The simultaneous measurement of water content and electrical conductivity of soils and KCl solutions was achieved using time domain reflectometry (TDR). Coaxial transmission lines varying in length from 90 to 300 mm contained either KCl solutions or soil of varied water and salt content. The water content of soil or dielectric constant of the water solutions was determined from the travel time. The measured dielectric constant of KCl solutions was unchanged from that of pure water (81) at those concentrations where there was sufficient reflected signal for measurement. Two analyses were used for determination of electrical conductivity, one based on signal attenuation after one “round‐trip” and the second based on a thin sample approximation for the signal reflection and attenuation. Reference measurements of conductivity were made on the same samples using low‐frequency conductance bridge measurements. These analyses of the TDR traces showed that for water solution both the thin sample analysis and the analysis after a signal had traversed one round‐trip yielded conductivity in agreement with bridge conductivity values. This indicated that the imaginary part of the complex dielectric constant was negligible. For soils the thin sample analysis was in general agreement with the bridge measurements. From the analysis of signal after one round‐trip in soils there was indication that the imaginary part of the dielectric constant should not be assumed negligible. Further investigation of the frequency dependence of the dielectric constant and attenuation will be required to identify the relative contributions of the real and imaginary parts of the dielectric constant to measurement by TDR. The effect of impedance‐matching transformers on conductivity measurements in the field has yet to be ascertained.

measurement of soil water content using a portable TDR hand probe. Can. J.Soil Sci. 64:313-321.The time-domain reflectrometry (TDR) technique had previously been shown to measure the volumetric water content of soil accurately when applied to long-term installations of parallel transmission lines. In this study a hand probe was used with a portable TDR instrument to measure water content of soil down the wall of soils pits to a depth greater than I m. In a separate experiment the water content of the surface soil at two sites was measured repeatedly in increasing depth increments of 50 (Fig. l) On the plastic holder above the point of TDR connection was a region for grasping the probe. Above that was a block which allowed the use of force, such as hammering, to insert the probe into hard soil (Fig. 1). Delrin was used because of its ability to withstand hammering without shattering or distortion.The balun consisted of an RF pulse transformer mounted on a fiberglass board. This allowed connection of the unbalanced coaxial cable from the TDR instrument to one side of the transformer and the balanced output side was fed to banana plugs mounted on the edge of the board. These plugged into the sockets on the hand probe assembly after it had been inserted into the soil.The determination of water content involved four steps (three of which are depicted in Fig. 2): (1) inserting the probe as required by the experiments described below; (2) plugging in the balun, turning on and setting the TDR to get the TDR trace on the oscilloscope; (3) taking a po-

No abstract

An improved technique for measuring soil water desorption curves of a large number of soil cores was tested. A procedure for improving contact between the soil and a "tension medium" resulted in rapid extraction of water from 7.6 × 7.6-cm cores at pressure heads from 0 to −500 cm of water. The data for clay and sandy loam cores showed that equilibrium was reached in less than 200 h at all pressure heads. The "tension medium," used essentially as a large porous plate, was carefully chosen with a narrow pore size distribution. This provided a high hydraulic conductivity and high air-entry values, both necessary for efficient desorption over the pressure head range 0 to −500 cm of water. A tensiometer-pressure transducer combination for establishing equilibration time proved more reliable than the traditional weight-loss criterion. The consistency and reproducibility of desorption curves was demonstrated using data for hysteretic loops as well as standard deviations of water contents at each pressure head on similar soils.

Infiltration, drainage, and chemical leaching are strongly influenced by the magnitude and spatial distribution of the field‐saturated soil hydraulic conductivity (Kfs). The Guelph permeameter (GP) method shows promise as an effective means for field measurement of Kfs and its spatial distribution, but its accuracy in medium‐ and fine‐textured soils is not well established. To further assess its accuracy and effectiveness, the GP method was compared with the auger hole (AH) method at the 0.5‐m depth at 68 grid locations in a texturally uniform silty clay soil that had stable but spatially variable structure. The two methods yielded similar geometric mean Kfs values (P < 0.001), as well as similar semivariograms. The two methods were also positively correlated (r = + 0.6565, P < 0.0001). We therefore concluded that the two methods gave equivalent estimates of Kfs at this field site, and that the GP method is capable of providing valid estimates of Kfs in at least some fine‐textured soils. The Kfs values were not correlated with soil texture, organic C content, or soil surface topography, but were negatively correlated (r = −0.7240 for GP method, r = −0.6070 for AH method, P < 0.0001) with antecedent volumetric water content (θa) measured in situ prior to the GP measurements using a down‐hole time domain reflectometry probe. The semivariogram for θa was similar to those for Kfs. These results suggest that the magnitude, range, and pattern of variability of the Kfs measurements were controlled primarily by the well‐developed and stable soil structure at the field site, rather than by texture, organic C content, or surface topography.