Behnam, Ken;
Wind speed units are in km/d in all DSSAT potential ET models.
Regards,
Cheryl

-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On Behalf Of Boote,Kenneth J
Sent: Sunday, January 22, 2012 1:46 AM
To: [log in to unmask]
Subject: Re: RH and Wind Units in DSSAT

Behnam,

What version of DSSAT are you using?  We don't recommend Penman-24 any more in the current version as it is too ET-demanding (not correct).  The FAO-56 was added (configured) after about 2004, as FAO (1998 publications) decided that ET was not as high as originally proposed.  I thought DSSAT was using correct units for both (km/day).  So I have three questions:

1)  Cheryl Porter, would you investigate the units for windrun per day in both FAO-56 and Penman-24 (old non-recommended)?

2)  Behnam, tell me what version of DSSAT you are using?  What crop?  If it is maize and you have drought, the "old" model prior to 2012 had a CO2 sensitive aspect of ET on canopy resistance that may cause under-prediction of transpiration under drought when LAI was low.  That could be part of the issue, that you wanted really high ET (for arid desert environment) and the FAO-56 coupled with this feature on the maize model could have been giving you less ET than you wanted.   So it may not be an error in the windspeed units.

We don't use relative humidity in the ET computations in DSSAT.  We use dewpoint temperature.  If you want it, you will need to compute it as you did.

TAVE is the season-long mean air temperature (average of monthly Tmean, from (monthly Tmax+Tmin)/2).

TAMP is the difference (range) from the highest monthly mean air temperature to the lowest monthly mean air temperature.

TAVE and TAMP can be computed from the climate "file" that Weatherman creates for each weather site, but it IS NOT AUTOMATICALLY OUTPUT to the weather file.  Hence your problem and problem for others.

These two parameters are used in the soil temperature prediction, and affect the maize crop phenology until the 5-leaf stage.  So it is very important that you (and everyone else) does not ignore this.  In absence of values (-99 default), the model has default of 20.0 TAVE and xxxx (I don't know) TAMP.

Good luck.
Ken Boote

________________________________________
From: DSSAT - Crop Models and Applications [[log in to unmask]] on behalf of Behnam Ababaei [[log in to unmask]]
Sent: Saturday, January 21, 2012 8:16 AM
To: [log in to unmask]
Subject: RH and Wind Units in DSSAT

Dear Users,

I was using WeatherMan for creating the weather data files for DSSAT. But,
some issues brought some questions up  for me:



1) WeatherMan uses the unit KM/day (=M/Sec*86.4) for wind speed. In this
case, when I use Penmann24 method for ET calculation, the simulation results
seem ok. But, when I use FAO56, the yields become near zero. When I changed
the values of wind speed into M/Sec, the results seemed more acceptable and
very similar to Penmann24. What is the correct unit for Wind?

2) Why WeatherMan doesn't export Relative Humidity data into DSSAT weather
files? I had to write an algorithm in MATLAB to create DSSAT weather files
for all weather variables.

3) What are the exact meaning of TAV and AMP in weather data files? Is these
correct:

                TAV : Mean value for Average Temperature through the whole
data series.

                AMP : ?