Time domain reflectometry field measurements of soil water content and electrical conductivity

Simultaneous measurements of volumetric water content and bulk electrical conductivity were made using time domain reflectometry (TDR) with a single parallel transmission line (PTL) imbedded in the soil. Sixty PTL's were installed at five depths in 12 existing lysimeter plots, irrigated with different amounts of water at two salt concentrations (1.3 and 3.1 dS/m). The water content measurements obtained with TDR showed a good relationship to gravimetric determinations (r = 0.84) and were comparable to neutron probe measurements. The TDR measurements of bulk soil electrical conductivity were similar to the measurements of the same soil physical property measured with the four-probe electrode technique. The relation between bulk soil electrical conductivity and the conductivity of the soil solution is discussed. The importance of measuring both soil water content and salinity in a nondestructive manner on the same sampling volume, enabling repeated in situ measurements is stressed. Additional Index Words: soil salinity, neutron scattering, spatial variability, soil solution conductivity, four-electrode probe. Dasberg, S. , and F.N. Dalton. 1985. Time domain reflectometry field measurements of soil water content and electrical conductivity. Soil Sci. Soc. Am. J. 49:293-297. T CONVENTIONAL METHOD for measuring soil salinity is by taking soil samples and determining the electrical conductivity of the extract of a saturated soil paste (Rhpades, 1982). These measurements can be converted into soil solution salt concentration by correcting for the soil water content at the time of sampling. The soil solution can be sampled directly by porous suction cups. This method, however, is limited to a narrow range of soil moisture between (approximately) field capacity and saturation and the small sample volume tends to make the measurements variable (Broadbent, 1981). Soil water content can be measured by destructive sampling and gravimetric determination or by the in situ neutron scattering method which has the advantage of measuring soil water content on a volume basis. The limitations 1 Contribution from the U.S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. 2 Visiting Soil Scientist and Soil Physicist. The permanent address of the first author is Institute of Soils and Water, ARO, The Volcani Center, Bet Dagan 50-250, Israel. of the neutron method are its relatively large sampling volume, the inability to measure close to the soil surface, the radiation hazard involved, and the need for individual soil calibration (Graecen, 1981). The fact that water content and salinity are usually obtained from separate samples with different geometry introduces an additional error in soil salinity assessment. Recently, Dalton et al. (1984) proposed the simultaneous measurement of both soil water content and salinity for studies on water and salt management, using time domain reflectometry (TDR). The relative dielectric constant of soil is primarily related to its water content. Measurement of the dielectric constant in the time domain, by measuring the propagation velocity of a voltage pulse was introduced by FellnerFeldegg (1969). Topp et al. (1980) showed a unique relationship between the relative dielectric constant« and the volumetric water content 6 for a large range of soils and soil-like materials. In later work, parallel transmission lines (PTL) constructed of metal rods were used to measure water content in the field with TDR (Topp et al., 1982). The relative dielectric constant t is calculated from the measured transit time / of a voltage pulse through the length C of soil material as given by the electrode length according to = [ct/29] [1] where c = velocity of light in free space (3 X 10 m/s). The empirical relationship between relative dielectric constant e and soil volumetric water content 6 was found to be (Topp et al., 1980) 0 = -5.3 X 1CT + 2.92 X I0~e 5.5 X 10-V + 4.3 X 10~ [2] It was found that this relationship was nearly independent of soil texture, soil density, temperature and salt content. It was noted, however, that soil salinity did influence the TDR signal in the soil medium (Topp et al., 1980). Dalton et al. (1984) in attempting to account for the signal attenuation which occurs in saline soils, used results from transmission line theory to calculate the electrical conductivity of moist soil. Figure 1 shows the assumed equivalent circuit of a differential length of the parallel transmission line. L and R are the inductance and series resistance of the electrodes. C and G are the equivalent shunting capacitance Published March, 1985