Archive AGMODELS-L: file log9405, part 1/1, size 52680 bytes:

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POT-MOD-L An Internet Discussion List for Potato Simulation Modeling

This list is a means for exchanging ideas and program modules
relating to potato simulation modeling. It is also a forum for
discussing other issues relating to potato modeling.

The list is open to anyone associated with or interested in
potato cultivation (very broad category).

Questions about this discussion group should be sent to
potmod@beta.tricity.wsu.edu

To join Pot-Mod-L, send e-mail to "listserv@unl.edu" and place

sub pot-mod-l YOUR_FIRST_NAME YOUR_LAST_NAME

as the body of the text. This should be the only line in the message.

i am seth asare, a phd candidate in water resources engineering at univ.
of guelph, canada. my research is in preferential flow through agricultural
soils. specifically it involves characterising macropores by
scanning undisturbed soil blocks in a CT and analysing with Iris explorer
2.1 on a SGI platform.
right now i am just getting familiar with the huge explorer program. any
help, contributions, suggestions contributions whatever will be very much
appreciated.

What do CT and SGI stand for? Are these universally known soil acronyms?
I'll be glad to know. Thanks.

>
>
> i am seth asare, a phd candidate in water resources engineering at univ.
> of guelph, canada. my research is in preferential flow through agricultural
> soils. specifically it involves characterising macropores by
> scanning undisturbed soil blocks in a CT and analysing with Iris explorer
> 2.1 on a SGI platform.
> right now i am just getting familiar with the huge explorer program. any
> help, contributions, suggestions contributions whatever will be very much
> appreciated.
>
>
>

unsubscribe. Thank you

Dr. Armand Van Wambeke
1014 Bradfield Hall, Cornell U.
Ithaca, NY 14853
PHONE: 1-607-255-1738
FAX: 1-607-255-8615
EMAIL: arv1@cornell.edu

Jensen (1968) yield function has been used in many papers

Y/Yp = (T1/TP1)^a1 * (T2/TP2)^a2 * (T3/TP3)^a3

where Y : actural grain yield
Yp : potential grain yield
Ti : total transpiration of growth stage i
TPi : total potential transpiration of growth stage i
ai : water stress factor of growth stage i

This equation is used to consider water stress effects on crop yields.
However, solar radiation is another important factor to affect crop yields.
In my opinion, this factor should be included in the "potential yield Yp"
term. Thus, there are different potential yields due to different received
solar radiation during growing seasons. My question is

"How can potential "grain" yields be estimated?"

Thanks for your opinions in advance!

*-*
Chingpin Tung
ct16@cornell.edu
*-*

Hi,I'am French student,and I don't know how to use this list.Could any body help
me ? Thanks.

The equation you are looking at is an empirical model. If you want to
do a more thorough job of predicting grain yield, I would recommend
you look at the many simulation models available. These models range
from very empirical to very mechanistic. You need to fit the goals of
your project to the complexity of the model you select.

+---------------------------------------++---------------------------------+
| Dr. Jon D. Hanson || Comm: (303)490-8323 |
| USDA, Agricultural Research Service || Fax: (303)490-8310 |
| Great Plains Systems Research || jon@gpsrv1.gpsr.colostate.edu |
| 301 S. Howes, P.O. Box E || FTS2000: a03jonhanson |
| Fort Collins, Colorado 80522 || |
+---------------------------------------++---------------------------------+

I am about to move to another university, so please temporarily
unsubscribe me. Thanks!

tmack@ag.auburn.edu

Hi! I'm not exactly centre-field for this discussion list, since I'm mainly
a soil erosion modeller, with a particular interest in the effects of
climate change on soil erosion by water, on agricultural land. However, models
such as EPIC are both erosion and crop models, and every erosion model needs a
crop growth component if it is going to deal with agricultural systems. Are
there any other kindred souls out there?

I have a question. What evidence is there that CO2 enrichment changes the rate
at which crops move through the various phenological stages? Or does it just
change the yield (in some cases) without affecting growth rates? I'm
interested because any change in the time required for crops to reach a
critically protective cover (say 30%) would have big implications for erosion.

Regards to all.

Dave Favis-Mortlock

Address: Environmental Change Unit
University of Oxford
1a Mansfield Road
Oxford OX1 3TB
UK
e-mail : david.favismortlock@environmental-change.oxford.ac.uk
fax : (+44) (0)865 281181
phone : (+44) (0)865 281180

Hi Dave, I don't have any evidence, but a crop like potato might
start rapid tuber growth earlier because of a lower N:C ratio due
to CO2 enrichment. The best place to check on evidence would be
with Bruce Kimball (USDA-ARS, the FACE project-free air CO2 enrichment)
in Phoenix, Arizona. They did cotton for several years and now are doing
wheat.

BTW, reaching a %cover level faster is not necessarily speeding up
phenology, to me that would involve starting formation of new organs
earlier such as double ridge in wheat or tassel initiation in maize.

Tom Hodges

Tom Hodges Cropping Systems Modeler ___ ___
USDA-ARS / \_/ \
Rt. 2, Box 2953-A Telephone: 509-786-2226 | |
Prosser, WA 99350 Fax: 509-786-4635 \______/^\/
USA potato tuber
============= thodges@beta.tricity.wsu.edu ========================
...photosynthesis makes the world go around... Mr. Potato Head
> From: Dave Favis-Mortlock <DAVE@nsa.ecu.ox.ac.uk>
>
> I have a question. What evidence is there that CO2 enrichment changes the rate
> at which crops move through the various phenological stages? Or does it just
> change the yield (in some cases) without affecting growth rates? I'm
> interested because any change in the time required for crops to reach a
> critically protective cover (say 30%) would have big implications for erosion.

Dave,
This paper has some evidence of a relationship between phenological
stages and CO2:
J.T. Baker et al., 1990, Developmental responses of rice to
photoperiod and carbon dioxide concentration. Agricultural
and Forest Meteorology, 50:201-210.

----- Forwarded message begins here -----
From: <agmodels-l@unl.edu>
Thu, 12 May 1994 06:38:28 -0500
To: Multiple recipients of list <agmodels-l@unl.edu>
Subject: AGMODELS-L digest 96

Contents:
Another introduction (Dave Favis-Mortlock <DAVE@nsa.ecu.ox.ac.uk>)

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

Date: Thu, 12 May 1994 12:36:20 GMT+1
From: Dave Favis-Mortlock <DAVE@nsa.ecu.ox.ac.uk>
Subject: Another introduction

Hi! I'm not exactly centre-field for this discussion list, since I'm mainly
a soil erosion modeller, with a particular interest in the effects of
climate change on soil erosion by water, on agricultural land. However, models
such as EPIC are both erosion and crop models, and every erosion model needs a
crop growth component if it is going to deal with agricultural systems. Are
there any other kindred souls out there?

I have a question. What evidence is there that CO2 enrichment changes the rate
at which crops move through the various phenological stages? Or does it just
change the yield (in some cases) without affecting growth rates? I'm
interested because any change in the time required for crops to reach a
critically protective cover (say 30%) would have big implications for erosion.

Regards to all.

Dave Favis-Mortlock

Address: Environmental Change Unit
University of Oxford
1a Mansfield Road
Oxford OX1 3TB
UK
e-mail : david.favismortlock@environmental-change.oxford.ac.uk
fax : (+44) (0)865 281181
phone : (+44) (0)865 281180

------------------------------
End of Digest
************************
------ Forwarded message ends here ------
============
Adrienne Roelofs
Bioresource Engineering
Oregon State University, Corvallis, OR 97331-3906
Telephone: 503-737-3218 FAX: 737-2082

I agree with Tom. CO2 enrichment acts as a fertilizer for plants. By
merely increasing growth rate, cover will increase. This does not
necessarily imply that plants move more rapidly through their
phenological development.

+---------------------------------------++---------------------------------+
| Dr. Jon D. Hanson || Comm: (303)490-8323 |
| USDA, Agricultural Research Service || Fax: (303)490-8310 |
| Great Plains Systems Research || jon@gpsrv1.gpsr.colostate.edu |
| 301 S. Howes, P.O. Box E || FTS2000: a03jonhanson |
| Fort Collins, Colorado 80522 || |
+---------------------------------------++---------------------------------+

In my opinion, since crop rates of development depends on temperature and
temperature may increase due to CO2 enrichment, phenology might be affected
by CO2 enrichment.

*-*
Chingpin Tung
ct16@cornell.edu
*-*

This aspect raises some interesting questions. Many crop simulation models
assume that phenology (timing of events) and growth are independent processes,
and that the first is controlled by temperature and daylength, while the second
is by solar radiation and to a lesser extent temperature, at least in the
normal ranges encountered by most field crops. CO2 effects are normally
accounted for by their effect on photosynthesis parameters or RUE, whichever
approach is used, and are assumed not to affect phenology.

However, recent work done here at IRRI on rice by Ingram and others
(about to be published) shows that there is an effect on development rates by
increased CO2. Unfortunately, the response is complicated in that at normal
temperatures (around 25 deg. C) the time to flowering is delayed by higher CO2
(660 ppm), but that at higher temperatures (29, 33 deg. C) time to flowering is
shortened. If this is a general and repeatable phenomenon then obviously it is
necessary to link growth and phenology in some way.

===============================================================================
Robin Matthews Tel: +63 2 818-1926 (ext.436)
International Rice Research Institute Fax: +63 2 818-2087
P O Box 933 Email: R.MATTHEWS@CGNET.COM
1099 Manila Tlx: (ITT) 45365 RICE PM
PHILIPPINES.
===============================================================================

In our modeling, particularly the RZWQM, I have closely linked phenological
development and plant growth. I do believe these linkages must be
considered.

+---------------------------------------++---------------------------------+
| Dr. Jon D. Hanson || Comm: (303)490-8323 |
| USDA, Agricultural Research Service || Fax: (303)490-8310 |
| Great Plains Systems Research || jon@gpsrv1.gpsr.colostate.edu |
| 301 S. Howes, P.O. Box E || FTS2000: a03jonhanson |
| Fort Collins, Colorado 80522 || |
+---------------------------------------++---------------------------------+

Hi! Many thanks for the reference.

Regards

Dave Favis-Mortlock

Address: Environmental Change Unit
University of Oxford
1a Mansfield Road
Oxford OX1 3TB
UK
e-mail : david.favismortlock@environmental-change.oxford.ac.uk
fax : (+44) (0)865 281181
phone : (+44) (0)865 281180

Many thanks. I would appreciate the reference to the IRRI work which you
mentioned, when available.

Regards

Dave Favis-Mortlock

Address: Environmental Change Unit
University of Oxford
1a Mansfield Road
Oxford OX1 3TB
UK
e-mail : david.favismortlock@environmental-change.oxford.ac.uk
fax : (+44) (0)865 281181
phone : (+44) (0)865 281180

If CO2 enrichment lowered C/N ratio of
plant, then increased production, nitrogen application (I mean larger
amount of nitrogen application than normal) decrease production?

Takuro Shinano
Fac. Agric. Hokkaido Univ.
(Sorry for my poor English)
Internet ID: kichiemon@a1.hines.hokudai.ac.jp

It seems to me that greater growth (potential) due to CO2 enrichment
would mean that crops could respond to higher levels of n applications
assuming nothing else (water, temperature) was limiting.
Tom
Tom Hodges Cropping Systems Modeler ___ ___
USDA-ARS / \_/ \
Rt. 2, Box 2953-A Telephone: 509-786-2226 | |
Prosser, WA 99350 Fax: 509-786-4635 \______/^\/
USA potato tuber
============= thodges@beta.tricity.wsu.edu ========================
...photosynthesis makes the world go around... Mr. Potato Head

> If CO2 enrichment lowered C/N ratio of
> plant, then increased production, nitrogen application (I mean larger
> amount of nitrogen application than normal) decrease production?
>
> Takuro Shinano
> Fac. Agric. Hokkaido Univ.
> (Sorry for my poor English)
> Internet ID: kichiemon@a1.hines.hokudai.ac.jp

Huh? Am I missing something? I thought CO2 enrichment was supposed
to increase the C/N ratio . . . because high CO2 generally makes the
Calvin cycle (a big N-grabber in plant chemistry) cheaper . . . and
thus lets plants increase production of N-poor tissues, such as structural
tissue, storage organs, seeds, or whatever at the cost of N-rich
leaf protein. Does anyone have evidence to the contrary?

Jennifer M. Robinson

Through ~15 June 1994
UFZ-ALOE, Permoserstr. 15, 04318 Leipzig, GERMANY
Post Fach 2, 04301 Leipzig
(Applied Landscape OEcol., Environ. Res. Inst. Leipzig-Halle)
tel. +49-341-235-2784, fax 235-2511
robin@alok.ufz.de <or> robin@essc.psu.edu
_________________________________________________________________

Thereafter: NCGIA--SUNY
301 Wilkeson Quad
Box 610023
Buffalo, NY 14261-0023
(National Center for Geographic Information & Analysis
State Univ of New York Buffalo)
tel. +1-716-654-2545
jrobin@ubvms.cc.buffalo.edu

I post this question couple weeks ago, but I still can not work out the
problem. Thus, I hope I can obtain more help from you.

What I concern is how to estimate crop potential yields. If water and
nutrient conditions are not limited, is it possible to estimate potential
yields according to solar radiation or temperature data during growing
season? Any opinion or direction to reference paper is welcome. Thanks in
advance.

*-*
Chingpin Tung
ct16@cornell.edu
*-*

Chingpin Tung:

There are a lot of models around that simulate potential production of crops. A
useful starting point might be Penning de Vries et al. (1989) "Simulation of
ecophysiological processes of growth in several annual crops", Pudoc,
Wageningen. 271pp. which also discusses other levels of production as well. I
don't know what crop you are working with, but we have a rice potential
production model that we are sometimes using here at IRRI which is based on
the Wageningen SUCROS model. Another is Horie's SIMRIW model from Japan. Other
models such as CERES-Rice can also simulate potential production if the water
and nitrogen routines are switched off.

===============================================================================
Robin Matthews Tel: +63 2 818-1926 (ext.436)
International Rice Research Institute Fax: +63 2 818-2087
P O Box 933 Email: R.MATTHEWS@CGNET.COM
1099 Manila Tlx: (ITT) 45365 RICE PM
PHILIPPINES.
===============================================================================

Loomis had a paper in the 60's, perhaps Robin Matthews references
would cite it? It was on potential growth and yield of various
crops based on photosynthetic efficiency and harvest index.

Tom Hodges Cropping Systems Modeler ___ ___
USDA-ARS / \_/ \
Rt. 2, Box 2953-A Telephone: 509-786-2226 | |
Prosser, WA 99350 Fax: 509-786-4635 \______/^\/
USA potato tuber
============= thodges@beta.tricity.wsu.edu ========================
...photosynthesis makes the world go around... Mr. Potato Head

I don't know the 1960's paper Tom Hodges referred to; the only one I know of
that might be relevant is

Loomis & Gerakis, 1975. Productivity of agricultural systems. In: J.P. Cooper
(ed.), Photosynthesis and productivity in different environments, pp145-72.
Cambr. Univ. Press, 715pp.

===============================================================================
Robin Matthews Tel: +63 2 818-1926 (ext.436)
International Rice Research Institute Fax: +63 2 818-2087
P O Box 933 Email: R.MATTHEWS@CGNET.COM
1099 Manila Tlx: (ITT) 45365 RICE PM
PHILIPPINES.
===============================================================================

Thanks for those who concern my question. I need time to digest the
information which I have got, but incoming information is still welcome.
After studying, I wish I can feedback to the list. Thanks again!

*-*
Chingpin Tung
ct16@cornell.edu
*-*

unsubscribe

In message <Pine.3.89.9405181841.B19962-0100000@beta.tricity.wsu.edu>
Tom Hodges writes:

>Loomis had a paper in the 60's, perhaps Robin Matthews references
>would cite it? It was on potential growth and yield of various
>crops based on photosynthetic efficiency and harvest index.

The paper is Loomis & Williams (Crop Sci. 3:67-72, 1963). The example
they work out for maize is:

1. Total solar radiation = 500 cal/ cm^2
2. Visible solar radiation (400-700 nm) = 44% = 222 cal/cm^2
3. Total quanta in visible range (approx. 19.5 uE/cal) = 4320 uE/cm^2
a. Albedo (reflection loss) 6-12% in visible spectrum = -360 uE/cm^2
b. inactive absorption loss = 10% (e.g., cell walls) = -432 uE/cm^2
4. Total quanta usefully absorbed in the visible spectrum
and available for photosynthesis = 3528 uE/cm^2
5. Amount of CO2 reduced (10 quanta/CO2 molecule reduced) = 353 umol CO2/cm^2
6. Respiratory loss of CO2 (33%) = -116 umol CO2/cm^2
7. Net production of CH2O (1 CH2O/CO2 reduced) = 237 umol CH2O/cm^2
8. Conversion of umoles/cm^2 to g/m^2
a. 237 umol/cm^2 = 0.000237 mol/cm^2 = 2.37 mol/m^2
b. CH2O = 30 g/mol * 2.37 mol/m^2 = 71 g/m^2/day
9. If CH2O is 92% of dry weight and inorganic constituents are
80%, total dry matter = 71 g/m^2/day/0.92 = 77 g/m^2/day

This gives a crop growth rate. Any number of things can be done with this.
For instance, assuming this CGR for a grain filling phase of X duration
will approximate grain yield. Likewise, multiplication of this CGR by the
duration of the linear phase of crop growth (from 95% canopy light
interception to grain fill attenuation) approximates total biomass
accumulation, from whence a harvest index of your chosing can be used
to determine final dry matter allocation to grain. In an introductory
crop physiology course, I use a similar exercise to integrate various
key concepts. The above assumptions can be encoded in a spreadsheet such
that for each day of a crop season actual radiation figures are taken
as input (assumption no. 1 above) and the resulting daily CGR computed.
Students can adjust the following variables: PAR loss due to reflectivity,
PAR loss due to light saturation, CO2 loss due to respiration, quantum
requirement. In addition, a photorespiration condition can be added for
C3 species. One use of such a "CGR calculator" is to have students plot
dry matter accumulation and predict total yield. Another is to have
students calculate photosynthetic efficiency (the ratio of one computation
where realistic values for light use are input to a second computation
where no light is lost due to reflectivity, transmission or light saturation).

Ricardo Salvador | Internet: rjsalvad@IASTATE.EDU | "Thou art a little
1126 Agronomy Hall | BITNET: a1.rjs@ISUMVS | soul bearing about
Iowa State University | Voice: (515) 294-9595 | a corpse."
Ames, IA 50011-1010 | FAX: (515) 294-8146 | -Marcus Aurelius-

I wrote "If CO2 enrichment lowered C/N ratio of
plant, then increased production, nitrogen application (I mean larger
amount of nitrogen application than normal) decrease production?"
however, it was mistake. I want to know that "Increasing C/N ratio of plant
by CO2 enrichment is essential for increasing productivity?".
I tried some experiments of CO2 enrichment last year, and I found that though th
e
dry matter production increased under 700ppm (CO2), the C/N ratio of plant
did not differ between 360ppm and 700ppm. I examined maize, wheat, and soybean.
So, I think that if application of nitrogen is not limited the C/N ratio
does not change under CO2 enriched condition. Am I correct?

Takuro Shinano
Fac. Agric. Hokkaido Univ.
Sapporo Japan
kichiemon@a1.hines.hokudai.ac.jp

Loomis and Williams [1963] gave 8.64 uE/cal for visible radiation in
their paper, but, according to the Table 2 of the paper, the value should be
about 19.5 uE/cal. The value of 19.5 is used in the reply mail to "Yield
Potential?" by Dr. Salvador. Is the value of 8.64 an error? Thanks for your
concern!

*-*
Chingpin Tung
ct16@cornell.edu
*-*

unsubscribe

unsubscribe

Dear agmodels-l Listmember! Hamburg,27.5.94
I've three points:
1. A rather short introduction
2. Some questions about plant phenology and a comment on plant phenology and
climate change
3. An annoyed comment about "unsubscribe" and people who do not give their
addresses in e-mails they sent out.

1.
My name is Joerg Kaduk and I'm currently working on my PhD at the Max-Planck-
Institute for Meteorology in Hamburg, Germany. I'm trying to piece a model
of carbon cycling in land vegetation together.
2.
Related with this I have a couple of questions to the members of agmodels-l:
I should like to include in my model a scheme for the determination of some
plant phenological stages in particular budburst and the beginning of leaf fall
Of further interest is the duration of leaf elongation after budburst and the
duration of leaf fall. This would be two points in time and two periods after
these points. So my questions are:

What are the main influences determining these times and periods?

Since I'm interested in global modelling, how do biomes differ with respect to
these influences? Which concepts of agricultural models would carry over to
natural vegetation?
(I use a very crude classification of natural vegetation; the
biomes are named: 1 Tropical dry forest/Savanna, 2. Trop. seasonal forest,
3. Trop. rain forest, 4. Xerophytic woods/shrub, 5. Hot desert, 6. Warm grass/
shrub, 7. Broad leaved evergreen/mixed forest, 8. Temperate deciduous, 9. Cool
mixed, 10. Cold mixed, 11. Cool coniferous forest, 12. Cool grass/shrub, 13.Cold
deciduous, 14. Boreal forest, 15. Tundra, 16. Semidesert, which are those
emerging from the BIOME model of C.Prentice et al.)

How do you and/or other people model these times and time spans?

As I have to content myself with limited meteorological input, are there any
good and (across plant types) stable correlations between monthly weather data
and plant phenology?

Are there any recent good reviews or data collections pertaining to these
questions? What literature would you think is indispensable to read (except
Woodward's Climate and Plant distribution, and Lieth's Phenology and seasonality
modelling?

My ideas are so far: temperate biomes (except grasslands) require a certain
amout of GDD for budburst and for leaf elongation. (But how much?) Does this
hold for coniferous forests as well?
Woodward states that temperate deciduous forests start dropping leaves in the
minimum temp. falls below -5 Deg. C. Is this good for all temperate and boreal
forests (except the coniferous forests clearly)?
Trop./warm biomes and grasslands grow whenever the conditions are good. They
drop leaves whenever the ratio of actual to potential evapotranspiration falls
below a certain threshhold. Is this a reasonable approach? What would be a good
threshhold?

I'd be happy about any comments, criticism, hints, suggestions.... !!!

Concerning the debate two weeks ago about phenology, CO2, and climate change...
I'm quite convinced as well that one needs a link between the growth dynamics
and CO2 and driving phenology by temperature alone might be misleading.
I do have in mind that there is (or was?) some discussion about sink and source
strenghts in plants influencing carbon allocation. I do not know nothing about
this but wouldn't one expect that a changed source strength (what is one
possibility with more CO2 in the atmosphere) wouldn't eventually influence
plant phenology as well?
Another point is the one about CO2 and temperature. Although I do believe that
we will see a greenhouse effect due to anthropogenic emmision of greenhouse
gases (CO2, CH4, CFC's, N2O,...) there will also be some other things going on.
Esp. we might see changes in precip and soil moisture patterns. These are
much more difficult to predict and might have a larger influence on plants
and the global carbon cycle than temperature....

3.
Although it is easily automated that whenever you enter your mail system, you
look for the messages saying "unsubscribe" first, delete them and then look at
the rest (you might even invent a script scanning your mail box or so) this
is an annoying point. I suggest that everybody (who is not aware of this
tiny difference) ponders a bit about the difference of the listserver, the list
and the list owner... Sorry for being a bit cynical...
And please specify your e-mail address in ALL your e-mails, since the listserver
takes them off your mail and plugs in its own... So in all e-mails you
get from any list member via the list server the original sender cannot be iden-
tifyed (by a normal user) from the mail head...

Looking forward to hear from you!

Regards,

Joerg Kaduk *
***
MPI for Meteorology kaduk@dkrz.d400.de *****
Bundesstr. 55 Phone + 40 41173 282 ***
D-20146 Hamburg Fax + 40 41173 298 #
---------------------------------------------------------------------- ###

I made a samml mistake in the description of the e-mail address modification
process. It is not the address of the listserver which is plugged in, but the
address of the list (so far the subtle differences ... :-) )
(That's also the reason why one should be carefull with the usage of auto-reply
... :-) )
Sorry,
regards,
Joerg Kaduk *
***
MPI for Meteorology kaduk@dkrz.d400.de *****
Bundesstr. 55 Phone + 40 41173 282 ***
D-20146 Hamburg Fax + 40 41173 298 #
---------------------------------------------------------------------- ###

Joerg Kaduk raises many interesting questions which will require
much thought, discussion, and data to answer. I will make just one
point:

Different plant species will respond differently on all the questions
you posed. Presumably modeling 1000s of species is not reasonable :-),
it may be necessary to identify groups of key species types that
dominate each biome and do simple models for those types. For some
photoperiod may be most important, for others drought may be vital
or temperatures above or below critical levels.

Also with regard to weather variables, from monthly mean temperatures
and precipitation, it may be reasonable to construct pseudo-variables
for water/ET ratio, snow cover, start of active growing season and
heat units since start, etc. Vikki French did something like this to
model yields of small grains for Eastern Europe in the '80s. Of
course photoperiod is "free" since it is calculated from latitude and
day of year.

If you are interested I could send you Vikki's address and phone.

Tom Hodges

Tom Hodges Cropping Systems Modeler ___ ___
USDA-ARS / \_/ \
Rt. 2, Box 2953-A Telephone: 509-786-2226 | |
Prosser, WA 99350 Fax: 509-786-4635 \______/^\/
USA potato tuber
============= thodges@beta.tricity.wsu.edu ========================
...photosynthesis makes the world go around... Mr. Potato Head

------------------------------ Cut here ------------------------------