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A Time Domain Reflectometry Coaxial Cell for Manipulation and Monitoring of Water Content and Electrical Conductivity in Variably Saturated Porous Media

^a Dep. of Plant, Soils, and Biometeorology, Utah State Univ., Logan, UT 84322-4820
^b Dep. of Civil and Environmental Engineering, Univ. of Connecticut, Storrs, CT 06269-2037

View larger version (64K): [in a new window]   Fig. 1. Cutaway schematic of time domain reflectometry (TDR) pressure cell showing components: (a) input/output port for P.M. solution exchange (coarse membrane), (b) temperature control port (circulating water bath), (c) input/output port for P.M. solution exchange (fine membrane), (d) 50 ohm coaxial cable, (e) porous stainless steel inner tube (0.5 um pore size), (f) solid stainless steel sleeve, and (g) porous stainless steel outer tube (40 µm pore size). Inset picture shows actual cell with cap removed.

View larger version (18K): [in a new window]   Fig. 2. Bulk permittivity (b) and electrical conductivity (ECb) measured in sand (0.18–0.295 mm) as a function of volumetric water content. Effluent mass provided water content determinations from draining cells oriented either horizontally (H) or vertically (V). The Topp et al. (1980) empirical model (Eq. [4]) represents measured data only at higher water content and more so in a vertically oriented cell. Deviations in expected b from Eq. [4] at reduced water content are attributed to gravitational water distribution (i.e., dependent on particle size, distribution, and sample thickness) and the resulting effects on TDR signal propagation.

View larger version (11K): [in a new window]   Fig. 3. Time domain reflectometry (TDR) measured bulk electrical conductivity (EC) (ECb) as a function of water content in sand for a solution ECw of 3 dS m–1. The Mualem and Friedman (1991) model (Eq. [2]) was plotted for comparison. The length of time allotted between changes in matric potential based on attainment of equilibrium conditions is also plotted as a function of water content.

View larger version (15K): [in a new window]   Fig. 4. Time domain reflectometry (TDR) measured bulk electrical conductivity (EC) (ECb) as a function of solution ECw, measured independently with an EC meter. Effluent ECe was also measured using the same meter and is compared to ECw with a 1:1 line. The saturated sand (0.18–0.295 mm) porosity was 0.45 and model prediction is from Eq. [2]. Inset displays time to equilibrium with changes in ECb.

View larger version (12K): [in a new window]   Fig. 5. Time domain reflectometry (TDR) ECb values for three different water contents (ECw = 3 dS m–1) as a function of temperature. Model predictions are given by combining Eq. [2] and [3].

View larger version (12K): [in a new window]   Fig. 6. Measured and modeled bulk permittivity of sand as a function of temperature at three different volumetric water contents. Modeled results employed a three-phase mixing model (Roth et al., 1990) assuming temperature-dependent water permittivity with solid- and air-phase permittivity values of 5 and 1, respectively. The parameter was set to 0.45.