Dear DSSAT users,

Hello, I had two quick questions related to simulation starting date.

The first is related to the fact that the model seems quite sensitive to the
starting date variable (SDATE)--the earlier you start, the higher yields
tend to be. I had assumed this was because more water was being stored in
the soil (rainfall accumulates for longer), but when I compared two maize
runs on the same soil, both with planting date of 80320, one started on
80001, the other on 80244, the 80001 start had a higher yield (5784 vs 4547)
but lower values for SWTD, SWXD, SW1D, etc. on the day of planting.

I am thus unclear as to what causes this fairly large yield discrepancy? I
was thus hoping someone could inform me as to the reason for this
discrepancy.

The second question I had relates to the above. Given these differences,
what is the "soundest" starting date to work with when running simulations?
What I am doing is running simulations year after year, but not in
sequeuntial mode (i.e. I am not doing rotations or interested in seeing how
soil properties change over time).  I thus code the X file to begin on 1st
year, e.g. 1980, and then let it run for 20 years or so (this is mostly to
avoid writing enormous numbers of X files).  e.g.

@N GENERAL     NYERS NREPS START SDATE RSEED SNAME....................
SMODEL
 1 GE             19     1     S 80001  2150 Sname

I had figured I would let it start at the beginning of each year, but given
the sensitivity, I was hoping for advice as to a good rule of thumb for
starting dates (if one exists)? Where I am working, the growing season
starts anywhere from October to December, with spring rains following a dry
winter starting Sept-November.

Thanks in advance for any advice.

Best, Lyndon