Benham,

My opinion is also that the models should be able to calculate
irrigation management depth based on root growth.  A few months ago, I
have modified the model to pass the root depth parameter from the
CERES-Wheat model into the irrigation management subroutine.  Then I
have used the model to automatically irrigate based on a threshold for
soil water depletion in the root zone.  If you will be attending either
the ASA meeting in Long Beach, CA in November or the ASABE Irrigation
Meeting in Phoenix, AZ in December, I will be presenting a talk on this
work.

To answer your question about irrigation depth, the model uses your soil
inputs for drained upper limit and lower limit (specified in your *.SOL
file) and the current soil water status as calculated by the model.  The
soil water depletion depth below the drained upper limit is calculated
for all the soil layers within the automatic irrigation management
depth.  Irrigation management depth (cm) is specified as the IMDEP
parameter in your management file.  To automatically manage irrigation
by root depth, I simply overwrite this parameter with the crop root
depth each day.  Automatic irrigations are triggered with the ITHRL
parameter in your management file.  This is the percent soil water
content (not depletion, but actual percent water content) within the
irrigation management depth at which irrigations are triggered.
Depending on your automatic irrigation option, you can either irrigate
up to the drained upper limit within the management depth or apply a
specified depth of water, as specified with the IRAMT input in your
management file.  As far as I can tell, the ITHRU input parameter is
currently not implemented in the released version of the code.  I find
this also limiting, so I have reimplemented the ITHRU parameter such
that I can specify the soil water content that I want to irrigate up to.
This allows for simulations of deficit irrigation.

Good luck,
Kelly Thorp

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Kelly Robert Thorp, Ph.D.
Research Agricultural Engineer
USDA-ARS
U.S. Arid-Land Agricultural Research Center
21881 North Cardon Lane
Maricopa, AZ  85138
520-316-6375 (phone)
520-316-6330 (fax)
[log in to unmask]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-----Original Message-----
From: DSSAT - Crop Models and Applications
[mailto:[log in to unmask]] On Behalf Of Behnam
Sent: Monday, August 30, 2010 2:40 AM
To: [log in to unmask]
Subject: Why Management Depth Instead of Rooting Depth?

Dear Users,
I once asked a question about how DSSAT calculates irrigation depth
automatically? But I didn't receive any complete answer. Some users
replied my question. But the main question remained unanswered. Why
does DSSAT calculate irrigation depth (automatically) using Management
Depth instead of Rooting Depth? What is the equation being used to
calculate the irrigation depth? How should we find proper management
depth? Why the irrigation depth is nearly constant during the period,
but the irrigation intervals get smaller in time?
Any response is greatly appreciated.

Best
Behnam Ababaei