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Calibration of Capacitance Probe Sensors in a Saline Silty Clay Soil

^a USDA-ARS, George E. Brown, Jr. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507
^b USDA-ARS, Water Management Res. Lab., 9611 S. Riverbend Ave., Parlier, CA 93648
^c Alterra-ILRI, P.O. Box 47, 6700 AA Wageningen, The Netherlands

View larger version (26K): [in a new window]   Fig. 1. Relative permittivity as a function of volumetric water content for the empirical models of Topp et al. (1980) and Malicki et al. (1996), and the dielectric mixing models of Dobson et al. (1985), Birchak et al. (1974) (phases in series and in parallel), and Friedman (1998) (solids, water, air configuration and air, solids, water configuration).

View larger version (27K): [in a new window]   Fig. 2. Scaled frequency as a function of volumetric water content as measured in the field (dots) and as calculated by combining the circuit model for nonlossy media with five models that describe the relative permittivity as a function of soil water content (lines). The model of Birchak et al. (1974) is given for phases in series and for phases in parallel. The model of Friedman (1998) is given for the solids, water, air configuration, and the air, solids, water configuration.

View larger version (15K): [in a new window]   Fig. 3. Measured versus calculated bulk electrical conductivity (EC) at 25°C using the tortuosity factor of Rhoades et al. (1976) (Eq. [16], open circles) and using the tortuosity factor derived from the gas diffusion model of Marshall (1959) (Eq. [17], filled circles).

View larger version (22K): [in a new window]   Fig. 4. Scaled frequency as a function of volumetric water content as measured in the field (dots) and as calculated with a combination of the electric circuit model for lossy media and the empirical () relationship of Malicki et al. (1996) (lines). Calculations assume no dielectric losses and w = 30, 60, and 90 dS m–1 combined with s = 60 dS m–1. The tortuosity factor of Rhoades et al. (1976) and the tortuosity factor derived from the gas diffusion model of Marshall (1959) are used to calculate bulk electrical conductivity.

View larger version (19K): [in a new window]   Fig. 5. Scaled frequency as a function of volumetric water content as measured in the field (dots) and as calculated with a combination of the electric circuit model for lossy media and the empirical () relationship of Malicki et al. (1996) (lines). Calculations assume no dielectric losses and w = 30, 60, and 90 dS m–1 combined with s = 60 dS m–1. Exponent c = 1.25 is used in the calculation of the tortuosity factor derived from Marshall (1959).

View larger version (16K): [in a new window]   Fig. 6. Measured versus calculated volumetric water content. Calculated values are obtained by combining the circuit model for lossy media with the empirical () relationship of Malicki et al. (1996). Exponent c = 1.21 is used in the calculation of the tortuosity factor derived from Marshall (1959).

View larger version (16K): [in a new window]   Fig. 7. Measured versus calculated volumetric water content. Calculated values are obtained with the factory calibration for the EnviroSCAN sensors.