Re: matric suction in sand vs height of soil wrt water table

[Mark Dale]

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Hey, I am with Cliff on this one.  Granted that above field capacity, the
profile will drain as the water table falls but from field capacity the pore
size will determine the suction pressure when moisture is lost.  I have used
moisture sensors (Watermatic by Ken Cuming) that makes use of this
principle.  The sensor is embedded in a porous block made from an inert
sand, heated until the grains just fuse.  Depending on the grain size,
blocks can be manufactured to correspond to any desired matric potential.
They work well and are very robust.

Mark Dale
Dale Irrigation  Services for irrigation management and skills training

E-mail       preferred address: dale@ncable.com.au
                              alternative: dale@sunnet.net.au

-----Original Message-----
From: owner-sowacs@aqua.ccwr.ac.za [mailto:owner-sowacs@aqua.ccwr.ac.za]
Sent: Monday, 4 June 2001 6:20 PM
Subject: Re: matric suction in sand vs height of soil wrt water table

At 7:01 AM +0000 6/3/01, owner-sowacs@aqua.ccwr.ac.za wrote:

Sorry to say, but Cliff Hignett's reply below is in error

>  > Reply to Ginny Carrera
>>
>>  A sand, particularly a pure sand of dune origons will have a very
>>  uniform grain size.   That means it also has a uniform pore size which
>>  in turn means that if a  column of saturated  sand is subjected to
>>  higher and higher suctions (raised further above the water table) , it
>>  will not change water content very much at all until a particular
>  > suction is reached, then nearly all the water drains at once.

This is physically impossible!

If such a column of sand is TRULY saturated (all its pores are full
of water) most of that water will IMMEDIATELY begin to drain from the
pores as the water table drops. However the residual capillary water,
wetting the surfaces along contacts of sand grains, will drain at an
increasingly slower rate.

Len Ornstein


>   I have a
>>  sand which drains at 49.5cm suction precisely leaving the sand water
>>  content virtually zero..    As the silt and clay content increase, the
>>  material will drain slowly over a wider range of suctions and will not
>>  drain completely.
>>
>>  The suction at which it wets will also be very precise - but it is
>>  unlikely to be the same suction as that at which it drains.
>>  cliff.hignett@soilwater.com.au
>>
>>  owner-sowacs@aqua.ccwr.ac.za wrote:
>>
>>  > NOTE:  To get off this list, send email to majordomo@aqua.ccwr.ac.za
>>  > with the body of the message containing the line:
>>  > unsubscribe sowacs
>>  >
>>  > Jiny Carrera asks
>>  >
>>  > In a sand (free of osmotic suction) what is the
>>  > relationship between matric suction and the height of
>>  > the soil with respect to the water table?
>>  >
>>  > Is this relationship the same for both, wetting and
>>  > drying process?
>>  >
>>  > __________________________________________________
>>
>>  --
>  > Cliff Hignett
>>  Soil Water Solutions
>>  45a Ormond Ave
>>  Daw Park
>>  South Australia 5041
>>  pH 61 (08) 8276 7706
>>  WWW.SOILWATER.COM.AU
>>
>>
>>
>>
>>