The effect of heterogeneous soil-water distribution on the TDR measurement of soil-water content

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  • 不均一な土壌水分分布がTDR 土壌水分測定値に及ぼす影響について
  • effect of heterogeneous soil water distribution on the TDR measurement of soil water content

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Abstract

This study investigated the shape of the TDR waveforms and compatibility of TDR support softwares to analyze the waveforms for dielectric constant, ε, to estimate volumetric water content, θ, in sands with heterogeneous distribution of water. When a dry layer was at the bottom, the waveform shifted upward due to reflection of the TDR pulse at the wet-dry interface before the final reflection of the pulse at the end of the sensor. The program algorithm incorrectly located the point of final reflection at the beginning of the dry sand layer when the water content of the dry layer, θdry, was less than 0.15m3m-3. Selection of inflection points at the wet-dry interface resulted in underestimation of ε and θ. For adry = 0.22 m3 m_3,the magnitude of the upward shift was not significant. The program then correctly determined the reflection point at the end of the sensor. Thickness of the bottom dry sand layer, Ldry, had no effect on the wave analysis. When the bottom layer was quite dry {0Ary =0.05m3m-3), the inflection point was always selected just after the pulse reflected back at the wet-dry sand interface regardless of Ldry values, resulting in underestimation of e and θ. On the other hand, TDR always underestimated ε and θ when water contents were different between two rods of the sensor. The degree of underestimation was higher when the core wire of the sensor was in dry sand.

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