Hydraulic Potential in Unfrozen Soil in Response to Diurnal Freezing and Thawing of the Soil Surface

FREEZING of soil water during nocturnal radiation frosts influences the field soil moisture regime by providing a moisture sink which attracts water from the sub-soil to the freezing front. Water movement and associated hydraulic potentials in soils during nocturnal soil freezing and daytime thawing cycles are not readily managed unless they can be measured in relation to soil and crop residue treatments that may affect heat transfer at the soil surface. Ceramic cup tensiometers at nine depths in the top 60 cm of a field soil were used to measure hydraulic potential response of unfrozen soil to daily freezing and thawing of the soil surface at hour intervals. Maximum soil frost penetration was 1.6 cm in the bare surface plot; soil frost did not form under a standing stubble surface cover. Hydraulic potential response to surface freezing was greatest at 2 cm in the bare surface plot. During one freezing night, volumetric water content at 0.25 cm increased from 0.31 at 1800 hours to 0.57 mVm3 at 0600 hours in the bare surface plot. In the unfrozen stubble-covered plot, water content increased from 0.34 to 0.4 mVm3 during this same time.