Graduate Thesis Or Dissertation
 

Simulation and field validation of heat and water flow during soil freezing and thawing

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  • Freeze/thaw cycles are numerous in agricultural soils of the Pacific Northwest. Potential loss of soil by erosion is high whenever a soil thaws from the surface creating a saturated soil overlying an impermeable frozen layer. Field studies were conducted to investigate the effect of surface residue on soil freezing. Residue treatments were: 1) bare surface, B; 2) standing stubble with no straw mulch between the rows, SSNM; and 3) standing stubble with straw mulch between the rows, SSM. Water migration to the freezing surface and freezing depth were partially controlled through the mulch cover which thermally insulated the soil. The SSM and SSNM treatments reduced the depth of frost penetration by an average of 35% compared to the B treatment, even in the absence of snow cover. During one diurnal freeze/thaw cycle the soil froze to 1.5 cm on the B treatment; the soil on the SSM treatment did not freeze. Water content of the 0- to 0.5-cm layer increased from 0.31 at 1800 hours to 0.57 cm³ /cm³ at 0600 hours on the B treatment. On the unfrozen SSM treatment, water content increased from 0.34 to 0.40 cm³ /cm³ during this same time. Soil heat and water flux during a 19-hour freezing and thawing cycle was numerically simulated using finite differences. The model couples the heat and water flux equations at sub-zero temperatures by the change in soil ice content. An experimental relationship between unfrozen water and sub-zero temperature was used to couple the heat and water flow equations and estimate the change in soil ice content. Field measurements of soil temperature, water content and frost depth were used to validate the model. Measured versus simulated soil temperature at the 1-cm depth resulted in an r² = .995. Standard error of the estimate was .63 °C. Simulated ice and water content of the 0- to 1-cm soil layer, at the time of maximum frost penetration, was .48 g/g, which compares to a measured value of .44 g/g. Simulated maximum frost depth was 2 cm, which compares to a measured depth of 1.5 cm. These results validate the usefulness of this model in simulation studies and also support the methods used to obtain the heat and water transport coefficients for this field soil.
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