|Spaans, Egbert - UNIVERSITY OF MINNESOTA|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 30, 1995
Publication Date: N/A
Interpretive Summary: Soil water does not freeze completely at a single temperature as free water does. Instead, it freezes gradually, with the water in large pores freezing at higher temperatures than the water in small pores and the water absorbed in films. Thus there is a liquid phase present in frozen soil, with the relative proportions of water and ice depending on temperature and pore size distribution. This soil-specific functional relationship is known as the freezing characteristic (SFC). The SFC is important in predicting the movement of water and solutes during freezing and thawing and during spring snowmelt, but it has heretofore been extremely difficult to measure, particularly in the field. We developed a system, using time domain reflectometry and precision thermistors, that allows accurate in situ measurement of the SFC at multiple depths. In the process, we have shown that it is possible to use the information from SFC to estimate the dry portion of the soil moisture characteristic (the relationship between soil water content and soil water potential) which is critical for predicting water flow in unfrozen soil. This portion of the moisture characteristic has historically been very difficult to measure. Hence, the methodology developed in this project should be useful to hydrologists, agronomists, and others concerned with water flow in soil, because it allows simultaneous measurement of both the SFC and a substantial portion of the soil moisture characteristic.
Technical Abstract: The soil freezing characteristic (SFC) is 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 model the transport of water, heat, and solutes in frozen soil. 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 accurate soil temperature measurements using 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 well when ice was assumed at atmospheric pressure, and when water-ice interfacial forces could be neglected. Determination of the SMC is reliable at high matric potentials, but becomes increasingly inaccurate 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.