Hi,

I see at least two shortcomings, when it comes to improving the root growth
dynamics in a model:
First of all, only a few studies might actually have the precise data on
root distribution during the whole cropping phase as you have it. And even
if you have them, I would question the 1:1 relationship between soil water
extraction and root biomass or root length density, especially under water
stress conditions.

Second, you use soil water depletion as a measure of the goodness of the
root growth dynamics. But do you have exact information about the soil
physical properties as well as evapotranspiration? It might simply be that
your drainage rate of a certain soil layer is set wrong. Thus, the model
drains more/less water out of this layer than actually is the case, which
you blame on the root distribution. Furthermore, your transpiration might
simply be to high or low creating an erroneous soil water depletion demand.

Consequently, there are at least three "valves" (root distribution, soil
physical properties, evapotranspiration) you can twist to adjust your model.
As I personally face it right now, applying a crop model you most times
haven't a good idea about even a single of these variables.
Currently it is probably beyond the scope of a crop model to give
100%-precise results on soil water depletion. You might then be better off
with a "stand-alone" soil water model (provided that you have good data for
that...).

Regards,

Rolf Sommer
Natural Resources Program
CIMMYT (International Maize and Wheat Improvement Center)
Tel. +52 (55) 5804 2128
Fax +52 (55) 5804 7558/59
www.cimmyt.org
CIMMYT is a Future Harvest Center of the CGIAR





> -----Original Message-----
> From: Uday Khodke [mailto:[log in to unmask]]
> Sent: Thursday, November 07, 2002 12:17 AM
> To: [log in to unmask]
> Subject: Re: Simulation of root dynamics
>
>
> Dear All,
>
> I also found this difficulty in CROPGROW during simulating
> water balance for
> peanut crop.
>
> During calibration I tried with a factor SLGF (root growth factor) of
> SOIL.SOL file. I hope this factor is soil as well as crop dependent.
> Observed root growth  data indicated that very few roots of
> peanut(local
> variety) crop grow in a bottom soil layer of the
> rootzone(45-60 cm) at the
> later stage of vegetative growth(75 DAS), so I set SLGF as
> 0.10 for this
> layer.
>
> However model shows root growth in this layer at the earlier
> stage (30 DAS
> onwards) which resulted in extraction of soil water form this
> layer quite
> earlier.
>
> Can we set this root growth factor as time dependent so that the root
> dynamics and consequently soil water balance can be simulated more
> precisely.
>
> I am eager to hear whether there is some way to overcome this
> problem or
> DSSAT v4.0 has made improvements in this regard.
>
> Thanking you in advance,
>
> Uday Khodke
> ..............................................................
> ..............
>