Fidel,
If you are using the V4.0 or later versions of DSSAT, you can use the FAO-56 option. It is available. You just need to select it via XBUILD, in the simulation control section for the ET method. BUT, I assume that means that you have data on windspeed and dewpoint temperature (we use dewpoint rather than relative humidity).
P.S. The FAO-56 option will "run" in DSSAT without windspeed and dewpoint temperature in weather file, but it has conservative defaults on wind and Tdew (assumes Tdew close to Tmin). So be cautious there.
I think Rolf is right. You probably have low solar radiation inputs. In my experience, Priestley-Taylor frequently overestimates ET, rather than underestimates it. What maize yield levels are you simulating? With SLPF at the "high" default of 1.00 and adequate water and N, you should be getting more than 10 metric ton grain yield. If not, then you have low solar radiation inputs.
Good luck,
Ken Boote
-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On Behalf Of Munoz, Giovanni (FAOSEC)
Sent: Thursday, January 20, 2011 3:37 AM
To: [log in to unmask]
Subject: Re: ET
Dear Fidel,
Rolf's advise is quite sensible. For that purpose you can use FAO's ETo
Calculator available at:
http://www.fao.org/nr/water/eto.html
If you want to see more irrigation options and better understand water
management and scheduling you could use CROPWAT 8.0 available at:
http://www.fao.org/nr/water/infores_databases_cropwat.html
Both software use FAO Penman-Monteith methodology and you can obtain
reference evapotranspiration (ETo) values even if you only have temperature
data.
Regards,
Giovanni Munoz
FAO
-----Original Message-----
From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] On
Behalf Of Sommer, Rolf (ICARDA)
Sent: 20 January 2011 09:48
To: [log in to unmask]
Subject: Re: ET
Dear Fidel,
I guess you are talking about 400 mm/yr potential Priestley-Taylor (PT) ET,
right?
If you think this is too low, most likely there is something wrong with your
radiation (correct units?) or temperature data.
The best would probably be to calculate PT-ET yourself, and compare the
results with what DSSAT produces.
(I am sure the error lies on the input side...)
By the way, the PT method is not consider very precise - some even call it
the Priestley-Failure method.
Regards,
Rolf
> -----Original Message-----
> From: DSSAT - Crop Models and Applications [mailto:[log in to unmask]]
> On Behalf Of Fidel Peņa Ramos
> Sent: Wednesday, January 19, 2011 23:00
> To: [log in to unmask]
> Subject: ET
>
> I'm running DSSAT v 4.0.2 in three regions of Coahuila Mexico for growing
> corn
> Radiation is ok
> Temperature is ok
> The potential evapotranspiration is estimated with the method of Prisley-
> T. ET values are very low (<400) for the cultivation of high yield in the
> three regions
>
> Why is this happening?
> What should I consider?
>
> _____________________
> Fidel Maximiano Peņa Ramos
> e-mail: [log in to unmask]
> [log in to unmask]
>
>
>
>
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