Arnold,

You have not interpreted this correctly, and it is not "new".  You should NOT sum up the indices for photosynthesis and growth.  That would be wrong and could give you more than 1.0, up to 1.5 or so of "stress".  I'm not sure you can sum the stresses up over the growing season either.

The photosynthesis factor is computed from the ratio of actual root water supply to transpirational demand (but limited between 0 and 1.0), so if root water uptake is limiting then photosynthesis (DM gain) is limited directly by that ratio.  The other "growth" factor is computed from the same two variables, but with a re-scalar of 1.5, thus making it more sensitive to water limitation.  The "growth" factor does not affect DM gain directly, it only affects expansive processes such as leaf area expansion, height & width increase.

Note that the "stress" reported to the OUTPUT is equal [1.0 minus the computed ratio].  For that reason, numbers above 0.0 indicate stress.

I recommend that you take the DSSAT course in May, to learn the questions to these questions.

Ken Boote

-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On Behalf Of Arnold Salvacion
Sent: Saturday, March 12, 2011 4:57 PM
To: [log in to unmask]
Subject: Stress Factor in New DSSAT version

Dear Colleagues,
Good day!
I noticed that there is now a stress factor on the "Overview Output" of DSSAT 4.5 with an index from 0 to 1. Just want to ask on how or what were the formulas used to compute those index? On the part of water stress, there is a separate index for photosynthesis and growth, is it safe or logical to assume that the sum  of these two is the total water stress the crop has? Furthermore, based on the output the two index was computed in DSSAT according to growth stage, is it safe or logical to assume that the summing up these values represents the total water stress the crop experience during the entire growing season?
Thanks in  advance.
Best regards,
Arnold