Dear all:

Sorry for writing everyone because I think there may be others who are
interested in such a problem as Carlos is having in his simulations.

First: The simulation described by Carlos seems to be very near to
simulating what the Wageningen school calls "Potential yield" for potato.
Limited only by genetics, radiation and temperature.

Second: Actual yields divided by potential yields can be defined as a
"production efficiency" . According to Beukema and van der Zaag (1990)
potential yield in the state of Washington (USA) and the Netherlands can be
as high as 140 and 100 tons / ha with an actual production of 65 and 45 tons
/ ha leading to production efficiencies of 0.46 and 0.45. So a potential
production of 110 ton / ha and an actual production of 40 - 50 ton / ha
leading to production efficiencies of 0.36 - 0.45 in the United Kingdom is
perfectly possible and not an error of the SUBSTOR potato model.

Third: In principle you don't even need SUBSTOR for calculating potential
yields of potato in the UK. That can be done from my desk using stationary
crop models such as FAO Agroecological Zones Model (Doorenbos and Kassam,
1979) or van der Zaag model (Beukema and van der Zaag, 1990). If you are
interested in having the necessary tables and formulas for using these
approaches I would be in the best of disposition of making them available
for you.

Fourth: It is normal that SUBSTOR is reporting N stress when you lower your
fertilizer amounts under 400 kg / ha. To be able to obtain such a high
potential production the potato plant requires a lot of mineral nutrients!

With my very best wishes,

Roger E. Rivero Vega

Beukema, H. P.; D. E. van der Zaag  (1990): Introduction to potato
production. Pudoc Wageningen, Netherlands, 207 pp.

Doorenbos, J. and A. H. Kassam (1979): Yield response to water. FAO
Irrigation and Drainage Pap. 33, FAO, Rome, 193 pp.



----- Original Message -----
From: "Berna Esteban, Carlos" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, February 06, 2008 7:01 AM
Subject: DSSAT v4. Double yield than 'normal' values



--------------------------------------------------------------------------------


> Dear DSSAT users,
>
> My name is Carlos Berná and I am doing a PhD research in Cranfield
> University (UK). The topic of my research is climate change impacts on
> agriculture water management on potato crop in UK.
>
> Currently I am on the process of calibrating/validating DSSAT model for my
> study area.
> The issue I am facing is that I am getting not very 'real' numbers coming
> out from the model comparing to the average 'normal' year for UK potato
> growers (40-50t/ha)... I am getting double Tuber fresh weight than that
> (110 t/ha). I am applying 400 kg/ha of N in 4 application along the
> growing season. Irrigation scheduling is set as 'automatic when required'
> (applying 30mm every time % of max water availability is down to 60% -
> this seems to work well). It is true that it might be a bit high levels of
> fertilizer, but if I put them down I get N stress... it is also true that
> I am not considering diseases or pests, and the plant is not water
> stressed.
>
> Maybe I am missing something or doing something wrong... Any idea or
> comment will be welcome.
>
> Please, feel free to comment on it.
>
> Best regards,
> Carlos
>