Water content measurements in the deep vadose zone using a new design and installation technique of TDR probes

A common method for obtaining continuous water content data in unsaturated soil relies on Time Domain Reflectometry (TDR). However, TDR probes are difficult to install in deep soil horizons, and often the soil is disturbed in the process. Given the importance of accurate determination of water content in many vadose zone monitoring programs, we have developed a new design and installation technique for TDR probes, enabling water content monitoring of deep vadose zone horizons in relatively undisturbed soil conditions to any desired depth. The method uses flat, flexible waveguides made of stainless foil strips, attached to the ceiling of a slanted borehole. The TDR waveguides are attached to a flexible sleeve made of rubber or PVC liner, filled with a liquid resin. Prior to its curing, the resin (e.g., a two-component urethane) generates a hydrostatic pressure that forces the flexible waveguides against the borehole wall, ensuring a close fit to the irregular shape of the borehole walls. The attachment and installation can be performed using either a standard TDR technique or a water content reflectometer (e.g. model CS505, Campbell Scientific, Inc., Logan, UT), which is sampled using a micrologger. Laboratory calibration results from both probes indicate high sensitivity to water content changes and high correlation coefficients (r2 = .988 using 2nd order form with standard TDR probe, and r2 = .989 for linear curve with CS-505). The probe was then installed in boreholes at two locations. One was an agricultural field subjected to three episodes of flood irrigation, and the second was in an ephemeral wash in the Mojave Desert, where we were studying the stability of long-term measurements in an area without significant rainfall. Data from the first experiments showed that the flexible probes detected changes in water contents for each flood episode to a depth of 2.9 m, including the rise in a shallow water table elevation. The results of the second experiment shows only steady water content reading with minor water content changes as the experimental site is located in desert area and yet no significant rain event occurred since the probes were installed. The experimental results shows that the method is reliable and capable of providing accurate water content measurements in deep vadose zone horizons.