The Use Of Time Domain Reflectometry For Vertically Profiling The Water Content Through The Unsaturated Zone

Abstract

Standard, continuous-rod time domain reflectometry (TDR) probes, installed vertically at the ground surface, measure the average water content from the ground surface to the ends of the rods. Alternative probes have been described that allow for vertical profiling of the water content beneath a single surface point. However, no single probe design is optimal for all measurement needs Based on analyses of the properties of published TDR probe designs, we make suggestions regarding the conditions for which several probes are best suited. We suggest the use of two-rod, standard probes wherever possible. Often, interval differencing of the responses of a series of vertically installed probes of different lengths can provide an approximate measure of the water content profile in a relatively homogeneous medium. Horizontally installed standard probes can be used to improve the resolution of the water content profile if the disturbance caused by their placement is acceptable. Two surface probe designs have been presented that can measure the water content very near the ground surface. Two access tube designs have been presented that can measure the water content over several preselected depth intervals. However, probes with coated rods and probes that place the rods entirely within an access tube will underestimate the water content if the water content varies along the rods. In addition, the sample volume of these probes will vary with the water content of the medium, becoming restricted to a region immediately adjacent to the coatings or access tubes in higher water content conditions. INTRODUCTION Time domain reflectometry (TDR) is a widely accepted method of measuring the water content of soils both in the laboratory and in the field. Standard TDR probes, composed of two or three parallel metal rods, are inexpensive, simple to construct, and easy to install in the field. The correlation between the relative dielectric permittivity measured with these probes and the average volumetric water content over the length of the rods is described well by a single relationship for a wide range of soils [Topp, 19801. This property allows for simple, efficient reconnaissance mapping of the water content in plan view in the field with accuracies within a few percent water content. Only studies that require highly accurate water content measurements or measurements made in the presence of ice or in high clay content soils require site-specific calibration [Stein and Kane, 19831. Despite the usefulness of standard, vertically installed TDR probes for describing the spatial distribution of the water content in plan view, there are limits to the application of these probes. Continuous-rod probes measure the average water content from the 1 ground surface to the ends of the rods. Therefore, these probes are ideally suited to monitoring the average water content with high temporal resolution. However, these / probes cannot determine the water content distribution along their length, limiting the