Current thinking

[David TerraNova]

Dear Members, I need guidance on the best current technology for rapid
soil moisture measurement. I need to measure the water content of large
numbers of soil samples and soils exposed in the field. Thus I need a
portable measurement system that will give reasonably precise
volumetric soil water measurements quickly - although I accept that I
may have to compromise precision for portability and speed. I also
accept that some calibration is likely to be needed - though I want to
do the least I can, to be able to take more readings.

I know a little about TDR and capacitance technologies and have seen a
number of systems over the years - but I also know that questions have
been raised about the true performance of these sytems, in practice,
and I want to find out what will truly best meet my needs, without the
hard sell. I've seen, for example, a Percometer system in use recently
and thought it was exactly what I needed - quick and simple and very
portable, with a small, circular measuring head which is pressed
against the soil surface - but does it really give reliable results in
a variety of soil textures and moisture states?

Could members please offer advice?

Many thanks

[Wojciech Skierucha]

Dear Sir,
There are many systems available on the market.
Recently the company Easy Test presented new devices for TDR soil moisture
measurenment. Please visit the WEB site: www.easytest.lublin.pl which is
still under construction. You can see a modern portable TDR soil moisture
meter FOM/mts and data loggers MIDL with DLOG meters equipped multiplexed
TDR meter.
Best regards,

Dr. Wojciech Skierucha
Institute of Agrophysics
Polish Academy of Sciences
20-290 Lublin, Doswiadczalna 4
Poland
tel. +48 81 7445061 ext. 125
fax. +48 81 7445067
e-mail. skie...@demeter.ipan.lublin.pl

[David TerraNova]

Dr Skierucha,

Thank you - but I need to be able to take tens to hundreds of readings
a day - and I'd prefer to do so on precisely defined soil volumes
(<10ml). So two questions - if I repeatedly push the TDR probe into
firm clay how long will it last before the probe breaks? Can I use such
a probe system to measure only a small soil volume, for example by
advancing the probes only 2cm into the soil surface and adjusting the
reading acordingly?

I'd be grateful for experience of other list members on these practical
matters too. The best kit in the world is no good if it breaks in
normal use so how robust are TDR systems in uses such as mine where I
need to make lots of repeated measurements in soils of all kinds? You
can see why I liked the Percometer measuing head because you only have
to clean the soil surface and press the head to it to get a reading -
there's very little to break and the system is inherently robust.

Thanks,

David

[Priyantha B Kulasekera]

Have you tried using theta probes?

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[WENCESLAU TEIXEIRA]

Dear David,

We used the Easy test device to measure hundreds point in a project here
in the Amazon some year ago. It worked well. The equipment are still
working.
In the begining we used to pushed the TDR probe into the soil. It didnt
work well. The problem is not with the resistance of the probes that are
quite good, but the soil gets disturbed and the measurements were too
bady. We bought more probes and installed them fixed at the interest
point. Only the display was movel and we had two of them.
In another project here in the Amazon it was used the Trime sensor (I
believe it is more like FDR) that is similar to neutron probes and you
must install access tubes. It works also good here.

About the measured soil volume for TDR probes ... it is quite
complicated to calculate it.
For the standard Easy test probe I calculated it. It is published in the
SAMPLING AND TDR PROBE INSERTION IN THE DETERMINATION OF VOLUMETRIC
WATER CONTENT
Revista Brasileira de Ciencia do Solo, 27:575-582, 2003

If you have problem to get this article and can send as pdf format to you.

I hope it helps

Regards from the Amazon.

Wenceslau Teixeira

[a.we...@unimelb.edu.au]

David,

We have used TDR on an all terrain vehicle with differential GPS
systems to map soil moisture over the last decade. Depending on
conditions, terrain, sampling spacing etc, we would do 100-500 samples
a day (we routinely mapped a 10ha site on a 10*20m grid - 500 points in
8 to 12 hours in hard setting clay loam duplex soils with an ironstone
layer. This was difficult during very dry conditions but having a
hydraulic insertion system on our setup was a really major advantage -
better quality data (less gaps around probes) faster with much less
effort compared with manual insertion. If you have a look at
www.civenv.unimellb.edu.au/~western/tarrawarra/tarrawarra.html you will
get an idea of the sort of data we've collected and also the gear. We
use soil moisture equipment corp TRASE system - reasonably robust for
this sort of work and you can replace the rods rather than the whole
probe.

Calibration - the necessity of this depends on the soil. Bound water is
probably the key issue so high clays (particularly certain
mineralogies) and/or high organic matter are something to look out for.
I think the jury is still out on the impact of salinity on
calibrations but I haven't looked at the literature for a while. I
would definitely go with true TDR rather than other dielectric systems
as in my experience the calibration is more stable to soil type.
Obviously your required accuracy depends a lot on your application.

You asked about getting a profile measurement with vertically inserted
tdr. This is not possible by partially inserting the probe but I have
seen papers where the time trace of voltage is analysed and the authors
claim to be able to back out the moisture profile - I don't know how
accurately. I certainly have had situations where a relatively dry or
wet upper layer is obvious on the voltage-time response but have never
looked at this quantitatively.

If you want more info email me.

cheers Andrew

[ral...@kimberly.uidaho.edu]

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