FIGURE 2.5 The three conductance pathways for the ECa measurement. (Modified from Rhoades, J.D., Manteghi, N.A., Shouse, P.J., and Alves, W.J., Soil Sci. Soc. Am. J., 53, 433-439, 1989. With permission.

flow are illustrated in Figure 2.5. Rhoades et al. (1989) formulated an electrical conductance model that describes the three conductance pathways of ECa:

where Qws and 8wc are the volumetric soil water contents in the soil-water pathway (cm3 cm-3) and in the continuous liquid pathway (cm3 cm-3), respectively; 8ss and 8sc are the volumetric contents of the surface-conductance (cm3 cm-3) and indurated solid phases of the soil (cm3 cm-3), respectively; ECws and ECwc are the specific electrical conductivities of the soil-water pathway (dS m-1) and continuous-liquid pathway (dS m-1); and ECss and ECsc are the electrical conductivities of the surface-conductance (dS m-1) and indurated solid phases (dS m-1), respectively. Equation (2.16) was reformulated by Rhoades et al. (1989) into Equation (2.17):

(Qss +Qws )


(Qss ' ECws )

+ (Qws' ECs)

where 8w = 8ws + 8wc = total volumetric water content (cm3 cm-3), and 8sc- ECsc was assumed to be negligible. The following simplifying approximations are also known:

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