Re: Soil Moisture Measurement in Shallow Saline Soil

[Steve Evett <srevett@ag.gov>]

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Dear Mark,

The TDR technique as practised by many is indeed relatively insensitive to
salinity as long as the salinity level is low enough that a useful wave
form is returned! The big problem is that, as salinity levels increase, the
signal reflection from the ends of the rods in the TDR probe is lost. This
occurs because of conduction of the signal through the saline soil between
the rods. The amount of conduction increases as the soil wets. Thus, one
can obtain quite good wave forms in a very dry saline soil and useless wave
forms in the same soil when wet. To backtrack a little, when people say
that TDR is insensitive to salinity they mean that the travel time is
little affected by salinity. So, as long as the wave form can be
interpreted, the water content estimation will be good. That fact doesn't
help much if the wave form is not interpretable due to signal loss. There
have been many attempts to solve this problem by coating the probe rods.
However, coating the rods introduces some other problems (change in
calibration, loss of sensitivity, wear of the coating affecting results,
etc.). I think there are others out there that could address the
possibilities associated with using coated rods. Hope this helps a little.

Steve

At 10:15 AM 9/27/1999 +0000, you wrote:
>NOTE:  To get off this list, send email to majordomo@aqua.ccwr.ac.za
>with the body of the message containing the line:
>unsubscribe sowacs
>
>Dear List Members,
>
>I have received a request from a client for information about appropriate
>soil moisture sensors for a shallow, saline soil.  The soil is not only
>extremely shallow (10cm in places), but is underlain by Class II carbonate,
>otherwise known as sheet calcrete (solid rock).  As a result, installation
>and operation of neutron probes is extremely problematic.
>
>I had recommended that TDR sensors would be appropriate, based on
>information on the SOWACS web page that TDR are unaffected by salinity.
>This has been contradicted, however, by a third party, who is also an
>independent source like myself, so I have no reason to doubt his sincerity.
>
>Consequently, I am confused, and also at a loss to help my client.  I am
>quite possibly opening a can of worms, but I am sure that others out there
>have had the same problem.  Please offer your opinions, preferably with
>supporting evidence, on both the sensitivity of TDR to salinity, and
>alternative devices to overcome my problem.
>
>I will be away for the next week, but I hope to return to a full e-mail
>basket.  Thanks in advance for your participation in this discussion.
>
>Mark Skewes
>Senior Water Management Consultant
>Irrigated Crop Management Service
>Primary Industries and Resources SA
>Ph. 08 8595 9149
>Fax 08 8595 9180
>E-mail skewes.mark@saugov.sa.gov.au
>
>
>
>

___________________
Steve Evett, Soil Scientist
USDA-ARS, 2300 Experiment Staton Rd., Bushland, TX 79012 USA
806-356-5775, FAX: 806-356-5750
srevett@ag.gov, http://www.cprl.ars.usda.gov/programs/