Hi

Priestley-Taylor developed their formular for large regions with wet
vegetation and assumed that the passing air decreases saturation vapor
pressure deficit until an equilibrium evaporation rate is reached.
McAneny and Itier published a nice paper about the operational limits of
the Priestley-Taylor formular. They concluded that under arid conditions
(VPD > 10 g m-3) there is little choice than to use the Penman-Monteith
formular which forms the scientific basis of the FAO-56 approach.

Explanations can be found in their article:

McAneny, K.J. and Itier, B. (1996) Operational limits to the
Priestly-Taylor formula. Irrigation Science 17:37-43

The documentation of the FAO 56 approach is a noteworthy example how
model theories can be made clearly understandable to the interested
audience. You can read it at following website:

http://www.fao.org/docrep/X0490E/X0490E00.htm

Good luck,
Matthias Langensiepen


Bahram Andarzian wrote:
> 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
>
>
>
>
>
>