THEORY AND SYSTEM OF AUTOMATIC DETERMINATION OF SOIL MOISTURE CHARACTERISTICS AND UNSATURATED HYDRAULIC CONDUCTIVITIES

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Abstract

The moisture characteristic and the unsaturated hydraulic conductivity are calculated from the measured water pressures at different depths in a soil core and the weights during evaporation at the top of the core. The measured water pressure is assumed to represent the mean pressure in a layer of a certain thickness, and the moisture characteristic is assumed to be described with sufficient accuracy by a polynom. The tangent of the polynom in each pressure interval is solved from a set of equations.

Assuming the hydraulic conductivity-water pressure relationship can be described by an exponential relationship with varying constants in the different pressure ranges, the constants can be solved from measured water pressure at two depths and three sequential moments, and the rate of change in water content below some defined depth in the core during two sequential time periods.

Up to ten cores can be handled at the same time. Measuring at a core can be started and stopped at any time. The water pressure at different depths is measured with one pressure transducer, connected with a scannivalve; the weight is measured with a straingauge load cell; and the data are recorded on a magnetic cassette tape. The recorded data are processed by a computer and stored on a magnetic disc.

If the saturated conductivity is relatively high, a steady-state infiltration method is applied to determine the unsaturated hydraulic conductivity at high water contents.

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