The effect of heterogeneous soil-water distribution on the TDR measurement of soil-water content
2002
"Mojid, M.A. (Saga Univ. (Japan). Faculty of Agriculture) | Toride, N. | Cho, H. "
"This study investigated the shape of the TDR waveforms and compatibility of TDR support softwares to analyze the waveforms for dielectric constant, e, to estimate volumetric water content, th, 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, theta sub(dry) , was less than 0.15cube m m sup(-3) . Selection of inflection points at the wet-dry interface resulted in underestimation of e and th. For theta sub(dry) = 0. 22cube m m sup(-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, L sub(dry), had no effect on the wave analysis. When the bottom layer was quite dry (theta sub(dry) =0.05 cube m m sup(-3) ), the inflection point was always selected just after the pulse reflected back at the wet-dry sand interface regardless of L sub(dry) values, resulting in underestimation of e and th. On the other hand, TDR always underestimated e and th. 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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