Re: Affect of accuracy of capacitance soil moisture technology

[Cliff Hignett]

I endorse some of  the comments of Moshe Meron - with the exception of
his first comment - In my experience it is unwise to trust the
calibration provided by manufacturers.   In general manufacturers are
electronics developers - often with few skill in soil science.    Few
manufacturers understand the difference between soil water and soil
water potential  - even fewer understand the difference between
gravimetric water content and volumetric water content.    Sometimes the
manufacturer is aware of there in expertise and get someone who is
skilled, to do the job.   DONT take calibrations at face value.  Enquire
who did them, what soil type they were done in, and was the effect of
temperature checked.  (as a minimum check list).

I recently participated in a review of soil water measurement methods
organised by the IAEA (along with Clarke Topp(Canada), Steve Evett(USA)
with IAEA and European scientists and a number of manufacturers
representatives).   The conclusions of this meeting were unanimous.
That even with a plethora of electronic devices to choose from, if you
wish to measure soil water in the field, the neutron moisture meter
requires far fewer readings to give a result with any precision.  The
reasons were (1) that there are still many situations where the
'unknowns' of electronic methods cause problems (2) the tiny volume of
measurement of electronic methods (3) the very real risk (because of
(2), that electronic methods are reading water in a volume of soil which
has been disturbed and is therefore not typical of the field.

Results of this meeting are soon to be published in the IAEA 'TECHDOc'
series.

Cliff Hignett

owner-sowacs@aqua.ccwr.ac.za wrote:

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> We may trust the sensor manufacturers for their test bench
> calibrations, and
> even for acceptable uniformity of the product. The variability of of
> calibration seems to be inherent to all types of sensors which read
> low
> volumes of soil water content, like TDR and FDR, for another reason:
>
> Water moves and equilibrates within the soil by potential gradients,
> not by
> water content. Soil pore volume distribution (hence soil water
> retention
> characteristics) is quite variable on the micro-scale even in
> undisturbed
> soils, not to say after all what we do to the soil during the process
> of
> sensor insertion. The chance to preserve the same water retention
> characteristics around the sensor as in the surrounding native soil is
> quite
> small.
>
> In fact what we read with the TDR&FDR sensors is the soil water
> content of
> the disturbed soil around the sensor, which is equilibrated with the
> soil
> water potential, but not the soil water content of the bulk soil
> around. So
> under similar bulk soil moisture content in a field we may read a wide
>
> variety of values on the TDR&FDR sensors, because the measurement is
> on the
> micro-scale, with a tremendous variability of pore size distribution.
>
> Add to this the natural variability in soil water content, and soil
> water
> potential on the micro-scale in the soil. For instance lately obtained
> (and
> not yet published) results by Martin Shmitz from the FAL-Braunshweig,
> Germany, (SCHMITZ@BT.FAL.DE) showed similar, and quite sizable,
> variability
> for TDR, FDR and granular matrix sensors readings in a very small
> grass
> plot. Common to all of those sensors was the measurement of a very
> small
> soil volume.
>
> It looks like the key of useful soil water content / potential
> measurements
> is at large volume measuring devices, or alternatively at numerous
> (and
> cheap...) small soil volume measuring sensors. There is still some
> place for
> technological advances in soil water measurement.
>
> Moshe Meron
> =====================================================================
> MIGAL Galilee Technology Center - Crop Ecology Laboratory
> Kiryat Shmona,  PO Box 90 000 Rosh Pina 12 100 ISRAEL
> Phone: +972-6-6953559  Fax:+972-6-6944980     Email:MERON@migal.co.il
> ====================================================================
>
>  .


--
Cliff Hignett
Soil Water Solutions
45a Ormond Ave
Daw Park
South Australia 5041
pH 61 (08) 8276 7706
WWW.SOILWATER.COM.AU