Re: Field x laboratory

[Martin Parkes]

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Cliff and Wenceslau

Soil tension in the lab decreases with increasing temperature, while in the
field
the opposite is observed! In trying to understand this, I have concluded
that in
the field there are at least 3 contributory effects. The opposing effects
explain
the time lag in the observed peak values of diurnal tension fluctuation.  The
summary of a research note is given below:

Previous reports suggested that diurnal fluctuations of field tensiometer
readings
did not always occur, with any fluctuations being less than 0.2 kPa.
Variations of
matric potential with temperature, measured in the laboratory, suggest that
air
entrapment or changes in the meniscus film are reasons for the variation
exceeding
that attributed to surface tension changes. The variation of tension with
temperature in the laboratory is the inverse relationship to that seen in the
field. At soil depths within the range of diurnal soil temperature
fluctuations,
it is possible that entrapped air pressure variations lead to diurnal
changes of
tensiometer reading of the order -2 kPa at 7.5 kPa tensions. Large diurnal
fluctuations of tensiometer measurements of  +8 kPa, at 50 kPa tensions, are
likely to be related to a combination of differential expansion of tensiometer
casing and water contained in it, together with fluctuations of root water
potentials. A reduction of tension is apparent first, as the casing expands
during
daylight hours. The subsequent heat transfer to the tensiometer water creates
causes a reduction of the tension reading in the early morning. The change of
entrapped air conditions within the soil, with change of temperature, is
likely to
be a subsidiary influence.  This contributes to the lag in measurement of peak
tension. So several of the potential contributions to diurnal tensiometer
readings, suggested by Remson and Randolph, contribute in differing amounts
and
differing ways within reported data.

Martin Parkes
Wuhan University (WUHEE)
8 Donghu Nanlu, Wuchang
430072 WUHAN
P R CHINA


> Tensiometers often misbehave in the field due to temperature fluctuations
> (especially day /night) - these cause the water and air in the
tensiometer to
> expand and contract - try insulating the tensiometers or reading then
early in
> morning (before they warm up)
>
> Cliff.hignett@soilwater.com.au
>
> owner-sowacs@aqua.ccwr.ac.za wrote:
>
> > Dear list members.
> >
> > I am working with experimental water retention data to fit parametric
> > equations, which I'd like to use in numerical simulations of water fluxes.
> >
> > I have determined in-situ volumetric soil water content by TDR readings
and
> > soil pressure head by tensiometers. However, measured field data disagree
> > with the determined on undisturbed soil cores samples collected at the
same
> > site.
> >
> > The problem of hysteresis was partially circumventing collecting the field
> > data in the drying brunch of the hysteretic curve (it was carried out
on an
> > instantaneous profile method experiment).
> >
> > The volumes sampled by TDR and the cups of our tensiometers are not the
> > same and, not only the volumes were different but they are separated
> > because of installation procedures. Moreover, the soil samples for the
> > laboratory measurement were collected about 2 meter apart.These factors
are
> > probably some explanation for the differences, but our data set is large
> > with many replications and, I believe that some other factors besides soil
> > variability are contributing to that difference ...
> > I have found a limited number of field studies (where in situ measurements
> > were carried out) in which undisturbed soil cores were taken.
> >
> > Someone knows studies concerning the problem of field x laboratory
> > determination of the soil water retention curves ? or have some
> suggestions ?
> >
> > Any help on this matter will be greatly appreciated
> >
> > Best regards.
> >
> > Wenceslau Teixeira
> >
> > _____________________________________________________________
> > Wenceslau Geraldes Teixeira
> > Embrapa - AmazŁnia Ocidental - Brasil
> >
> > Universit”t Bayreuth
> > Bodenkunde - Abt. Bodenphysik
> > D - 95440 - Bayreuth - Germany
> > Fax: +49 (0)921 552246
>
> --
> Cliff Hignett
> Soil Water Solutions
> 45a Ormond Ave
> Daw Park
> South Australia 5041
> pH 61 (08) 8276 7706
> WWW.SOILWATER.COM.AU