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Time Domain Reflectometry Coil Probe Measurements of Water Content during Fingered Flow

Environmental Engineering Lab., Dep. of Civil Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark

L. W. de Jonge and O. H. Jacobsen

Dep. of Soil Science, Danish Inst. of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark

^* Corresponding author (i5hn{at}civil.auc.dk).

ABSTRACT

The study of fingered flow and other unstable flow phenomena in soil requires a nondestructive technique for measuring changes in volumetric water content () with high temporal and spatial resolution. In this study, a small-scale time domain reflectometry (TDR) coil probe is tested and used to measure the spatial and temporal development of stable and unstable wetting fronts in two sandy soils and a 0.0005- to 0.002-m size fraction of a sandy soil. The small outer dimensions of the coil probe (0.015-m length, 0.0036-m diameter) allowed the use of a Hele-Shaw cell in which progressive wetting fronts could be observed both visually and by TDR measurements of . The sample volume originates at the outer surface of the coil and extends 0.002 to 0.003 m into the soil. Despite this small sample volume, the accuracy of the coil probe relative to the gravimetric method was ±0.017 m³ m^-3 (across a water content range of 0.01 0.135 m³ m^-3). Coil probes, which were small enough to be located at different positions in several of the wetting-front fingers, indicated lateral movement of water from the center of the finger toward the outer fringe surrounding the finger. Profiles of the vertical soil water distribution within a finger measured with the coil probe compared well with a profile from literature measured by the moisture-content-visualization technique. It was concluded that the TDR coil probe is capable of obtaining direct measurements of with high spatial and temporal resolution.

Received for publication September 24, 1997.

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