Using time domain reflectometry in stony forest soil

Forest soils often contain many large coarse fragments making it difficult to insert probes to measure soil water content. The ability of time domain reflectometry (TDR) to give reliable measurements of water content in soil with up to 40% coarse fragments was evaluated at a site in the southern interior of British Columbia, Canada. A commercial time domain reflectometer was used with 0.3-, 0.5- and 0.75-m-long probes to measure soil water content of the profile and layers within the profile. A probe had a shorting diode at the surface and two 3-mm-diameter stainless steel rods inserted vertically, 30 mm apart, as the waveguide. Diverging rods or profile discontinuities resulted in erroneous readings that required a review of the recorded signals and recalculation the travel time. Soil physical and hydrologic soil properties were determined and the soil calibrated for TDR. An accuracy of +/-0.02 m(3) m(-3) was obtained with measurement of soil bulk density and minimizing probe and travel time errors. Variation in water content between probes reflected the variability in coarse fragment content; however, the ranking of the probes stayed constant with time and rates of change were similar between probes. One standard deviation on the measured change in the volume of water between measurement days for the 0 to 0.5 m depth was +/-6 mm (n = 20), equivalent to 0.012 m(3) m(-3). Measurements of water content of the layers had one standard deviation of 0.02 m(3) m(-3).