At the end of this message I have included an introduction from 
Dr. Ioan C. Paltineanu as a welcome to SoWaCS.

Having so many experts on board, I would now like to restart
discussion on some of the more practical aspects of TDR
installation, use and data related considerations. (Yes, I am
about to go into the field at last).

Please don't feel shy - add your two cents' worth and we may
eventually add up to a Dollar!

                      * * *

1/ First and foremost, I have heard conflicting advice when it
comes to insulating the electrodes on TDR sensors.
The presence of soil salinity is said to affect the soil water
content measurement by its attenuating influence on the signal.
Various coatings have been proposed to electrically insulate the
electrodes from the soil solution.
Obviously the coating should be as thin as possible, and have a
very low dielectric constant.
Heat-shrink has been suggested as a practical option, but perhaps
a tough paint or resin would do a better job?

Are there any other suggestions or experience in this regard?

2/ Placement of sensors.
The Campbell CS615 sensors are 300 mm in length, and so integrate
the soil water content over that length. For a good areal
estimate (With a limited number of sensors), units should thus be
placed horizontally at a number of levels below the surface. But
for the highly variable top soil region of the soil, perhaps a
vertical sensor would be more appropriate, rather than sensing at
just one, depth.
(When comparing soil water content in the top soil of adjacent
experimental treatment plots, for example, observed treatment
differences could actually be the result of slightly different
depth placements)?

3/ The practical side of placing sensors below ground is
complicated if trenches are required.
The back-filling of such trenches should be done in an attemp to
restore the original conditions of bulk density, etc. But soil
disturbance is known to promote preferential root growth, and for
this reason the sensor placement needs to be done some time
before the measurements are required. (In studies of water use of
plantation forestry - such as I am doing - this means that an
already long-term experiment becomes even longer!)

4/ The practicalities of running wiring in the field from sensor
to logger is also a problem. Fire, rodent and termite damage,
vandalism and agricultral operations can put the wires and
sensors at risk.
I am considering using platic conduiting buried just below the
surface. But with several hundred metres of cabling to run, I am
thinking again... Any experiences of this nature and advice would
be welcomed.


5/ The capabilities of dataloggers allow an overwhelming amount
of data to be collected, and I am therefore looking to do some
real-time in-field reduction.
For a water balance, is one value per day sufficient information?
(After all, we usually have only one daily total rainfall
measurement!)
If so, should this value be a mean of various measurements during
that day, or is one chosen time period adequate? When should this
be - midnight, sunrise or midday?
Battery-powered sensors and logger is a consideration of course.

6/ How does one go about calibrating these type of sensors?
Obviously using pressure pot equipment such as I did with smaller
sensors is out.. I presume large weighed containers of the soil
to be used would be one way. Comparison with other measurment
methods (neutron probe, psychrometry etc.) peresumably all have
their place.

7/ Is it acceptable to extend the 5-core wiring of the CS615
sensors up to about 100 m? It is not shielded, has a wire
supplying current, one to enable (turn on), a signal wire and a
common? signal-current ground. The fifth is the earth.

8/ Swelling and shrinking soils are especially problematic when
it comes to adequate soil:sensor contact, as TDR and FDR based
sensors do not operate adequately with even the smallest of
airgaps. Any practical advice on how to minimise or eliminate
such problems? Aslo, how does one know if it is happening?

Come on, those of you with opinions on these points let's have
some illuminating debate...

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp           SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209
Rep. of South Africa             Voice:27 331 62314
E-mail: bruce@icfr.unp.ac.za       FAX:27 331 68905
URL http://www.icfrnet.unp.ac.za/~metele
                                        /SoWaCS.html
                                        /RR.html
Host of SoWaCS (Soil Water Content Sensor) Discussion List.


From:	ipaltin (ipaltin)
To:		bruce@icfr.unp.ac.za
Date:	Friday, May 17, 1996  7:07 pm
Subject:	I have subscribed

Dear Bruce, It will be my pleasure and honor to subscribe sowacs.
As I have been working with the neutron thermalization method for
almost 30 years and with the capacitance technique and TDR for
the last 3 years, subscribing to your list will be a good
opportunity to exchange ideas on the merits and disadvantages of,
at least, these three methods and techniques for the real-time
soil water content monitoring. Sincerely yours, 

Dr. Ioan C. Paltineanu- visiting scientist from Romania,
UDSA-ARS-BARC-NRI-Environmental Chemistry Laboratory, 10300
Baltimore Ave. Bldg. 007, Room 224, BARC-West, Beltsville, MD,
20705-2350
Tel: (301) 504-5503, Fax:(301) 504-5048