Dear Dr. Hansen,

This is exactly what we were looking for.  Thanks a lot.

Best regards,

Young

-----Original Message-----
From: James Hansen [mailto:[log in to unmask]]
Sent: Wednesday, April 10, 2002 12:02 PM
To: [log in to unmask]
Subject: Re: Root Weighting Functions for the CropGro Model


Dear Dr. Hong,

For a deep, uniform soil with no physical impediments, I have seen two
root weighting functions:

Ritchie and Crum, 1989:

SRGF(i) = exp(-0.02 * z(i))

Jones et al., 1991:

SRGF(i) = (1 - z(i) / 300)^g,  where g = 0.2 for soybean and maize

In both cases, z(i) is the depth in the middle of layer i (cm I believe,
although units are not stated clearly in Ritchie and Crum).  Mechanical
impediments and chemical stresses reduce root weighting at particular
depths.  Both references discuss adjustments for measurable factors.
The former is implemented in the DSSAT soil parameter estimation software.

The source-driven root weighting factor approach is not realistic.  In
this approach, soil stresses (other than water and N content) affect the
vertical distribution of new root growth, but do not reduce the total
amount of root growth.  The implications become clear if one considers a
soil with a shallow but favorable (i.e., high SRGF) topsoil over a deep
but unfavorable (i.e., low SRGF) subsoil.  Contrary to expectations,
reducing topsoil depth to simulate erosion can improve simulated crop
yield by encouraging deep root growth.  Several colleagues have proposed
functional root growth models that combine the source-driven root
weighting factor with a "hospitality factor" that reduces absolute root
growth.  I don't know what the current status of the effort is.

Ritchie, J.T., Crumb, J., 1989. Converting soil survey characterization
data into IBSNAT crop model input. In: Bouma, J., Bregt, A.K. (Ed.),
Land Qualities in Space and Time. Proc. ISSS symposium, Wageningen,
22-26 Aug 1998. Pudoc, Wageningen, The Netherlands, pp. 155-167.

Jones, C.A., Bland, W.L., Ritchie, J.T., Williams, J.R., 1991.
Simulation of root growth. Hanks, J., Ritchie, J.T. (Ed.), Modeling
Plant and Soil Systems. ASA-CSSA-SSSA, Madison, WI, USA, pp. 91-123.

I hope this helps.

Hong, Suk Young wrote:

> Hello, all.
>
> Could anybody recommend some literatures on the root weighting functions for the cropgro model ?
>
> Young
>
> Cropping Systems and Water Quality Research
> USDA-ARS
> Columbia, MO 65211
>
>

Regards,
--
Dr. James W. Hansen
Associate Research Scientist - Agricultural Systems
International Research Institute for Climate Prediction
P.O. Box 1000
Palisades, NY 10964-8000
USA

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