The Soil Freezing Characteristic: Its Measurement and Similarity to the Soil Moisture Characteristic

Abstract A soil freezing characteristic (SFC) represents the relationship between the quantity and the energy status of liquid water in frozen soil. The SFC is the analogue to the soil moisture characteristic (SMC) and is essential to modeling the transport of water, heat, and solutes in frozen soil. This paper presents a new, automated technique to measure an SFC in situ, for which there has previously been no method. Liquid water content in frozen soil was measured with time domain reflectometry. The corresponding energy status was inferred from accurate soil temperature measurements with a generalized form of the Clapeyron equation. Since both SFC and SMC describe water retention properties in soil, their similarity was investigated. The SMC and SFC agreed to within 1% moisture content across a wide range of matric potentials. Determination of the SMC is reliable at high matric potentials but becomes increasingly inaccurate and time consuming as soil dries. By contrast, the SFC determination becomes more accurate and rapid at lower matric potentials. We thus propose that water retention properties at high matric potentials are best obtained from draining and at low matric potentials from freezing.