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

Standard, continuous-rod time domain reflectometry (TDR) probes, installed<br>vertically at the ground surface, measure the average water content from the ground<br>surface to the ends of the rods. Alternative probes have been described that allow for<br>vertical profiling of the water content beneath a single surface point. However, no single<br>probe design is optimal for all measurement needs Based on analyses of the properties of<br>published TDR probe designs, we make suggestions regarding the conditions for which<br>several probes are best suited. We suggest the use of two-rod, standard probes wherever<br>possible. Often, interval differencing of the responses of a series of vertically installed<br>probes of different lengths can provide an approximate measure of the water content<br>profile in a relatively homogeneous medium. Horizontally installed standard probes can be<br>used to improve the resolution of the water content profile if the disturbance caused by<br>their placement is acceptable. Two surface probe designs have been presented that can<br>measure the water content very near the ground surface. Two access tube designs have<br>been presented that can measure the water content over several preselected depth<br>intervals. However, probes with coated rods and probes that place the rods entirely within<br>an access tube will underestimate the water content if the water content varies along the<br>rods. In addition, the sample volume of these probes will vary with the water content of<br>the medium, becoming restricted to a region immediately adjacent to the coatings or<br>access tubes in higher water content conditions.