Unfrozen Soil Research Articles

Heave and Heaving Pressure in Freezing Soils: A Unifying Theory

A unifying theory is presented for the process of heave in freezing soils. Out of the Cold Regions Research and Engineering Laboratory (CRREL) school of D.M. Anderson came the concept of the segregation potential. Out of the Cornell school of R.D. Miller came the model for the heave rate. Here ideas from both schools are put on a fundamental thermodynamic footing, leading to the definition of a new heave index. Both schools use the temperature gradient in the frozen fringe as the driving force for heave. We argue and demonstrate that this choice leads to erratic results. The driving force should be the temperature gradient over the complete layer of soil that is at sub‐zero (°C) temperature, that is, the combined frozen zone plus the frozen fringe. The value of the heave index is completely dominated by the unsaturated hydraulic conductivity function of both the unfrozen soil below the frozen fringe and the soil layer at sub‐zero temperature.

Determining Hydraulic Conductivity for Air‐Filled Porosity in an Unsaturated Frozen Soil by the Multistep Outflow Method

Accurate simulation of snowmelt infiltration and runoff in frozen soil is important for many environmental and engineering issues in cold regions. It is well known that infiltration in frozen soil is dramatically reduced due to the impedance of ice crystals; however, it is very often inaccurately predicted because of limited data to parameterize the related processes. In this study, the hydraulic conductivity K(h) for air‐filled porosity in an unsaturated frozen soil was investigated by employing a multistep pressurized outflow method using antifreeze liquid. Comparisons of water flow in both partially frozen and unfrozen soils indicated that frozen soil significantly reduced K(h) due to the blocking effects of ice crystals. Based on one common K(h)‐based hydraulic equation, an impedance parameter for liquid‐filled porosity was extended to an apparent impedance parameter for air‐filled porosity. The apparent impedance factor Ωa is about 4 ranging from 0.5 to 6.5 as a function of matric potential. These findings represent significant new progress for estimation of Ωa by an experimental method that can be used for the estimation of snowmelt infiltration. We suggest that the current applied measurement method should be used and further evaluated for a variety of soil types.

Application of Multiphase Dielectric Mixing Models for Understanding the Effective Dielectric Permittivity of Frozen Soils

The time domain reflectometry (TDR)–measured effective permittivity in frozen soil conditions is affected by many complex factors including bound water effects on soil water permittivity, phase changes, soil microstructure and relative positions of soil constituents with respect to each other. The objective of this study was to improve understanding of some of the factors affecting the effective permittivity of frozen soils through the use of dielectric mixing models. Published datasets and frozen and unfrozen soil data measured on western Canadian soils were investigated with multiphase discrete and confocal ellipsoid models available in the literature. The results revealed that adjusting model parameters allowed the mixing models to describe the frozen soil permittivity equally well when bound water effects and temperature‐dependent water permittivity effects were included or not included. Measurement of freezing and thawing curves on western Canadian soils showed significant hysteresis and some mechanisms for this observed hysteresis and its influence on the interpretation of published datasets are discussed. When independent measurements of liquid water, ice and effective permittivity are available, it is possible to find one set of model parameters that reasonably predict effective permittivity for both frozen and unfrozen conditions. In frozen soils the predictive capability of the models is constrained to scenarios where the initial water content prior to freezing (i.e., the total water content) in the sampling volume is constant.

Review of the influence of freeze-thaw cycles on the physical and mechanical properties of soil

Seasonally frozen soil is a four-phase material and its physical-mechanical properties are more complex compared to the unfrozen soil. Its physical properties changes during the freeze-thaw process; repeated freeze-thaw cycles change the characteristics of soil, which can render the soil from an unstable state to a new dynamic equilibrium state. The freezing process changes the structure coupled between the soil particle arrangements, which will change the mechanical properties of the soil. The method of significance and interaction between different factors should be considered to measure the influence on the properties of soil under freeze-thaw cycles.

Water Infiltration into a Frozen Soil with Simultaneous Melting of the Frozen Layer

Understanding water infiltration into frozen soil is important for preventing soil erosion and managing soil water and nutrients. In this study, we performed a column experiment on infiltration through frozen soil using a variably‐saturated silt loam. Three soil columns (7.8 cm i.d., 35 cm long), with three different initial soil water contents, were cooled from the top to form a frozen layer of the same thickness. The columns were instrumented with 34 thermocouples, seven time‐domain reflectometry (TDR) probes, and seven tensiometers. Water at a temperature of 3.5°C was applied to the top of the columns with a 15‐cm constant head. We monitored ice and liquid water contents, temperatures, and the position of the infiltration front. Three phases of infiltration were observed: (i) no infiltration at the beginning, (ii) slow infiltration as the infiltration front advanced through the frozen layer, and (iii) increased infiltration as the infiltration front advanced through the unfrozen soil below the frozen layer. The duration of each phase became longer with increasing initial soil water content as the infiltration rate of each phase decreased. The volumetric ice content and thickness of the frozen layer controlled the infiltration process. We use a capillary bundle model to characterize the hydraulic conductivity as a function of ice content during infiltration. Based on our experimental data and results, we mechanistically describe the water infiltration into frozen soil.

Coupling of a simultaneous heat and water model with a distributed hydrological model and evaluation of the combined model in a cold region watershed

AbstractSnow and frozen soil prevail in cold regions worldwide, and the integration of these processes is crucial in hydrological models. In this study, a combined model was developed by fully coupling a simultaneous heat and water model with a geomorphologically based distributed hydrological model. The combined model simulates vertical and lateral water transfer as well as vertical heat fluxes and is capable of representing the effects of frozen soil and snowmelt on hydrological processes in cold regions. This model was evaluated by using in situ observations in the Binggou watershed, an experimental watershed for cold region hydrology of the Watershed Allied Telemetry Experimental Research Project. Results showed that the model was able to predict soil freezing and thawing, unfrozen soil water content, and snow depth reasonably well. The simulated hydrograph was in good agreement with the in situ observation. The Nash–Sutcliffe coefficient of daily discharge was 0.744 for the entire simulation period, 0.472 from April to June, and 0.711 from June to November. This model can improve our understanding of hydrological processes in cold regions and assess the impacts of global warming on hydrological cycles and water resources. Copyright © 2012 John Wiley & Sons, Ltd.

N2O emissions during the freezing and thawing periods from six fields in a livestock farm, southern Hokkaido, Japan

In many countries, high nitrous oxide (N2O) emissions have been observed during soil freezing and thawing periods. Quantification of those emissions is crucial to evaluate annual N2O emissions. For this study, we measured N2O and nitric oxide (NO) fluxes along with soil N2O concentrations at a corn (Zea mays L.) field and five grasslands (Phalaris arundinacea L. and Phleum pratense L.) during a winter-spring period in southern Hokkaido, Japan. We also measured denitrification activities of the soils from those sites. During the observation period, the soils froze to a maximum depth of 370 mm under saturated conditions and the lowest soil temperature at a 50 mm depth was −4.5°C. After 6 March 2005, daily air temperature rose above 0°C, but the soil temperature remained approximately 0°C for about two weeks. These two weeks were defined as the “transition period,” while the periods before and after the transition period were defined as the “freezing” and the “thawing” periods, respectively. During the freezing and transition periods, N2O concentration increased in the frozen soils relative to the unfrozen soils, and the highest values were observed in the frozen soils during the transition period. During the thawing period, the N2O concentration in the soils decreased. N2O emissions were much higher during the thawing period than during the freezing and transition periods, and remarkably higher N2O emissions were observed at the corn site compared to those at the grassland sites. NO emissions were also observed during the thawing period but at much lower levels than N2O emissions at all the sites. N2O-N/NO-N ratio exceeded one at all the sites during the entire period, indicating N2O production through denitrification. At the corn site, denitrification activity was much lower and N2O/(N2O+N2) was much higher than at the grasslands. The result indicated that high N2O emissions at the corn site were caused by complementary processes: (1) high accumulated N2O through denitrification in the frozen soil during the freezing and transition periods, and (2) low N2O reduction rate during the thawing period.

Relationships between Soil Compaction and Harvest Season, Soil Texture, and Landscape Position for Aspen Forests

Although a number of harvesting studies have assessed compaction, no study has considered the interacting relationships of harvest season, soil texture, and landscape position on soil bulk density and surface soil strength for harvests in the western Lake States. In 2005, we measured bulk density and surface soil strength in recent clearcuts of predominantly aspen stands (Populus grandidentata Michx. and Populus tremuloides Michx.) in the Chippewa National Forest in northern Minnesota. We stratified these clearcuts by the season harvested, soil texture, and topographic position. In nearly all cases, we observed higher bulk density and surface soil strength following harvesting compared with adjacent and similar but unharvested stands. Within harvested sites, fine-textured soils generally had higher surface soil strength (more compaction) than coarse-textured soils when harvested in the summer, and fine-textured sites harvested in the summer had higher surface soil strength than those harvested in the winter. Landscape position was an important factor only in fine-textured soils. Both summit and toeslope positions had higher surface soil strength following summer harvesting compared with winter harvesting. Overall, our results indicate that fine-textured soils located on both lower and upper slope positions and harvested during unfrozen soil conditions are most susceptible to compaction during logging.

Intra- and inter-annual changes in chemistry of Australian glacial lakes

The chemical characteristics of five seasonally ice-covered lakes in the Snowy Mountains were measured monthly from 2006 to 2009. Although N and P concentrations were significantly higher in rainfall than snowfall, concentrations peaked in lakes in winter rather than summer. This was linked to continuous winter nutrient flow into the lakes from melting snowpack and continued biogeochemical processes in unfrozen soil at a time when biological activity beneath the lake ice was depressed. In contrast to high altitude lakes elsewhere, there was no spring ionic pulse of nutrients. Lake pH fluctuated throughout the ice-free period between 6.9 and 6.5, falling to 6.1–6.0 beneath ice cover, before rising abruptly after ice break-up. Earlier ice break-up in recent years has resulted in an earlier increase in pH, and decrease in concentrations of NH3-N and NOx-N. In years with least snowfall and early ice break-up, winter peaks of NH3-N were lowest whereas both PO4-P and NOx-N showed winter peaks of various concentrations in medium years rather than extreme years. Rising winter and/or spring temperatures resulting in changes in precipitation from snow to rain could lead to increased nutrient deposition, with rain carrying an order of magnitude more nutrients than does snow.

Determining Water Retention in Seasonally Frozen Soils Using Hydra Impedance Sensors

The soil freezing characteristic, defined as the relationship between freezing soil temperature and unfrozen water content, can be used to determine the soil water retention curve in situ. The objective of this study was to investigate whether freezing characteristics measured with Hydra impedance sensors (Stevens Water Monitoring Systems, Portland, OR) result in accurate depthwise soil water retention curves. Hydra sensors measuring the complex permittivity and soil temperature were installed at five depths at five sites in southeastern Wyoming and monitored for approximately 2 yr. A dielectric mixing model was calibrated using unfrozen soil data to predict the liquid water content and ice content in frozen soils. The total water potential in the frozen soils was calculated from the Hydra sensor soil temperature measurements using the Clapeyron equation. Soil water pressure heads were calculated from the total soil water head by subtracting the osmotic head. Comparison of the resulting Hydra sensor water retention data with depthwise laboratory retention data using a dew point potentiometer and a pressure plate extractor showed mixed results (coefficient of determination 0 ≤ R2 ≤ 0.94). The best results were obtained for the shallowest sensors at the five sites (0.74 ≤ R2 ≤ 0.93) because of the more significant and more prolonged soil freezing at these depths, resulting in relatively wide ranges for the calculated soil water pressure heads. Fitted curves for the Hydra sensor water retention data yielded unreliable parameters because of insufficient information on the wet end of the water retention curves.

Estimations of moisture content in the active layer in an Arctic ecosystem by using ground-penetrating radar profiling

Abstract We applied high-frequency GPR at a study site in the high arctic ecosystem of Northeast Greenland to evaluate its usefulness in assessing depth of, and water content in, the active layer at Zackenberg Valley (74°N; 20°W) to evaluate its usefulness in the high arctic ecosystems. The study site includes different vegetation types, and it well represents of the entire valley, for which we aimed to determine the conditions and characteristics that influence the GPR performance in the active layer. The spatial distribution of moisture content along the transect studied was estimated using GPR data (400 MHz antenna), depth to permafrost, soil samples and vegetation observations. Vertical distribution of the water content in the unfrozen soil bulk was predicted for several points on the transect by combining data that influence the behavior of the radar waves with that of capacitive moisture probes. The statistical models resulted to be highly significant, thus assuming common conditions of the soil to the classified vegetation, we can obtain from the GPR data, truthful estimations of water content, and, moreover, we can predict the distribution to the bottom of the active layer. Hence, we conclude that GPR is a viable option for improving active layer spatial quantification of water contents that can be used to assess changes in the active layer in arctic regions.

Experimental study on unfrozen water content and soil matric potential of Qinghai-Tibetan silty clay

A new soil moisture content sensor coupled with a new matric potential sensor that can operate in the subfreezing environment was used to measure the moisture content and soil matric potential dynamics of Qinghai-Tibetan silty clay. Combined with nuclear magnetic resonance (NMR) technique and thermal resistor temperature probe, the characteristics of unfrozen water content and soil matric potential, and their relationships with temperature were analyzed. The results show that initial water content has an impact on the freezing point and unfrozen water content. The decrease in the initial water content results in a depression in the freezing point. The Qinghai-Tibetan silty clay has more similar unfrozen water content characteristic to clay than to silt. There is approximately 3% of unfrozen water content retained when the soil temperature drops to −15°C. The change of soil matric potential with temperature is similar to that of the unfrozen water content. The matric potential value of the saturated silty clay is approximately −200 kPa when the soil temperature drops to −20°C. The measured matric potentials are significantly lower than the calculated theoretical values based on the freezing point depression. Moisture migration experiment indicates that soil matric potential controls the direction of moisture movement and moisture redistribution (including ice and liquid water) during the soil freezing.

Dynamic responses of Qinghai-Tibet railway embankment subjected to train loading in different seasons

Compared with unfrozen soils, frozen soil has the most important characteristic—under natural conditions its matrix, which is composed of ice and water, changes continuously with varying temperature, which implies the mechanical properties of frozen soil are dependent on temperature. Accordingly, dynamic responses of frozen embankment under traffic load are different with the alternation of seasons. In order to clarify the definite seasonal difference, an indirect moisture, heat and dynamic coupled model of embankment in permafrost regions is deduced on the basis of principal theories of numerical heat transfer, physics of frozen soil, frozen soil mechanics and soil dynamics, meanwhile, its corresponding numerical program is also developed. Then, a common embankment section of Qinghai-Tibet Railway (QTR) is taken as an example, and while four typical seasons (January 15, April 15, July 15 and October 15) in the 50th service year are selected to carry out dynamic analyses and to reveal the seasonal difference in dynamic response. The results show that acceleration, velocity and displacement responses to train loading on October 15 are all the greatest in these four seasons. What’s more, even after the train passes across the embankment section, there exist larger permanent displacement residing in the embankment and more damages to the toe of the embankment slope on October 15. Therefore, the deformation of QTR embankment should be monitored closely to ensure safe operation on October every year. Of course, the numerical model and results in this study may be a reference for design, maintenance and research on other embankments in permafrost regions.

Exploring the sensitivity of soil carbon dynamics to climate change, fire disturbance and permafrost thaw in a black spruce ecosystem

Abstract. In the boreal region, soil organic carbon (OC) dynamics are strongly governed by the interaction between wildfire and permafrost. Using a combination of field measurements, numerical modeling of soil thermal dynamics, and mass-balance modeling of OC dynamics, we tested the sensitivity of soil OC storage to a suite of individual climate factors (air temperature, soil moisture, and snow depth) and fire severity. We also conducted sensitivity analyses to explore the combined effects of fire-soil moisture interactions and snow seasonality on OC storage. OC losses were calculated as the difference in OC stocks after three fire cycles (~500 yr) following a prescribed step-change in climate and/or fire. Across single-factor scenarios, our findings indicate that warmer air temperatures resulted in the largest relative soil OC losses (~5.3 kg C m−2), whereas dry soil conditions alone (in the absence of wildfire) resulted in the smallest carbon losses (~0.1 kg C m−2). Increased fire severity resulted in carbon loss of ~3.3 kg C m−2, whereas changes in snow depth resulted in smaller OC losses (2.1–2.2 kg C m−2). Across multiple climate factors, we observed larger OC losses than for single-factor scenarios. For instance, high fire severity regime associated with warmer and drier conditions resulted in OC losses of ~6.1 kg C m−2, whereas a low fire severity regime associated with warmer and wetter conditions resulted in OC losses of ~5.6 kg C m−2. A longer snow-free season associated with future warming resulted in OC losses of ~5.4 kg C m−2. Soil climate was the dominant control on soil OC loss, governing the sensitivity of microbial decomposers to fluctuations in temperature and soil moisture; this control, in turn, is governed by interannual changes in active layer depth. Transitional responses of the active layer depth to fire regimes also contributed to OC losses, primarily by determining the proportion of OC into frozen and unfrozen soil layers.

Evaluation of the heat pulse probe method for determining frozen soil moisture content

Heat pulse probes (HPP) have been widely utilized to determine soil thermal properties and water content in unfrozen soils; however, their applications in frozen soils are largely restricted by phase change and the presence of unfrozen water. This study explores the possibility of using HPP to determine total water content of frozen soils by (1) establishing the optimum heat applications to limit melting, (2) improving the mathematical representations for frozen conditions, and (3) evaluating the applicability of HPP methods under various temperature and moisture conditions. A custom‐built HPP was tested at total moisture levels that varied from full saturation to oven dry and initial soil temperatures from 20°C to −11°C. The applied heat pulse durations varied from 8 to 60 s, with total heat strength varying from 100 to 2000 J m−1. Comparison of mathematical methods involved two analytical solutions and a one‐dimensional finite difference numerical model. While both analytical methods assumed no phase change, the numerical model considered ice melting and unfrozen water. Conclusions include the following: (1) the numerical model with phase change is the only appropriate method to represent the temperature change curve once melting occurs; (2) below −4°C, ice melting could be limited, and measurement errors of total moisture content were within ±0.05 m3 m−3; (3) application of HPP between −2°C and 0°C is difficult because of the retarded response of probe temperature to changing moisture contents and heat applications; and (4) probe spacing is a sensitive parameter requiring calibration once reinstallation of the probe or the thawing and freezing process occurs.

Prediction of Cryo-SWCC during Freezing Based on Pore-Size Distribution

AbstractThe freezing process in soils is generally recognized as a coupled problem of heat and moisture transfer. Especially in the process of transfer, it is essential to understand the status of unfrozen water. In this paper, the freezing process in unsaturated soils has been studied with the hypothesis that initially unsaturated soil maintains the apparent unsaturated condition during freezing. Accordingly, the concept of the soil-water characteristic curve (SWCC) of unfrozen soil could be also applied to soil in the freezing stage. Therefore, a methodology for estimating a correlation between the suction and the amount of unfrozen water content, named cryo-SWCC, which considers the volume change of soil pores owing to ice formation, is proposed with the assumption that any deformation in unsaturated frozen soils owing to freezing can be negligible. On the basis of a relationship between a statistical pore-size distribution and the SWCC represented by the Mualem model, the variation of characteristic p...

Characteristics of land surface heat and water exchange under different soil freeze/thaw conditions over the central Tibetan Plateau

AbstractFreezing and thawing processes at the soil surface play an important role in determining the nature of Tibetan land and atmosphere interactions. In this study, land surface water and heat exchanges under different freezing and thawing conditions over the central Tibetan Plateau were investigated using observations from the Coordinated Enhanced Observing Period/Asia‐Australia Monsoon Project on the Tibetan Plateau, and the Simultaneous Heat and Water Model. During the freezing and thawing stages, significant diurnal variation of soil temperature resulted in a diurnal cycle of unfrozen water content at the surface. Radiation and energy components and evapotranspiration averaged over four freeze/thaw stages also changed diurnally. On average, the surface albedo (0·68) during the completely frozen stage was sharply higher than those during the freezing, thawing, and completely thawed stages due to the snow cover. The Bowen ratios were 3·1 and 2·5 in the freezing and thawing stages, respectively, but the ratio was only 0·5 in the completely thawed stage. Latent heat flux displayed distinctly better correlation with unfrozen soil water content during the freezing and thawing stages than during the completely frozen and thawed stages. This implies that the diurnal cycle of unfrozen soil water, resulting from diurnal freeze/thaw cycles at the surface, has a significant impact on latent heat flux. A surface energy imbalance problem was encountered, and the possible sources of error were analysed. Copyright © 2011 John Wiley & Sons, Ltd.

Prefreeze Soil Moisture and Compaction Affect Water Erosion in Partially Melted Andisols

In this study, artificial rainfall was applied to a partially frozen Andisol soil, and the effects of the subsoil's physical conditions, prefreeze moisture condition, and soil compaction on surface runoff and soil loss were studied. The soil, an Andisol from a mountainous region of Japan, was packed into a plastic box to a thickness of 0.08 m to attain a predetermined moisture condition and dry bulk density, then stored in a freezer at −30°C overnight to create a sample representative of frozen subsoil. Twenty millimeters of unfrozen soil were added on top of the frozen subsoil to simulate a thawing surface layer. During the simulated rainfall (42 mm h−1) experiment, surface runoff, soil loss, and seepage from outlets at the front wall of the downslope end of the box were sampled periodically. In the early stage of rainfall, the impermeable frozen subsoil increased surface runoff and soil loss, and seepage occurred at the downslope end of the box, indicating saturation of the near‐surface, unfrozen soil. During the early stage of the rainfall, soil loss was particularly greater than during the latter half of the rainfall experiment although both showed a similar surface runoff rate. Saturation of the near‐surface soil was the reason for the greater soil loss rate, while a surface seal restricted soil loss by surface runoff during the latter half of the rainfall experiment.

Effects of snowmelt on phosphorus and sediment losses from agricultural watersheds in Eastern Canada

Snowmelt is the most important hydrological event in cold climates. However, snowmelt effects on suspended sediment (SS) and phosphorus (P) loss are poorly documented in Canada. Using two agricultural watersheds in Eastern Canada, this study aimed to quantify SS and P loss during the snowmelt period and to investigate how snowmelt contributes SS and P loss. Water samples were collected from the outlets of the Bras d’Henri watershed (BHW, 2007–2009) and Black Brook watershed (BBW, 2008–2009) and measured for SS and P concentrations. Hydrological parameters (precipitation, snow water equivalent, and runoff discharge), soil frozen status and soil temperature were also measured. Results revealed inter-annual variation of snowmelt conditions and SS and P losses in each watershed. The 2008 snowmelt in BHW and BBW mainly occurred on unfrozen soils, while the 2007 and 2009 snowmelts in BHW and 2009 snowmelt in BBW mainly on frozen soils. In BHW, 2008 snowmelt caused much higher median concentrations of SS, total P (TP), dissolved P (DP) and particulate P (PP) in stream water than 2007 and 2009; ratios of PP fractions in TP were variable with events but the median values were similar, suggesting both DP and PP important contrubutors to TP loss. In BBW, the median concentration of dissolved reactive phosphorus (DRP) in stream water was greater in 2008 snowmelt than in 2009 snowmelt; PP dominated TP loss. This study also suggests that soil state (i.e. frozen status) and rainfall were the most important factors influencing SS and P losses during snowmelt. Furthermore, snowmelt P export represented more than 20% of the total annual P export in BHW, and more than 12% of the annual DRP export in BBW. Thus, we strongly recommend adopting Best Management Practices (BMPs) that specifically target sediment and P loss during snowmelt.

Equipping a Conventional Wheeled Forwarder for Peatland Operations

In Finland, peatland logging is generally conducted during the winter due to the inherently low soil bearing strength under unfrozen soil conditions. Mild winters the past several years have raised the issue of operations on unfrozen peatlands. Modifying wheeled logging equipment such that it was able to operate cost-effectively on sensitive sites and then switch back to normal, base machine specifications at other times would be a significant advantage. The mobility and rut formation of a conventional 8-wheeled forwarder was studied with four different sets of chain/track equipment. Additionally, the forwarder was equipped with a rear, add-on axle resulting in a 10-wheeled forwarder. That modified forwarder was tested with the widest set of tracks on an abandoned peat field and on a pine bog. Results indicate that the forwarder modifications significantly increased mobility and decreased rut formation on the test soils. On the pine bog, the 10-wheeled forwarder had the best mobility and the least rut formation of all equipment tested.