When using the time domain reflectometry (TDR) technique in laboratory experiments, e.g., with packed soils columns, it is often of great importance to obtain high depth resolution with minimal disturbance of the soil and to be able to measure close to the soil surface. This requires the use of fairly small TDR probes that can be placed near each other. In laboratory experiments on packed soil, we have examined the importance of relations between the probe-rod diameter, probe-rod length, distance between probe rods, and distance from the probe to the soil surface for accurate determination of volumetric water content. The experiments were conducted on a coarse sand with three different rod diameters (1, 2, and 3 mm), three different rod spacings (10, 20, and 50 mm), two rod lengths (50 and 150 mm), and at distances to the soil surface varying from 5 to 50 mm. Theoretical work by Knight et al. (1994, Symposium and workshop on time domain reflectometry in environmental, infrastructure and mining applications, North-western Univ., Evanston, Illinois, Sept 7-9, 1994, pp. 93-104) has been used to evaluate the results. In general, theory and measurements agreed very well. Both measurements and theory showed that the volume of soil contributing to the measurement is highly dependent on the spacing of the rods and, to a lesser degree, on the rod diameter. For rod spacings of 10, 20, and 50 mm, measurements were accurately made as close to the soil surface as 10, 15, and 20 mm, respectively.
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