Time domain reflectometry coil probe measurements of water content during fingered flow

The study of fingered now and other unstable now phenomena in soil requires a nondestructive technique for measuring changes in volumetric water conte nt (theta) with high temporal and spatial resolution. In this study, a smal l-scale time domain reflectometry (TDR) coil probe is tested and used to me asure the spatial and temporal development of stable and unstable wetting f ronts 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 wettin g fronts could be observed both visually and by TDR measurements of theta. 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 accura cy of the coil probe relative to the gravimetric method was +/-0.017 m(3) m (-3) (across a water content range of 0.01 less than or equal to theta less than or equal to 0.135 m(3) m(-3)). Coil probes, which were small enough t o 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 wate r distribution within a finger measured with the coil probe compared well w ith a profile from literature measured by the moisture-content-visualizatio n technique. It was concluded that the TDR coil probe is capable of obtaini ng direct measurements of theta with high spatial and temporal resolution.