Dear Bahram and Abraham:

We have estimated ET using the FAO-56 methodology with missing data.  One of
the methodologies discussed in the paper is estimating dew points from
minimum temperatures.  They suggest decreasing the minimum temperature 1C in
semi-arid regions.  We were also forced to estimate wind run based on long
term daily averages since we were trying to simulate years prior to wide
spread wind measurements.

When we compared these methods with our current datasets (2000-2007) where
we had all of the measurements for proper FAO-56 calculations, we were
surprised at how close the two methods were.  We were estimating reference
ETs for western Kansas, Nebraska, Oklahoma and Texas and Eastern Colorado
and Wyoming, which is a relatively dry environment (semi-arid).  There are
numerous papers that compare the methods and may also give you an indication
of what your Priestly Taylor adjustment factor may need to be.

Regards
SAS

==============================================
Scott Staggenborg
Professor,  Cropping Systems
Department of Agronomy
2004 Throckmorton Plant Science Center
Kansas State University
Manhattan, KS 66503
785-532-7214 - office
785-532-6094 - fax
[log in to unmask]
==============================================
----- Original Message -----
From: "Abraham Singels" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, February 09, 2009 3:31 AM
Subject: Re: Evapotranspiration


> 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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