Soil water determination by time domain reflectometry: Sampling domain and geometry.

Abstract

This work investigates several aspects of time domain reflectometry (TDR) theory and application. One of these aspects is the study of the influence of TDR probe geometries on the travel time. No change in the travel time resulted from increasing either the diameter of wire or spacing. However, we found a linear relationship between the travel time and the length of the probe for measurements in water. Also we found the reflected voltage was inversely proportionally to the incident voltage in water. Another aspect is the volume of sensitivity for the TDR which depends on the electrical properties of the medium and the geometry of the probe. The sensitivity of TDR in soil is different than in water. The observations in soils indicate that soil with a high water content (θᵥ) has a smaller sample volume than the one with low θᵥ. A probe with a large wire diameter has a larger sample volume than a probe with a small wire diameter. Also, a simple model and a mixing model were investigated and compared to Topp's model, for relating θᵥ to the effective dielectric constant. The distance to wetting front over time was observed and calculated using an expression which relates the travel time in soil before and after water application. This was tested with probes of different geometries. The wetting front from a point source were monitored for two and three dimensions in a plexiglas tank using TDR. Contour maps for the calculated radius of wetting front vs. the depth over time were produced.