% soil moisture

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May 17, 2006, 1:56:54 PM5/17/06
to sowacs: soil water moisture content measurement systems and sensors
How is the % soil moisture determined with a Trase, System I, TDR? I
understand the formula that is used to produce Ka, but want to know how
the % moisture is calculated? Is a formula, a table, or some other
measure used to calculate it? I need to recalculate data for a
different length waveguide. If anyone knows how % moisture is
calculated using a TDR please tell me where to find the data or
Thank you,
Kelly Bolger
Research Asst.
The Morton Arboretum

Sally Logsdon

May 18, 2006, 12:49:34 PM5/18/06
to sow...@googlegroups.com

If you have Ka, the best way to convert to water content is to determine a site / sample specific calibration (by measuring volumetric soil water another way to pair with measured Ka). The square root of Ka usually produces a linear fit with water content. Sometimes there is a variation in the line at high or low water contents. You can used published equations, but they may not be correct for your system. You may be able to group several samples or sites with the same calibration equation. It is also important to keep the whole system intact between the calibration and the measurements (cables, multiplexers, etc.).

Sally Logsdon

>>> kbo...@mortonarb.org 05/17/06 12:56PM >>>


May 18, 2006, 8:07:33 PM5/18/06
to sowacs: soil water moisture content measurement systems and sensors

The Trase uses a table - there is a graph in the manual that you can
read the breakpoints from for linear interpolation or you can get a
download file and that has both the ka and moisture in it so you can
essentially make your own table from this as long as you have enough
readings. I could send you one directly if you wish that was taken
from our Trase 1 with the standard SMEC table.

Beware of the standard calibration for high moisture contents (above
50%) as there is a really odd jump in the calibration relationship.
I'm not sure why this is here but I don't think it is very realistic.
Of course also beware of the standard calibration as the true
calibration varies between soils to some extent.

Dr Andrew Western
Senior Research Fellow
eWater Cooperative Research Centre and
Centre for Environmental Applied Hydrology
Department of Civil and Environmental Engineering
The University of Melbourne
Victoria 3010

email: a.western "at" unimelb edu au, a.western "at" civenv unimelb edu

Frans Joseph

May 19, 2006, 2:13:44 AM5/19/06
to sow...@googlegroups.com

I am also about to read soil moisture content with TDR. Can you please send me that file. I would really appriciate it if you can send me that.


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