Dear All,
in case you are interested in computing ETo using the FAO-56 methodology
(also in case of missing data), I would like to let you know that FAO
recently released a software, called ETo Calculator, having this precise
objective. ETo Calculator Software and Reference Manual can be freely
downloaded at:
http://www.fao.org/nr/water/eto.html
The ETo Calculator can handle daily, ten-day and monthly climatic data. The
data can be given in a wide variety of units and data specified in commonly
used climatic parameters can be processed. When data for some weather
variables are missing, procedures are used for their estimation. Even where
the dataset contains only maximum and minimum air temperature, it is still
possible to obtain reasonable estimates for ten-day or monthly ETo.
Regards,
Gabriella Izzi
__________________________________________
Gabriella Izzi, PhD
Crop Water Productivity Programme
Water Development and Management Unit
Land and Water Division
Food and Agriculture Organization (FAO)
Viale delle Terme di Caracalla, 00153 Rome (Italy)
office B727
tel. (+39) 06 570 55605
-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On
Behalf Of Scott Staggenborg
Sent: 09 February 2009 14:36
To: [log in to unmask]
Subject: Re: Evapotranspiration
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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