crop yield dry weight vs. wet weight

[Evan]

I am trying to check my crop yields to see how close they are to USDA county-level data (averaged across the watershed). I see in the manual that SWAT gives yield in dry weight, so should I convert my USDA numbers from wet to dry weight? I'm not trying to calibrate yields, just checking to see if they are close. I'm not sure how much dry vs. wet weight affects the yields, and if that is going to cause erroneously high percent bias between USDA and SWAT. I'm not very familiar with the crop growing side of SWAT (mostly concerned with flow), so I would appreciate some advice/ explanation.

Thanks,

-Evan

[Jiajia Liu]

What is your plant? 

I know for pasture it varies by season but usually it's around 20%DM for every KG of fresh pasture. 

[CBoles]

Hi Evan, 

You'll need to convert the SWAT yields to yields at standard moisture to compare with USDA numbers. We use 60 lb/bushel for soybeans and wheat and 56 lb/bushel for corn. The standard moisture content assumed is 13% for soybeans/wheat and 15.5% for corn. 

-Chelsie

[Evan]

Thank you both! This makes more sense to me now, so I'll go back and correct my yield estimates so they're comparable. 

[Jim Almendinger]

Evan --

Just for further reference -- attached is an Excel workbook with a few tables that I've put together that summarize some basic data about crops and livestock that I've needed to configure or calibrate my SWAT models.  One table converts common US (English) units for crops to SWAT's SI units (with the same weights and moisture content as Chelsie gives below), and other tables deal with livestock manure production and content.  References should be in the footnotes.  Most of the information (for livestock, at least) was gleaned from the SWAT manuals themselves.  I think the crop information came from North Dakota State University. 

-- Jim

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From: "Evan" <evan...@gmail.com>
To: "ArcSWAT" <arc...@googlegroups.com>
Sent: Tuesday, September 29, 2015 7:16:16 AM
Subject: [ArcSWAT:7615] Re: crop yield dry weight vs. wet weight

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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

[Evan]

Jim, thank you for this great resource! 

[Naresh Pai]

One other important point is that the biomass simulated by SWAT represents both above and below ground biomass. At plant maturity, I think it assumes that 20% of the biomass is below ground. 

Naresh

[Evan]

UPDATE:  I just received a response from someone at the USDA about the peanut moisture content as reported by the USDA Quickstats county level data. Quote, "Our yields should already be equivalent to dry weight. We do not do any kind of conversions. The yield we publish should already exclude foreign material and excess moisture"     Assuming that this is true for all crops, then no corrections should be necessary for SWAT comparisons. 

[Jim Almendinger]

Unless they've changed policies recently, NASS reports many crop yields as bushels per acre, in which case you'll need to convert based on both density (pounds per bushel) and moisture content -- in addition to conversion to SI units.  Some hay, I think, was actually reported as dry weight per acre, which would require only the SI conversion. 

-- Jim

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From: "Evan" <evan...@gmail.com>
To: "ArcSWAT" <arc...@googlegroups.com>

Sent: Monday, October 19, 2015 1:48:32 PM
Subject: Re: [ArcSWAT:7653] Re: crop yield dry weight vs. wet weight

UPDATE:  I just received a response from someone at the USDA about the peanut moisture content as reported by the USDA Quickstats county level data. Quote, "Our yields should already be equivalent to dry weight. We do not do any kind of conversions. The yield we publish should already exclude foreign material and excess moisture"     Assuming that this is true for all crops, then no corrections should be necessary for SWAT comparisons. 

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