm.org
web: www.smm.org/SCWRS/

[Rosemary Records]

Hi Jamil, Jim and all:

Just a quick note, Jim is right that you can adjust curve number based
on plant evapotranspiration rather than antecedent soil moisture
condition. It may work better for shallow soils...See Amatya and Jha
2011 and Kannan et al 2007.

Cheers,

Rosie Records

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Rosemary Records
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Colorado State University
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[Adam Freihoefer]

Both Jamil and Jim are correct. The CNOP overrides the CN2 in HRUs where it is applied. Applying the CNOP for HRUs where the crop changes through a given year or when you want to apply a much higher CN for a pratice (tillage) are options for using CNOP. If you run the SWAT model on a daily time step you can track the change in CN2 or CNOP over time, which is very helpful. What you will should see is that even though you think you are simulating a CN2 of 60, the majority of the time the model is either simulating the value as antecedent moisture condition 1 or 3. I did some of this work several years back (Slide 12 and 16 of http://swatmodel.tamu.edu/video/a2-freihoefer.pdf). Hope our responses help you move forward.

Cheers,

Adam Freihoefer

Wisconsin Department of Natural Resources 

[Stalled Mower]

Thanks to all for your advice.
Just one further clarification.

I am firstly setting:
CNOP for the 'Plant/begin growing season' operation = 52; and then (For my winter wheat in October)
CNOP for the 'Harvest and kill' operation = 0. (For winter wheat harvest in June).

I am assuming that as CNOP is set to 0 the curve number defaults back to the CN2 until the beginning of the next growing season (i.e from June to October).
Is that correct, or, does SWAT assume that since I started using a CNOP value that parameter is now established and set the curve number to zero?

Regards,

Phil

[Jim Almendinger]

I would not assume that setting CNOP = 0 will reset things back to the original CN2.  I'm not sure what it will do, but I doubt that.  My guess it that it will try to use zero, which might give unpredictable result.  Or the program may choke if the parameter is outside of accepted bounds.  

I think you should set your last CNOP to something realistic.

-- Jim

[Jochen]

What I don't really understand which value I should use as IGRO = 1 and the plant has grown a bit at the beginning of the year.
For example:
For plants that grow in summer I take the CN2 value from the manual, change it for a specific operation (CNOP) and at the end of the period after plowing I set CNOP = CN2 so as if nothing would have happened. So for the next year (if I have the same plant on the field without crop rotation) IGRO = 0 would make sense.

For winter wheat I am not sure. In my understanding I lower the CN2 value when planting. So due to a planting operation I allocate a smaller CN value for CNOP until the next operation that changes the CN value.

But: As for winter wheat something is growing at the beginning of the year I assign the CNOP value to CN2. Later in the year, after having harvested and seedbed preparation I plant again on that field (no crop rotation). Again I change the CNOP value. CNOP now has the same value as in the general CN2 settings.

What do you think? Is this a proper way to deal with that?

[Phil Selby]

Jochen,

In theory as the crop grows the curve number should decrease (i.e. runoff should be reduced) and the CNOP value allows for this.

Between the last kill and the next planting I have set the CNOP value back to the CN2 for bare soil.

As I undertand it IGRO is only relevant right at the start of the simulation (it doesn't apply to each year). I would set IGRO=0 for all arable crops and make sure your warm-up period includes at least one planting and harvest.

Unfortunatley it seems the CNOP value is fixed for the whole season so you have to make a compromise, either:

1. Set the CNOP to a value representing the crop once it is grown and accept that your winter runoff maybe underestimated; or

2. Set the CNOP to CN2 for baresoil and accept some overestimation of runoff in the summer; or

3. Something inbetween.

It really depends on what you are trying to model and what else is influencing your runoff. I.e. if your soils are quite wet during the winter you should probably bias the CNOP towards the summer months.

I have wondered whether to stop the crop growing and start it again with some initial growth (using LAI_INIT and BIO_INIT I think) setting a different CNOP each time.

I haven't tried it but I think it might add more problems than it solves.

Phil

From: Jochen Jochen....@gmx.de
 

[Jochen]

Thanks for your response.

I don't know if I got it. Within the optional entries for the harvest/kill operation I set CNOP to the CN2 value of the specifig Hydrologic Soil Group of that specific plant until the next operation (be it a planting operation). Is that what you mean by "bare soil". SWAT offers higher values for the land use category "fallow", that would be different.

Im an a bit confused. So let's say I plant this winter wheat again in October (during the warm up period) on a soil which belongs to HydroGroup D. The default plant specific CN2 value is 84. In the first year of the simulaton I really use it is harvested in let's say July. Would you now set the CNOP value due to the harvest/kill operation to "bare soil". I thought the specific CN2 value from the crop database was made for that?

Regards,
Jochen

[Jim Almendinger]

The CN2 value for a specific crop, for a specific hydrologic soil group (HSG), is for that crop, not bare soil (otherwise all crops would have the same CN for each HSG).  It's a good question about what stage in the crop development that CN value applies, though.  I presume it is for a fully developed field of that crop, but I suppose it could be some sort of season-long average.  I don't really know.  

The CNOP value supplied by the user remains in effect until it is superseded by a another user-input value.  If you want to change it during the season you can add a non-effective operation (like adding a zero quantity of fertilizer or pesticide, or maybe a no-till operation), and change CNOP at that time.  Just beware that sometimes in SWAT when you add zero of something, SWAT sometimes assumes that you really mean "default" and it will add that amount instead.  

-- Jim

p.s. -- Yes, Jochen -- in my previous note it was because I was changing the crop in the same HRU.  

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[Charles Ikenberry]

Jim et al.,

I think the CN2 value does reflect a "mature" crop.  But I, too, could be wrong about that.

Also, I believe some of the hydrologic aspects of crop growth are reflected in the SWAT algorithms, but outside of the CN computation.  It is true that the CN2 is "static" and daily CN values are independent of crop growth using the standard soil moisture CN method.  CN changes every day due to changes in soil moisture, not crop growth.  However, if the alternative PlantET CN method is used, then the daily CN values would reflect crop growth indirectly via increased ET.  There are limitations for the PlantET CN method, but it is available to users concerned about hydrologic impacts of crop growth and hesitant to alter CNOP.

Lastly, wouldn't the primary concern with crop growth and hydrology be interception, which should be reflected in increased leaf area?

--Charles

---

From: Jim Almendinger <din...@smm.org>
To: Jochen <Jochen....@gmx.de>
Cc: swat...@googlegroups.com; Phil Selby <sel...@yahoo.com>
Sent: Wednesday, August 22, 2012 2:05 PM
Subject: Re: [SWAT-user:3522] Re: CNOP or CN2

[Jim Almendinger]

Charles, all --
I think you're right, that interception becomes an important factor in reducing runoff and increasing ET as the crop matures.  Do you know how SWAT handles this?  If the Penman-Montieth ET method is chosen, I think LAI is taken into account for stomatal resistance -- but does it include effects of interception as well?  And if the Hargreave's ET method is selected, does LAI matter at all? 
-- Jim

---

From: "Charles Ikenberry" <cdik...@yahoo.com>
To: "Jim Almendinger" <din...@smm.org>, "Jochen" <Jochen....@gmx.de>
Cc: swat...@googlegroups.com, "Phil Selby" <sel...@yahoo.com>
Sent: Wednesday, August 22, 2012 3:52:38 PM

Spam
Not spam
Forget previous vote

--

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

[Charles Ikenberry]

Actually, I did some digging, and it is a little vague, but Section 2:2.1 (page 124) of the SWAT2009 User Manual suggests to me that if CN infiltration is used, interception via canopy storage is accounted for by the Initial Abstraction parameter in the SCS methodology.  In other words, impacts of crop growth is limited to impacts on soil moisture (or plant ET if using that method).  However, it appears as though leaf area index is utilized to estimate interception if Green-Ampt is used.  But like most users, I seldom have subdaily precip at my fingertips, which is needed to apply Green-Ampt.

If I'm wrong about interception not being impacted by crop growth (i.e., increasing leaf area) in the CN infiltration routines in SWAT, I'd love for somebody to correct me.

--Charles

---

From: Jim Almendinger <din...@smm.org>

To: Charles Ikenberry <cdik...@yahoo.com>
Cc: swat...@googlegroups.com; Phil Selby <sel...@yahoo.com>; Jochen <Jochen....@gmx.de>
Sent: Wednesday, August 22, 2012 4:52 PM

[Jochen]

@ Charles, Jim:

Ok that allays my confusion, thanks a lot for your detailed answers.
But another question arises then. WOuld you set your initial CN2 to bare soil and due to a planting operation change the CNOP value to the CN2 value for that plant (listed in the crop data base)?
For me that would make sense but in the general section one also defines plant specific PHU and the plant ID (if IGRO=1), so this is inconsistent for me on the other hand.

Regards,
Jochen

[Charles Ikenberry]

It really because a question of judgement, in my opinion.  I think what you propose is valid approach.  But I can tell you that there are many peer-reviewed publications that have not adjusted CNOP and just used a static CN2, even though that is an over-simplification.  You could do a sensitivity analysis to see how your simulated daily flows compare with your observed flows using both methods.  If there is noticeable improvement by varying CNOP vs. using only CN2, then it's probably worthwhile.  I don't think annual flow would vary that much, but it could sure make a difference at a daily or even monthly time step.  If it does, a sensitivity analysis would be worthy of documentation in a publication, too - I'd like to know if I should be using CNOP values!

--Charles

---

From: Jochen <Jochen....@gmx.de>
To: swat...@googlegroups.com
Cc: Jim Almendinger <din...@smm.org>; Phil Selby <sel...@yahoo.com>; Jochen <Jochen....@gmx.de>; Charles Ikenberry <cdik...@yahoo.com>
Sent: Thursday, August 23, 2012 2:06 AM

Subject: Re: [SWAT-user:3522] Re: CNOP or CN2

[Jim Almendinger]

I agree with Charles -- the proposed approach makes sense, and might prove valuable, especially for daily to monthly output.  On the other hand, even using the crop's CN for the entire year, I seem to get plenty of runoff in the spring, and plenty of erosion from the bare fields (still considered as Corn or Soybeans, even though they were harvested and disked the previous fall).  Increasing CN for bare ground conditions didn't seem necessary to get enough runoff to match the monitoring data.

-- Jim

---

From: "Charles Ikenberry" <cdik...@yahoo.com>

To: "Jochen" <Jochen....@gmx.de>, swat...@googlegroups.com
Cc: "Jim Almendinger" <din...@smm.org>, "Phil Selby" <sel...@yahoo.com>
Sent: Thursday, August 23, 2012 8:35:41 AM

Subject: Re: [SWAT-user:3522] Re: CNOP or CN2

It really because a question of judgement, in my opinion.  I think what you propose is valid approach.  But I can tell you that there are many peer-reviewed publications that have not adjusted CNOP and just used a static CN2, even though that is an over-simplification.  You could do a sensitivity analysis to see how your simulated daily flows compare with your observed flows using both methods.  If there is noticeable improvement by varying CNOP vs. using only CN2, then it's probably worthwhile.  I don't think annual flow would vary that much, but it could sure make a difference at a daily or even monthly time step.  If it does, a sensitivity analysis would be worthy of documentation in a publication, too - I'd like to know if I should be using CNOP values!

--Charles

---

From: Jochen <Jochen....@gmx.de>
To: swat...@googlegroups.com
Cc: Jim Almendinger <din...@smm.org>; Phil Selby <sel...@yahoo.com>; Jochen <Jochen....@gmx.de>; Charles Ikenberry <cdik...@yahoo.com>
Sent: Thursday, August 23, 2012 2:06 AM
Subject: Re: [SWAT-user:3522] Re: CNOP or CN2

@ Charles, Jim:

Ok that allays my confusion, thanks a lot for your detailed answers.
But another question arises then. WOuld you set your initial CN2 to bare soil and due to a planting operation change the CNOP value to the CN2 value for that plant (listed in the crop data base)?
For me that would make sense but in the general section one also defines plant specific PHU and the plant ID (if IGRO=1), so this is inconsistent for me on the other hand.

Regards,
Jochen

[hanmei...@gmail.com]

Hi Adam,

I know you replied to this message about four years ago. 

I also want to have SWAT report the daily CN in output files so I can track and check, could you point how did you do this? I let the model run on daily scale, but none of the output has daily CN in it. 

Did I miss anything? Anybody else can help? Many thanks.

Mei