Sensible heat balance method to determine rates of soil freezing and thawing.

摘要

Soil freezing and thawing can have a significant impact on the winter hydrology of soil, and soil ice content is an important component of the winter hydrological cycle. However, transient soil ice contents have been difficult to quantify under field conditions. A sensible heat balance (SHB) method using a sequence of heat pulse probes (HPP) has the potential to measure in-situ soil ice content. Therefore, the objectives of this dissertation are to develop, to test, and to improve the SHB method for determining soil ice content. A series of numerical, field, and laboratory studies were conducted.;The SHB method for soil ice content determination contains two important assumptions; negligible convective soil heat fluxes, and non-concurrent water evaporation and water fusion. A numerical study shows that these assumptions are acceptable for soil layers deeper than 12 mm, and the SHB method in theory can accurately estimate soil ice contents. However, when applied to an actual winter field, the in-situ soil ice contents determined with the SHB method are inaccurate. The inaccurate soil ice contents are associated with errors in soil thermal conductivity measured with the HPP. A sensitivity analysis shows that at least 20% accuracy of soil thermal conductivity is required to accurately estimate soil ice contents with the SHB method. In addition, the use of short time steps in the SHB method (e.g., 15 min) can improve soil ice content estimation.;A numerical study was performed to understand the sensitivity of HPP measurement needle temperature changes to properties of partially frozen soil. The measurement needle temperature changes were sensitive to soil thermal conductivity and freezing characteristics but not to volumetric heat capacity. The HPP may not be able to determine volumetric heat capacity accurately when soil temperature is between -2°C and 0°C. The sensitivity analysis suggests that soil thermal conductivity and soil freezing characteristics are the best candidate parameters for determination by inverse analysis. The inverse analysis may improve the accuracy of soil thermal conductivity and soil ice content determination with the SHB method.;A laboratory study was performed to further evaluate the SHB method for determining soil ice contents with soil column freezing and thawing. The SHB method accurately described the latent heat associated with soil freezing and thawing and provided reasonable transient soil ice content values at soil temperatures between -3°C and 0°C. The SHB method was not sensitive enough to determine soil ice content at temperatures below -3°C. Latent heat values are relatively large at temperatures between -3°C and 0°C but latent heat values are small at temperatures below -3°C. The soil ice contents during extended freezing periods at temperature below -3°C could be accurately estimated with change in volumetric heat capacity determined with the HPP. Thus, a combination of the SHB method and HPP volumetric heat capacity determination can be used to determine soil ice contents for a wide range of temperatures.;The SHB method is a new approach that has some advantages compared to other methods for estimating transient soil freezing and thawing. This dissertation presents fundamental information on the SHB approach and provides guidance for further development of the SHB method. (Abstract shortened by UMI.).