Dear Bahram

From my experience the Priestley Taylor method seriously underestimates ET in semi-arid and arid climates.  The FAO-56 method would give a much better estimate, provided you supply it with measured dew point (or some other measured measure of humidity).  If you do not supply measured values, the DSSAT system will estimate vapour pressure deficit based on the assumption that dew point equals minimum temperature (100% humidity at Tmin), as far as I know.  This assumption is also questionable for semi-arid and arid regions and will lead to significant under-predictions of ET.
Good luck.

Abraham Singels
Principal Agronomist
South African Sugarcane Research Institute

e-mail: [log in to unmask]
Tel: +27315087446
Cell: +27836554092




-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On Behalf Of Bahram Andarzian
Sent: 09 February 2009 11:00 AM
To: [log in to unmask]
Subject: Evapotranspiration

Dear all of my
friends

Hi

In DSSAT models\ simulation
option there are 4 methods to calculate evapotranspiration. The calculated
results by these methods are different from each others. For example, in my
simulation with CERES-wheat, the Priesty-Taylor/Ritchie  method estimates evapotranspiration
very much than the FAO-56. If possible let me know, which manner is more appropriate
for warm and arid and semi-arid regions?

Best

Bahram Andarzian





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