Dear Pradeep,

The problem you are seeing is probably because the roots of the plant don't
do down as far as your irrigation management depth.  Let's say for argument
sake that your roots go down to 30 cm.  In your first case (Man. Depth =
30) the roots will quickly take up water in your entire management
zone.  This will cause  the automatic irrigator to trigger frequently and
keep the plant well watered this will allow for maximum ET with a number of
small frequent irrigations.

Lets take your last case (Man. Depth = 60).  The plant will quickly uptake
water from the top 30 cm of soil.  However, there is still plenty of water
in the lower 30 cm, the automatic irrigator still "sees" that the soil is
too wet to irrigate.  The plant will start to experience water stress and
ET will go down.  Eventually the top 30 cm will dry so much or enough water
will leave the bottom 30 cm to trigger irrigation.

In this second case you will get fewer bigger irrigations.  The total
amount of water will be decreased because, I believe, that the plant was at
higher water stress for longer periods of time.  This means less total ET
and therefore less irrigation needed.

Brian

At 02:25 PM 4/19/2001 +0530, you wrote:
>Dear members
>
>We ran SUBSTOR-Potato model to simulate the effect of different management
>depths on ET and Irrigation requirements (IR). We took irrigation
>management depths as 0-30, 0-45 and 0-60 cm. The model was run for
>different depletion levels using automatic irrigation (auto-refill
>profile). We found that total ET and IR reduces as the depth of management
>increases. Similar trend was noticed if we simulate considering various
>depletion levels. In my opinion total ET and IR should increase as the
>depth of management increases, but the simulations show other way round.
>What may be the possible reasons?
>
>With kind regards.
>
>Pradeep
>______________________________________________
>Pradeep Singh Kashyap
>Department of Agricultural & Food Engg.
>I. I. T., Kharagpur
>(WB) - 721 302
>India
>______________________________________________
>Tel:  091-3222-83138 (O)
>                      83139 (R)
>Fax: 091-3222-82244 & 55303
>Alternate e-mail: [log in to unmask]
>______________________________________________

==============================================================
Brian Baer                           [log in to unmask]
CSS Dept, Michigan State University  (517) 353-4702 (voice)
570 Plant and Soil Science Building  (517) 355-0270 (fax)
East Lansing, MI  48824-1325         http://nowlin.css.msu.edu