Dear Francesco,

I see that you work with the modelling group of Colorado State Univ., so I
gues you are familar with the CENTURY SOM model of Bill Parton and his
group, with whom I interacted a lot when linking the two models. I have the
DSSAT-CENTURY version ready, but am a bit hesitant releasing it already,
because we are still testing it and making (hopefully minor) modifications.
Also, we are now converting the DSSAT CROPGRO model into a modular format,
with two soil-organic-matter modules (PAPRAN- and CENTURY-based). I expect
to be done around Feb/March. But if you are now just comparing various
models, I could give you a pre-release.

Arjan

-----Original Message-----
From: francesco tubiello [mailto:[log in to unmask]]
Sent: 28 October, 1999 10:12 PM
To: [log in to unmask]
Subject: Re: Use of the DSSAT cassava model ...


dear arjian,

i would be very much interested in working with your SOM module asap. I had
recently recognized most of the problems you mentioned in your e-mail, and
greatly commend you for trying to improve the existing modules.  I am
interested in looking at SOM in a series of simulations i am running within
a  crop model intercomparison group that was recently started at colorado
state university. Is your new code available now?

bet regards,

francesco tubiello

-----Original Message-----
From: Gijsman, Arjan J. <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, October 28, 1999 11:13 AM
Subject: Re: Use of the DSSAT cassava model ...


>For simulating crop growth in smallholder systems it is important to
realize
>that low-input systems get almost all their nutrients from
>soil-organic-matter or residue turnover. The DSSAT models, however, were
>developed for systems that were not at all low-input oriented, so their
>organic-matter module is quite weak. Also, this module is based on the
>PAPRAN model, which was developed for annual pastures and small-grain crops
>in a semiarid environment, and its authors explicitly state that the use of
>PAPRAN should be limited to those systems. Now as part of DSSAT it is
>applied to any system. For the cassava model this may not be of much
>importance, but for the other DSSAT models it surely is.
>
>We are working with DSSAT in low-input systems in Central America, where
>fertilizer applications are quite limited. For such systems one has to
>realize the limitations of the PAPRAN-based SOM module of DSSAT: [1] it
only
>distinguishes one type of SOM (i.e. 'humus') and
>recently-added-but-not-yet-humidified residue; [2] it does not distinguish
a
>residue layer on top of the soil; [3] newly formed humus is given a fixed
>C:N ratio of 10; [4] though three litter C pools are distinguished, there
is
>only one litter N pool. [5] For residues with C:N ratio < 25, the three
>litter C pools decompose at a rate that is independent of the residue's N
>concentration; [6] SOM and residue flows are independent of soil texture.
>
>So we decided to link the SOM module from the CENTURY model to DSSAT and
>added a new residue layer on top of the soil; this addresses all the points
>mentioned. Modifications were also made to the senescence module of the
>CROPGRO submodel, so that senesced material is now added daily to the soil
>instead of collecting it till the end of the season as is done presently.
>This latter is particularly of importance for green-manure systems with a
>high input of senesced plant parts. This new SOM/residue module should make
>DSSAT much more suitable for low-input systems. It will be part of the next
>DSSAT version.
>
>
>Arjan Gijsman
>Centro Internacional de Agricultura Tropical (CIAT)
>Cali
>Colombia
>
>-----Original Message-----
>From: Roy Fawcett [mailto:[log in to unmask]]
>Sent: 28 October, 1999 10:27 AM
>To: [log in to unmask]
>Subject: Re: Use of the DSSAT cassava model ...
>
>
>Dssat cassava model is a very important contribution to the suite of
>models which form the basis of domestic subsitence throughout the
>tropics. Development of farming systems models can not take place
>without this valuable contribution.
>
>IERM Edinburgh is interested in the use of DSSAT models in small
>holder farming. and will integrate the use of this in on going systems
>work.
>Roy H Fawcett
>