Hi,

maybe inverse modeling (IM) will be helpful in this case.  i agree that
there are adverse limitations of PTF in estimating the soil hydraulic
properties (Espino, A., Mallants, D., Vanclooster and J. Feyen.  1995.
Cautionary notes on the use of pedotransfer functions for estimating soil
hydraulic properties.  Agricultural Water Management. 29: 235-253).
althhough (according to my knowledge) DSSAT does not use the pF curve in the
soil water dynamics but the DUL and LL and other sensitive properties could
be defined by IM.  In IM you can use the soil water measurements to
inversely estimate the DUL and LL, and therefore averting the uncertanties
laden in PTF use.  but of course there are also uncertainty issues in IM.


Maybe these documents can be of help:



Ines, A.V.M. and P. Droogers.  2002.  Inverse modeling in estimating soil
hydraulic functions: A Genetic Algorithm approach.  Hydrology and Earth
System Sciences.  6: 49-65.



Jhorar, R.K., Bastiaanssen, W.G.M., Feddes, R.A. and J.C. Van Dam.  2002.
Inversely estimating soil hydraulic functions using evapotranspiration
fluxes.  Journal of Hydrology.  258: 198-213.



http://www.sce.ait.ac.th/research/dissertation/wem/2002/~ines/



Thanks,

Amor V.M. Ines


----- Original Message -----
From: "Gerrit Hoogenboom" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, December 06, 2002 9:24 PM
Subject: Soil water limits


> >Hi,
> >
> >
> >
> >
> >
> >Estimating LL, DUL and SAT on the basis of a soil's texture and SOM
content
> >
> >is very uncertain and gives great differences depending on which
estimation
> >
> >method is used (and there are >150 methods!). If one really wants
reliable
> >
> >data there is nothing better than measuring it, though this still leaves
the
> >
> >uncertainty of measuring in the lab with a pressure plate apparatus or
> >
> >measuring in the field.
> >
> >
> >
> >I have been comparing eight estimation methods for LL, DUL, SAT,
especially
> >
> >focusing on the uncertainties in each method (and these were big!). I
> >
> >presented this last year at a crop-modeling symposium in Florence, Italy,
> >
> >from which the article just has been published (the title expresses the
high
> >
> >level of uncertainty in these methods....):
> >
> >
> >
> >A.J. Gijsman, S.S. Jagtap, J.W. Jones. Wading through a swamp of complete
> >
> >confusion: how to choose a method for estimating soil water retention
> >
> >parameters for crop models. European Journal of Agronomy, Volume 18,
Numbers
> >
> >1-2 (2002), pp. 75-105.
> >
> >
> >
> >Since the water retention parameters are so crucial for the outcome of a
> >
> >crop model simulation, I would recommend to pay due attention to this.
> >
> >
> >
> >Concerning your specific question: there is no need for having the same
(or
> >
> >even similar) difference DUL vs. SAT in all soil layers. Layers differ in
> >
> >texture and in soil organic matter (SOM) content, and will thus differ in
> >
> >water-retention parameters. Some of the methods I compared gave DUL < SAT
> >
> >(as it should be!) for certain texture combinations, but DUL > SAT for
> >
> >others, in which case I used DUL = 0.95 * SAT; this is not to say that
this
> >
> >is right, but it is surely better than DUL > SAT. For DSSAT, DUL has to
be
> >
> >greater than LL and SAT has to be greater than DUL (thus: SAT>DUL>LL), or
> >
> >the model crashes.
> >
> >
> >
> >If you have measured bulk density (BD), I would not modify this to tune
the
> >
> >water retention. Sometimes BD values may indeed be suspicious, as I found
> >
> >e.g. recently in an international soils database BD's greater then 2.50
> >
> >g/cm3; knowing that the particle density of granite (i.e. sand without
the
> >
> >pores) is about 2.65 g/cm3, such a value is highly unlikely, as it would
be
> >
> >a soil without pores and without SOM. In the above-mentioned article we
> >
> >describe also a method from the literature for calculating the
(theoretical)
> >
> >BD; this may not apply equally to all soil types, but it is the best one
can
> >
> >do, if measured data are missing or clearly wrong.
> >
> >
> >
> >
> >
> >Arjan Gijsman
> >
> >Univ. Florida / Centro Internacional de Agricultura Tropical (CIAT),
Cali,
> >
> >Colombia
> >
> >
>