The soil freezing characteristic: Its measurement and similarity to the soil moisture characteristic

Soils retain water by matric forces, which lower the energy status of the soil water (or matric potential) with respect to free water. The relationship between the amount of soil moisture and the matric potential is the soil moisture characteristic (SMC). The SMC is a soil specific property crucial to many biological and engineering investigations dealing with unsaturated soil. The same matric forces also allow liquid water to coexist with ice in frozen soil, since the reduced matric potential depresses its freezing point. The relationship between the liquid water content in frozen soil and its energy status is called the soil freezing characteristic (SFC). The SFC is essential to model transport of water, heat, and solutes in frozen soils. Since the SMC and SFC both describe water retention properties in soil, their similarity was investigated.; There has previously been no satisfactory method to measure the SFC in situ. A new, automated technique was developed to measure the SFC in situ. Liquid water content in frozen soil was measured with time domain reflectometry (TDR). The corresponding energy status was inferred from the soil temperature using a generalized form of the Clapeyron equation. A new procedure was developed to calibrate TDR for liquid water content in frozen soil, by means of a gas-dilatometer. A gas-dilatometer is a closed container with a soil sample inside, which is subjected to freezing and thawing cycles. The liquid water content is calculated from changes in air pressure inside the gas-dilatometer, which result from the expansion of water as it freezes.; The SFC and SMC agreed well when ice was assumed at atmospheric pressure, and when water-ice interfacial forces were neglected. Determination of the SMC is reliable at high matric potentials, but becomes increasingly inaccurate and time consuming as soil dries. In contrast, the SFC determination becomes more accurate and rapid at lower matric potentials, when thermal equilibrium in freezing soil is established more rapidly than hydraulic equilibrium in drying soil. I propose that water retention properties at high matric potentials are best obtained from draining, and at low matric potentials from freezing.