A multiple-rod TDR probe development with a view to measure water content and electrical conductivity(EC) of surface soil.

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A multiple-rod TDR probe was developed with a view to measure water content and electrical conductivity(EC) of surface soil. The probe consisted of several three-rod probes in which the outer rods of each three-rod probe were the common electrical earth guides for the adjacent probes. There was thus no extra space between the adjacent three-rod probes. This probe configuration enabled the TDR measurement to extend over a large area in the field. The characteristics of the multiple-rod probe were examined in sand in a container of 1. 1m×0. 81m×0. 20m. First, a one unit of probe consisting of three 1-m long steel rods of 0. 5 cm diameter with a spacing of 10 cm between the rods was tested for its detective performance for water and EC in sand. Then, a multiple-rod probe consisting of seven rods and with the same probe specifications was inserted horizontally in the sand of the container at 10 cm below the surface. The dielectric constant and EC of the sand were measured with during an evaporation test. Since TDR measurement depended on the air volume in the sampling volume of a probe, the unit probe test revealed the dependency of dielectric constant on the insertion depth of the probe. A cubic expression, such as the Topp' s one, provided a good relation between the average water content and dielectric constant of the surface soil. The insertion depth did not however affect the EC measurement. Moreover, the multiple-rod probe, through the evaporation test, accurately measured the change in water content and EC over 0. 88 m^2 in the surface region. Our results thus showed the possibility of extending the multiple-rod TDR measurement system to a larger area to make the remote sensing data more accurate.