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United States Department of Agriculture

Agricultural Research Service

Title: Time Domain Reflectometry Range of Accuracy for High Surface Area Soils

Author
item Logsdon, Sally

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 31, 2005
Publication Date: November 16, 2005
Citation: Logsdon, S.D. 2005. Time domain reflectometry range of accuracy for high surface area soils. Soil Science Society of America Journal. 4(4):1011-1019.

Interpretive Summary: Dynamic soil water content changes are important for understanding how rain moves into the soil, how crops use soil water, and how chemicals wash through the soil. Automated devices measure soil water content in the field every hour for several locations and soil depths. This study showed that the hardware attachments necessary for automated data collection impaired the signal used to determine soil water content. This can reduce the ability of the automated system to accurately measure soil water content. This information is important for scientists who need to know soil water content. This information is also important for field managers who use automated soil water content devices to trigger irrigation systems.

Technical Abstract: Time domain reflectometry (TDR) has been used to determine soil water content. Multiple attachments and long cables often result in unreliable data, especially for fine-textured, high-charge soils, but these attachments are necessary for automated field monitoring at multiple sites and depths. The objective of this study was to experimentally determine the effect of attachments on the TDR waveform for a parallel probe with a balun. Webster clay loam soil (Fine-loamy, mixed, superactive, mesic Typic Endoaquoll) was packed into two PVC pipes. The probe with parallel waveguides and balun was successively attached to the front panel of the cable tester (level 1), to a transient suppressor (level 2), to the first level multiplexer (level 3), or to a second level multiplexer (level 4). All components were placed in a temperature controlled chamber, except for the controlling computer which was outside the chamber. Waveforms were collected for 16384 points (0.1 m/div) for three temperatures and three soil water contents. Waveforms that included a transient suppressor (levels 2 to 4) were attenuated to about half of those at level 1. Multiplexers caused spurious reflections, and long cables resulted in more rounded waveforms. For level 4, increased temperature significantly increased travel time. Each attachment level significantly reduced frequency bandwidth (level 1 > level 2 > level 3 > level 4). The bandwidth for level 4 was < 0.5 GHz. Attenuations due to TDR hardware make it difficult to obtain useful waveforms for determining soil water content.

Last Modified: 4/16/2014
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