Core Ideas Unfrozen water content in frozen soils with flowing water is higher than in no‐flow cases. Unfrozen water content with flowing water is higher than the thermodynamic prediction. Frozen and unfrozen soils with the same water content have the same hydraulic conductivity. Understanding the hydraulic conductivity of frozen soils near melting temperature is important in agricultural management and water balance calculation in cold regions and in the use of artificial ground freezing techniques. However, measurement of the hydraulic conductivity of frozen soils has been limited. Therefore, it is often estimated from the unsaturated hydraulic conductivity of unfrozen soil using temperature and the Clausius–Clapeyron equation, with inadequate validation. In this study, we simultaneously measured the unfrozen water content and hydraulic conductivity of three frozen soils by passing water through them and compared the results with water characteristic curves and unsaturated hydraulic conductivities of the unfrozen soils determined using the evaporation method. Using measured temperature data and water characteristic curves, the Clausius–Clapeyron equation underestimated unfrozen water contents in the frozen soils, which also resulted in underestimation of hydraulic conductivities, particularly near melting temperature. However, the unfrozen and frozen soils had similar hydraulic conductivities when their liquid water contents were the same. The hydraulic conductivity of frozen soil should be estimated from that of unfrozen soil based on the liquid (unfrozen) water content instead of the temperature.

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Estimating water potential in a relatively dry soil is fundamentally important, not only for predicting soil water flows in arid areas or during the evaporation process, but also for understanding soil freezing processes. A micro-chilledmirror hygrometer (FINEDEW) that does not require a sampling chamber and has a rapid response time has recently been developed. The sensor head can be inserted into a soil sample. We confirmed that FINEDEW directly and quickly measures soil water potential in the range of less than -500 kPa in soil under near-equilibrium conditions and in the range of less than -1000 kPa in soil under evaporation at room temperature. The FINEDEW hygrometer was also applied to frozen soil at temperatures between -20 and -0.5°C. At equilibrium, regardless of soil type and freezing-thawing processes, the measured potential corresponded to the calculated potential determined using the Clausius-Clapeyron equation. Soil water potential was found to require time to reach equilibrium after a temperature change. This is thought to be because ice in the soil pores required time to acquire a new equilibrium geometry. E stimating water potential in relatively dry soil is fundamentally important for predicting soil water flows in arid areas or during the evaporation process and for obtaining accurate hydraulic conductivity models (Sakai and Toride, 2009). In frozen soil, some water remains in the liquid state at subzero temperatures, and the amount of unfrozen water decreases with temperature in a similar manner as for drying (Williams and Smith, 1989). The unfrozen water contributes to water and nutrient redistribution during freezing and thawing processes (Baker and Spaans, 1997) and may cause frost heaving (Rempel, 2010;Liu et al., 2012). To understand hydrologie and thermal soil processes in cold regions, it is important to accurately estimate the unfrozen water potential in soils under a range of temperatures.To measure soil water characteristics in a relatively dry range, soil water potential is often estimated as the corresponding centrifugal force or from soils equilibrated over salt solutions. These estimates, however, include potential errors caused by changes in bulk density, considerable pressure differences arising from slight temperature differences between soil and solution, or the prolonged time required to reach equilibrium (Campbell and Gee, 1986). The water potential in dry soil can also be estimated using a psychrometer (Andraski and Scanlon, 2002), which measures the relative humidity of water vapor in equilibrium with the soil water. The recent development of chilled-mirror hygrometers (e.g., the 'WP4 dew-point potentiometer by Decagon Devices) has enabled quicker and easier estimations of water potential in dry soil and increased measurement accuracy compared with conventional psychrometers (Kirkham, 2005, p. 249-256).Centrifugal tests and methods of equilibration over salt solution cannot be used for in situ measurements of soil water potential. Most currently available