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United States Department of Agriculture

Agricultural Research Service

Title: Comparing Ambient Temperature Effects on Heat Pulse and Time Domain Reflectometry Soil Water Content Measurements

Authors
item Olmanson, Ole - UNIVERSITY OF MINNESOTA
item Ochsner, Tyson

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 9, 2005
Publication Date: May 26, 2006
Citation: Olmanson, O.K., Ochsner, T.E. 2006. Comparing ambient temperature effects on heat pulse and time domain reflectometry soil water content measurements. Vadose Zone Journal. 5:751-756.

Interpretive Summary: Environmental and agricultural scientists often need to measure soil water content. Soil water content directly influences plant growth, soil biological activity, greenhouse gas emissions from soil, and contaminant transport. Time domain reflectometry (TDR) and the heat pulse method are both used to measure soil water content. Changes in ambient temperature have been shown to affect TDR measurements, but less is known about the behavior of heat pulse sensors in response to changes in temperature. This study directly measured and compared the temperature sensitivity of the TDR and heat pulse methods. Increases in temperature led to small increases in measured water content in most cases. The heat pulse method exhibited a greater temperature sensitivity than the TDR method, although the differences were not statistically significant. The temperature sensitivity of the heat pulse method can be attributed to the changes in the density and specific heat of water and specific heat of soil with respect to temperature. When the changes in these parameters are accounted for, the temperature sensitivity is largely eliminated. Scientists who measure soil water content will benefit from these findings. Our results may help them improve the accuracy of soil water content measurements in the field where soil temperature changes continuously.

Technical Abstract: Time domain reflectometry (TDR) and the heat pulse method are both used to measure soil water content. Changes in ambient temperature have been shown to affect TDR measurements, but less is known about the behavior of heat pulse sensors in response to changes in temperature. This study directly measured and compared the temperature sensitivity of the TDR and heat pulse methods. Both methods were used to estimate water content in silt loam and sand at two fixed water contents across a wide temperature range. An increase in temperature led to an increase in measured water content in most cases. Across the 40 degree Celsius temperature range, changes in measured water content were generally 0.04 m3m-3 or less for both methods. Weighted linear regression showed that in these soils the heat pulse method exhibited a greater temperature sensitivity than the TDR method, although the differences were not statistically significant. The temperature sensitivity of the heat pulse method can be attributed to the changes in the density and specific heat of water and specific heat of soil with respect to temperature. When the changes in these parameters are accounted for, the temperature sensitivity is largely eliminated.

Last Modified: 4/18/2014
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