ALBERT

Description: Soil water evaporation is an important component of the surface water balance and the surface energy balance. Accurate and dynamic measurements of soil water evaporation enhance the understanding of water and energy partitioning at the land-atmosphere interface. The objective of this study was to measure the cumulative soil water evaporation with time and depth in a bare field. Cumulative water evaporation at the soil surface was measured by the Bowen ratio method. Subsurface cumulative soil water evaporation was determined with the heat pulse method at fine-scale depth increments. Following rainfall, the subsurface cumulative evaporation curves followed a pattern similar to the surface cumulative evaporation curve, with approximately a 2-d lag before evaporation was indicated at the 3- and 9-mm soil depths, and several more days' delay in deeper soil layers. For a 21-d period in 2007, the cumulative evaporation totals at soil depths of 0, 3, 9, 15, and 21 mm were 60, 44, 29, 13, and 8 mm, respectively. For a 16-d period in 2008, the cumulative evaporation totals at soil depths of 0, 3, 9, 15, and 21 mm were 32, 25, 16, 10, and 5 mm, respectively. Cumulative evaporation results from the Bowen ratio and heat pulse methods indicated a consistent dynamic pattern for surface and subsurface water evaporation with both time and depth. These findings suggest that heat pulse sensors can accurately measure subsurface soil water evaporation during several wetting-drying cycles.

Description: In situ determination of soil freezing and thawing is difficult despite its importance for many environmental processes. A sensible heat balance (SHB) method using a sequence of heat pulse probes has been shown to accurately measure water evaporation in subsurface soil, and it has the potential to measure soil freezing and thawing. Determination of soil freezing and thawing may be more challenging than evaporation, however, because the latent heat of fusion is smaller than the latent heat of vaporization. Furthermore, convective heat flow associated with liquid water flow and occurrence of evaporation or condensation during freezing and thawing may cause inaccurate estimation of freezing and thawing with the SHB method. The objective of this study was to examine the applicability of the SHB concept to soil freezing and thawing. Soil freezing and thawing events were simulated with the simultaneous heat and water (SHAW) model. Ice contents were estimated by applying the SHB concept to numerical data produced by the SHAW model. Close agreement between the SHB-estimated and the SHAW-simulated ice contents were observed at depths below 24 mm. The main cause of inaccuracies with the SHB method was poor estimation of heat conduction at the 12-mm depth, possibly due to simplifications of temporal or vertical distributions of temperature and thermal conductivity. The effects of convective heat flow and concurrent evaporation or condensation and freezing or thawing on the SHB method were small. The results indicate that the SHB method is conceptually suitable for estimating soil freezing and thawing. Independent, accurate estimates of thermal properties must be available to effectively use the SHB method to determine in situ soil freezing and thawing.

Description: Soil ice content impacts winter vadose zone hydrology. It may be possible to estimate changes in soil ice content with a sensible heat balance (SHB) method, using measurements from heat pulse (HP) sensors. Feasibility of the SHB method is unknown because of difficulties in measuring soil thermal properties in partially frozen soils. The objectives of this study were (i) to examine the SHB method for determining in situ ice content, and (ii) to evaluate the required accuracy of HP sensors for use in the SHB method. Heat pulse sensors were installed in a bare field to measure soil temperatures and thermal properties during freezing and thawing events. In situ soil ice contents were determined at 60-min intervals with SHB theory. Sensitivity of the SHB method to temperature, heat capacity, thermal conductivity, and time step size was analyzed based on numerically produced soil freezing and thawing events. The in situ ice contents determined with the SHB method were sometimes unrealistically large or even negative. Thermal conductivity accuracy and time step size were the key factors contributing to SHB errors, while temperature and heat capacity accuracy had less influence. Ice content estimated with a 15-min SHB time step was more accurate than that estimated with a 60-min time step. Sensitivity analysis indicated that measurement errors in soil temperature and thermal conductivity should be less than ±0.05°C and ±20%, respectively, but the error in the soil heat capacity could vary by ±50%. Thus, improving the accuracy of thermal conductivity measurements and using short time steps are required to accurately estimate soil ice contents with the SHB method.

Description: Soil ice content is an important component for winter soil hydrology. The sensible heat balance (SHB) method using measurements from heat pulse probes (HPPs) is a possible way to determine transient soil ice content. In a previous study, in situ soil ice content estimates with the SHB method were inaccurate, due to thermal conductivity errors and the use of relatively long time steps for calculations. The objective of this study is to reexamine the SHB method for soil ice content determination. A soil freezing and thawing laboratory experiment was performed with soil columns and heat exchangers. Transient soil ice contents in the soil columns during soil freezing and thawing were determined with the SHB method. The SHB method was able to determine dynamic changes in soil ice contents during initial freezing and final thawing for soil temperatures between –5 and 0°C when latent heat values associated with ice formation or with thawing were relatively large. During an extended freezing period, when soil temperatures were below –5°C, the small associated latent heat fluxes were below the sensitivity of the SHB method, and the SHB method did not provide accurate estimates of ice contents with time. However, the soil ice contents during the extended freezing period could be estimated well from changes in volumetric heat capacity ( C ) determined with HPP. Thus, combining the SHB method for initial freezing and final thawing, with a change in C method for extended freezing periods, allowed determination of dynamic soil ice contents for the entire range of freezing and thawing soil temperatures investigated. HPPs were able to measure soil ice contents.

Description: Determining soil ice content during freezing and thawing is important and challenging for both engineering and environmental issues. The thermo-time domain reflectometry (T-TDR) probe, which can monitor unfrozen soil water content and soil thermal properties simultaneously, has the potential to measure ice content in partially frozen soils. The objective of this study was to identify an optimum heat application strategy for measuring soil thermal properties with T-TDR probes in partially frozen soil while minimizing ice melting during the process. The optimized heating schemes were then applied for monitoring soil ice content dynamics during freezing and thawing. The results indicated that the heat pulse method failed at temperatures between –5 and 0°C because of temperature field disturbances from latent heat of fusion. When soil temperatures were ≤ –5°C, ice melting during heat pulse applications could be limited effectively with a combination of 60-s heat-pulse duration and 450 J m –1 heating strength, or a 90-s heat-pulse duration and heating strength of 450 to 900 J m –1 . With the optimized heating scheme, T-TDR probes were able to measure soil ice content changes at ≤ –5°C during freezing and thawing, and the errors were within ±0.05 m 3 m –3 in sandy loams and within ±0.1 m 3 m –3 in soils with high clay content. At temperatures between –5 and 0°C, soil ice contents could not be measured accurately with the heat-pulse method directly, but they could be estimated coarsely from water content before freezing, TDR measured unfrozen water content, and T-TDR measured total water content at temperatures below –5°C.