Sowacs : Is temperature correction of TDR-determined soil water
content necessary?

The three abstracts below (sent out by Kevin O'Conner today on
tdr-l - see below), concern the temperature dependance of TDR
measurements in different media.

For continuous measurements of SWC, such a correction would be
required, presumably particularly close to the soil surface where
the temperature would fluctuate the most.

But if such soil temperature has not been collected, there are
probably still two ways of getting around the problem (apart from
accepting/ignoring the error):
1)
If one was to use only a single SWC value per day (taken at the
same time at say, midnight) then the temperature would be
reasonably stable and this correction not required.
2)
With knowledge of the air temperature from a nearby met station,
there can at least be some guess at the likely soil temperatures
at the required depths by simple models.


I wanted to mention this as a prelude to discussing the
temperature correction factors supplied by CSI with their CS615
sensor relative to other suppliers.

CSI recommend using the following temperature coefficient for a
range of volumetric water content (VWC) values:
= -3.46E-4 + 0.019VWC - 0.045VWC^2

They report that for a 5 degrees Celcius increase in temperature,
an error of +0.8% would result (without the correction).

This figure is very close to the figure reported below of 2% for
10^C by Verstricht (et al.) (see below).
Alven (et al.) report changes over a day as high as 5% - which
corresponds to a 25^C change in top soil temperature.


These figures seems to tie up rather well, and it would be
interesting to compare these to those of Ledieu (et al.) (1986).

It would also be useful to better understand why the corrections
apply fairly unirormly to all but clay soils...

Relevant parts of the abstracts of the papers mentioned follow:
(Thanks again to  Kevin O'Connor of the tdr-l list who has been
sending these useful abstracts out). To find out about tdr-l,
send INFO to listproc@piranha.acns.nwu.edu

---
Verstricht, J., B. Neerdael, P. Meynendonckx, G. Volckaert. 
1994. Clay Moisture Measurements in Radioactive Waste Disposal
Research. Proceedings of the Symposium on Time Domain
Reflectometry in Environmental, Infrastructure, and Mining
Applications, Evanston, Illinois , Sept 7-9, U.S.
Bureau of Mines, Special Publication SP 19-94, NTIS PB95-105789,
pp. 337-348

"Based on linear regression we found the travel time to increase
at the rate of 8.1 picoseconds per degree-C, so a temperature
increase of 10 degrees-C has the same effect as an increase in
water content of about 2%."
----
Alven-s, G., and M. Stenberg.  1995.  Problems in estimating soil
water content by TDR measurements.  Paper in the Proceedings of
the Symposium: Time-Domain Reflectometry Applications in Soil
Science, (Foulum, Denmark, Sept 16, 1994) SP Report No. 11, June,
Vol. 3, pp. 121-123.

"noticed a dependence of temperature on the estimated soil water
content. In particular, the daily oscillation in estimated soil
water content has been found to be synchronized with daily
temperature oscillation.  The difference between the highest and
lowest water contents during a clear day could be as high as 5%."
----
Halbertsma, J., E. van den Elsen, H. Bohl, and W. Skierucha.
1995. Temperature effects on TDR determined soil water content.  
Paper in the Proceedings of the Symposium: Time-Domain
Reflectometry Applications in Soil Science, (Foulum, Denmark,
Sept 16, 1994) SP Report No. 11, June, Vol. 3, pp. 35-37.

"It found that for sand, loam, and peat there exists a clear
temperature effect on the TDR-measured water content.  This
effect can be compensated for, using our model-based correction
or the correction of Ledieu et al (1986).  However, clay soils
behave differently - the TDR-measured water content showed no
temperature effect."

Regards

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
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
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