Micro‐Chilled‐Mirror Hygrometer for Measuring Water Potential in Relatively Dry and Partially Frozen Soils

Abstract

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

Effect of macropores on soil freezing and thawing with infiltration

“…Exposure of a ground surface to subzero temperatures leads to the formation of a frozen soil layer, in which gas, ice, and liquid water coexist (Dash et al, ). The amount of unfrozen water decreases with temperature, which can cause large negative water pressures (Schofield, ; Hohmann, ; Watanabe et al, ). Because negative pressure induces water flow from unfrozen soils at greater depths, the frozen soil layer tends to have a relatively high water content (Iwata et al, ; Tokumoto et al, ).…”

“…Soil water flow during the freezing of disturbed soils has been investigated thoroughly using laboratory experiments (Stähli and Stadler, ; Weigert and Schmidt, ) and numerical simulations (Harlan, ; Hansson et al, ). Although the infiltration of water into thawing soils has received less attention than redistribution of water in freezing soils, relationships between water content of the frozen layer and the timing and rate of infiltration have been investigated with field and laboratory observations (Gray et al, ; Watanabe et al, , ). However, ground is rarely uniform; it contains macropores, such as cracks, roots, and animal holes.…”

Matric Potential Sensor Using Dual Probe Heat Pulse Technique

“…Tensiometers have a limited measurement range from 0 kPa to -80 kPa and require frequent maintenance to refill their water (Hillel 1998). Only in-situ psychrometers measure matric potentials lower than -500 kPa (Watanabe et al 2012); however, their measurements are susceptible to the influence of temperature, and therefore, they cannot be used for subsurface soils experiencing dynamic temperature changes. Give that psychrometers measure the sum of matric potential and osmotic potential, they may overestimate the soil dryness when the soil contains solutes.…”

Discussion of the applicability of the generalized Clausius–Clapeyron equation and the frozen fringe process

“…Konrad (1989) supported the validity of the GCCE under high initial pore water pressure through measurements of the pore water pressure during soil freezing tests with applied backpressure. Watanabe et al (2012) demonstrated the validity of the GCCE in the equilibrium state by measuring the pore water pressure using a micro-chilled-mirror hygrometer. The second type of supporting experiment involves indirect measurements of heaving pressure.…”