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Dynamic measurement of hydrologic characteristics of a layered soi l during drainage, evaporation and subsequent wetting Chow, Thien Lien

Abstract

Instrumentation and methodology were developed and used for dynamic measurement of hydrologic characteristics of an undisturbed layered soil column over the soil water matric potential range between -0.1 and -90 bars. This instrumentation included the gamma radiation attenuation technique to measure water content and the tensiometer-pressure transducer and the three-terminal double loop thermocouple psychrometer techniques to measure water potential. The thermal stability of this psychrometer is approximately 40 times better than that of the two-terminal psychrometer (Spanner type) for ambient temperature fluctuations with a time rate of change greater than 0.2°C/min. Water potential measurements carried out with this psychrometer on silty clay and silt loam soil samples were within ± 0.4 bar of the porous plate extractor equilibrium values. Water content and the corresponding water potential of an undisturbed layered soil were measured simultaneously with this instrumentation as a continuous function of time and depth for the soil water matric potential between -0.1 and -90 bars. These measurements were carried out during water flow induced by drainage and evaporation, followed by rewetting. Water retention and flow characteristics were then calculated from these measurements. The same properties were also determined for the top layer separately. None of these properties showed system dependence. Both water retention, w(ψ[sub M]), and unsaturated hydraulic conductivity-matric potential function, K(ψ[sub M]), showed hysteresis effects at soil water matric potential (ψ[sub M]) between -0.1 and -3.0 bars, w(ψ[sub M]) showed no hysteresis at ψ[sub M]> -3.0 bars, whereas K(ψ[sub M]) showed inconclusive hysteresis in this range. The unsaturated hydraulic conductivity-water content function, K(w), and diffusivity-water content function, D(w), displayed inconclusive hysteresis. Propagated errors in both unsaturated hydraulic conductivity and diffusivity were estimated to be ± 7.8 to ± 20.0% and ± 8.1 to ± 38.2% respectively.

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