Temperature and water profiles during diurnal soil freezing and thawing: field measurements and simulation

Agricultural soils of the inland Pacific Northwest undergo frequent freezing and thawing during the winter and early spring. Repeated freezing and thawing of the soil surface may accelerate breakdown of soil aggregates, resulting in decreased water infiltration, and erosion resistance. The objective of this research was to simulate soil temperature and water distributions in a field soil during freezing and thawing. A finite difference numerical model was used to simulate soil temperature, depth of freezing, and water movement. Field measurements of soil temperature, ice and water content, and freezing depth in a bare surface treatment were used to validate the simulation model. The model couples heat and water flow equations by the change in soil ice content. An experimental relationship between unfrozen water content and degrees below freezing was used to estimate the change in soil ice content. Soil heat and water flux during 7 h of freezing followed by 12 h of thawing were simulated for two different days when the soil froze to a depth of 1.5 cm, Test 1, and 0.75 cm, Test 2. Simulated maximum frost depths were 1.4 and 0.8 cm for the respective tests. Simulated ice and water content of the 0- to 1-cm soil layer, at the time of maximum frost penetration was 0.46 and 0.41 cm3/cm3, which compares to measured values of 0.49 and 0.49 cm3/cm3 for the respective tests. Correlation of measured soil temperature and simulated soil temperature at the 1-cm depth was 0.99 for both tests. Standard deviation of the differences between measured and simulated temperature at the 1-cm depth was 1.04 and 0.29 degrees C for the respective tests. These results support the validity of this modeling approach for diurnal simulations of heat and water flux, near the surface, in freezing and thawing soil.