George,

Are you using the truly latest DSSAT V4.5 released since about July 2010?  The CO2 responsiveness of C-4 crops was reduced just prior to that, and my changes to species for C-4 crops may not have made its way into the species file.  Take a look to see if you have the new CO2 responses (I think there should be a note in the file, saying changed from ....)

On the other hand, the responses that you find for irrigated grain sorghum and irrigated wheat are pretty close to what I would expect.  We found that wheat response to CO2 matched what experiments showed, so the wheat coefficients were not changed.  I expect the response to be considerably larger for rainfed.  There is data to that effect from FACE-Sorghum CO2 enrichment studies in Arizona.  BUT.. your values of 50 to 100% are much too much, and I don't know what to say about them.  The reason for higher response is that high CO2 decreases transpiration which allows the crop to carry longer in dry periods.  I expect 50% more response to CO2 under drought than irrigated.  If 30% for wheat irrigated, I expect 45% or so for strong drought.

You could (but should not change things in the species file), especially since the change would affect both irrigated and rainfed.

Let's see what other responses we get.

Ken Boote

-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On Behalf Of George Paul
Sent: Friday, March 04, 2011 2:32 PM
To: [log in to unmask]
Subject: CO2 responses

Dear group,

I'm using version 4.5.
When I'm using a CO2 concentration of 600 ppm  the grain yield (Grain Sorghum/Wheat),  I'm getting very high grain yields as compared to baseline.  I'm incorporating a onetime application of CO2 through environmental modification.  The values I'm getting are in the range (yield increase):
7-12%   (irrigated Grain Sorghum),  50-100% (rainfed Grain Sorghum)
23-35% (irrigated wheat), 50-100% (rainfed wheat)

Queries:
Why is the high yield? Is there other way to do it?
Do I have to change anything in the species file to get the correct responses for C3 (wheat) and C4 (sorghum) crops?
Please share if anyone has an experience similar problem and have any suggestions.


Thanks for time and help

George Paul
Kansas State University