Coupled heat and water transport in frozen soils

The effect of freezing on soil temperature and water redistribution was examined in four Mesocosms maintained at different initial water content profiles. An innovative experimental setup involving use of a frozen base layer acting as a proxy to permafrost beneath an active layer made up of packed and undisturbed peat cores was used. The experimental setup was successfully validated for its ability to maintain one dimensional change in temperature and soil water content in frozen soil. There was a substantial amount of water redistribution towards the freezing front, enough to create an impermeable frozen, saturated zone within 15 cm of the soil surface. The water movement was likely due to the potential head gradients between colder and warmer regions created by temperature effects on matric potential of frozen soil. In addition, there is enough evidence that water migration in form of vapour contributed to moisture movement towards the freezing front. Initial moisture profiles appeared to have a significant effect on the freezing induced soil water redistribution likely through differences in moisture dependant hydraulic conductivity. Initial soil moisture profiles also affected the rate of freezing front movement. Frost propagation was controlled by latent heat for long periods, while soil thermal conductivity and heat capacity appeared to control the rate of frost migration once the majority of water was frozen. Using the observations of this study, a conceptual model was proposed to explain freezing of an active layer on a permafrost plateau assuming a variable moisture landscape at onset of winter. Further, a one-dimensional model based on coupled cellular automata approach was developed. The model is based on first order conservation laws to simulate heat and water flow in variably-saturated soil. Inside the model, Buckingham-Darcy’s -and Fourier’s heat laws are used to define the local interactions for water and heat movement respectively. Phase change is brought about by the residual energy after sensible heat removal has dropped the temperature of the system below freezing point. Knowing the amount of water that can freeze, the change in soil temperature is then modeled by integrating along the soil freezing curve. This approach obviates the use of apparent heat capacity term. The 1D model is successfully tested by comparing with analytical and experimental solutions

The emergence of hydrogeophysics for improved understanding of subsurface processes over multiple scales

Geophysics provides a multi-dimensional suite of investigative methods that are transforming our ability to see into the very fabric of the subsurface environment, and monitor the dynamics of its fluids and the biogeochemical reactions that occur within it. Here, we document how geophysical methods have emerged as valuable tools for investigating shallow subsurface processes over the past two decades and offer a vision for future developments relevant to hydrology and also ecosystem science. The field of “hydrogeophysics” arose in the late 1990s, prompted, in part, by the wealth of studies on stochastic subsurface hydrology that argued for better field-based investigative techniques. These new hydrogeophysical approaches benefited from the emergence of practical and robust data inversion techniques, in many cases with a view to quantify shallow subsurface heterogeneity and the associated dynamics of subsurface fluids. Furthermore, the need for quantitative characterization stimulated a wealth of new investigations into petrophysical relationships that link hydrologically relevant properties to measurable geophysical parameters. Development of time-lapse approaches provided a new suite of tools for hydrological investigation, enhanced further with the realization that some geophysical properties may be sensitive to biogeochemical transformations in the subsurface environment, thus opening up the new field of “biogeophysics”. Early hydrogeophysical studies often concentrated on relatively small ‘plot-scale’ experiments. More recently, however, the translation to larger-scale characterization has been the focus of a number of studies. Geophysical technologies continue to develop, driven, in part, by the increasing need to understand and quantify key processes controlling sustainable water resources and ecosystem services

Dynamic Modeling Of The Hydrologic Processes In Areas Of Discontinuous Permafrost

Thesis (Ph.D.) University of Alaska Fairbanks, 2006The overarching hypothesis of this dissertation is "in the sub-arctic environment, the presence or absence of permafrost is dominant influence on hydrologic processes." The presence or absence of permafrost is the defining hydrologic characteristic in the sub-arctic environment. Discontinuous permafrost introduces very distinct changes in soil hydraulic properties, which introduce sharp discontinuities in hydrologic processes and ecosystem characteristics. Hydraulic properties vary over short and long time scales as the active layer thaws over the course of a summer or with changes in permafrost extent. The influence of permafrost distribution, active layer thaw depth, and wildfire on the soil moisture regime and stream flow were explored through a combination of field-based observations and computer simulations. Ice-rich conditions at the permafrost table do not allow significant percolation of surface waters, which result in saturated soils near the ground surface and limited subsurface storage capacity, compared to well-drained non-permafrost sites. The removal of vegetation by wildfire results in short-term (10 years) drying of soils in moderate to severe wildfire sites is the result of an increased active layer depth and storage capacity. A spatially-distributed, process-based hydrologic model, TopoFlow, was modified to allow spatial and temporal variation in the hydraulic conductivity and porosity of soils. By continual variation of the hydraulic conductivity (proxy for permafrost distribution and active layer thaw depth) and porosity (proxy for storage capacity), the dynamic soil properties found in the sub-arctic environment are adequately represented. The sensitivity of TopoFlow to changes in permafrost condition, vegetation regime, and evapotranspiration is analyzed. The net result of the field observations and computer simulations conducted in this research suggest the presence or absence of permafrost is the dominant influence on soil moisture dynamics and has an important, but secondary role in the stream flow processes

EVALUATING FROZEN SOIL PROPERTIES WITH ELECTRICAL RESISTIVITY MEASUREMENT AND ELECTRO-MAGNETIC INDUCTION METHODS

Electromagnetic induction was utilized in the past by the United States Army Corps of Engineers as a method of detecting unexploded ordinance, while it has the potential to act as a novel method of investigating frozen soils in cold regions. In this study, we performed lab-scale 1D electrical resistivity measurements under freeze-thaw circumstances on frost-susceptible soils with varied soil properties. We implemented an empirical model from our experiments into a COMSOL finite element model at both laboratory and field scales to simulate soil electrical resistivity response under both short-term and long-term sub-freezing conditions. Dynamic temperature-dependent soil properties, most notably unfrozen water content, exert significant influences on the observed electrical resistivity. We also characterized the evolution of electrical resistivity during the freeze-thaw cycle with empirical models. Laboratory and field experiments were made to validate the effectiveness of the iFrost Mapper device in detecting typical patterns of metal, liquid, and soil samples of different concentrations and temperatures. The original data were processed by considering both inphase and quadrature responses. Meanwhile, simulation studies with similar parameters to the laboratory tests, including geometry, material properties, and physical conditions, and the samples were made in COMSOL Multiphysics to compare the analytical solutions and experimental data

Spatial variability of aircraft-measured surface energy fluxes in permafrost landscapes

Arctic ecosystems are undergoing a very rapid change due to global warming and their response to climate change has important implications for the global energy budget. Therefore, it is crucial to understand how energy fluxes in the Arctic will respond to any changes in climate related parameters. However, attribution of these responses is challenging because measured fluxes are the sum of multiple processes that respond differently to environmental factors. Here, we present the potential of environmental response functions for quantitatively linking energy flux observations over high latitude permafrost wetlands to environmental drivers in the flux footprints. We used the research aircraft POLAR 5 equipped with a turbulence probe and fast temperature and humidity sensors to measure turbulent energy fluxes along flight tracks across the Alaskan North Slope with the aim to extrapolate the airborne eddy covariance flux measurements from their specific footprint to the entire North Slope. After thorough data pre-processing, wavelet transforms are used to improve spatial discretization of flux observations in order to relate them to biophysically relevant surface properties in the flux footprint. Boosted regression trees are then employed to extract and quantify the functional relationships between the energy fluxes and environmental drivers. Finally, the resulting environmental response functions are used to extrapolate the sensible heat and water vapor exchange over spatio-temporally explicit grids of the Alaskan North Slope. Additionally, simulations from the Weather Research and Forecasting (WRF) model were used to explore the dynamics of the atmospheric boundary layer and to examine results of our extrapolation

Numerical Modeling Of Seasonally Freezing Ground And Permafrost

Thesis (Ph.D.) University of Alaska Fairbanks, 2007This thesis represents a collection of papers on numerical modeling of permafrost and seasonally freezing ground dynamics. An important problem in numerical modeling of temperature dynamics in permafrost and seasonally freezing ground is related to parametrization of already existing models. In this thesis, a variation data assimilation technique is presented to find soil properties by minimizing the discrepancy between in-situ measured temperatures and those computed by the models. The iterative minimization starts from an initial approximation of the soil properties that are found by solving a sequence of simple subproblems. In order to compute the discrepancy, the temperature dynamics is simulated by a new implementation of the finite element method applied to the heat equation with phase change. Despite simplifications in soil physics, the presented technique was successfully applied to recover soil properties, such as thermal conductivity, soil porosity, and the unfrozen water content, at several sites in Alaska. The recovered properties are used in discussion on soil freezing/thawing and permafrost dynamics in other parts of this thesis. Another part of this thesis concerns development of a numerical thermo-mechanical model of seasonal soil freezing on the lateral scale of several meters. The presented model explains observed differential frost heave occurring in non-sorted circle ecosystems north of the Brooks Range in the Alaskan tundra. The model takes into account conservation principles for energy, linear momentum and mass of three constituents: liquid water, ice and solid particles. The conservation principles are reduced to a computationally convenient system of coupled equations for temperature, liquid water pressure, porosity, and the velocity of soil particles in a three-dimensional domain with cylindrical symmetry. Despite a simplified rheology, the model simulates the ground surface motion, temperature, and water dynamics in soil and explains dependence of the frost heave on specific environmental properties of the ecosystem. In the final part, simulation of the soil temperature dynamics on the global scale is addressed. General Circulation Models are used to understand and predict future climate change, but most of them do not simulate permafrost dynamics and its potentially critical feedback on climate. In this part, a widely used climate model is evaluated and the simulated temperatures are compared against observations. Based on this comparison, several modifications to the Global Circulation Models are identified to improve the fidelity of permafrost and soil temperature simulations. These modifications include increasing the total soil depth by adding new layers, incorporating a surface organic layer, and modifying the numerical scheme to include unfrozen water dynamics

Remote Sensing of Environmental Changes in Cold Regions

This Special Issue gathers papers reporting recent advances in the remote sensing of cold regions. It includes contributions presenting improvements in modeling microwave emissions from snow, assessment of satellite-based sea ice concentration products, satellite monitoring of ice jam and glacier lake outburst floods, satellite mapping of snow depth and soil freeze/thaw states, near-nadir interferometric imaging of surface water bodies, and remote sensing-based assessment of high arctic lake environment and vegetation recovery from wildfire disturbances in Alaska. A comprehensive review is presented to summarize the achievements, challenges, and opportunities of cold land remote sensing

Characterization of Boreal-Arctic Vegetation Growth Phases and Active Soil Layer Dynamics in the High-Latitudes of North America: A Study Combining Multi-Year In Situ and Satellite-Based Observations

This dissertation examined the seasonal freeze/thaw activity in boreal-Arctic soils and vegetation physiology in Alaska, USA and Alberta, Canada, using in situ environmental measurements and passive microwave satellite observations. The boreal-Arctic high-latitudes have been experiencing ecosystem changes more rapidly in comparison to the rest of Earth due to the presently warming climatic conditions having a magnified effect over Polar Regions. Currently, the boreal-Arctic is a carbon sink; however, recent studies indicate a shift over the next century to become a carbon source. High-latitude vegetation and cold soil dynamics are influenced by climatic shifts and are largely responsible for the regions atmospheric carbon fluxes. Under a warming climate, soils are thawing for extended periods of time, allowing for heightened aerobic decomposition of organic matter in the soil, increasing soil carbon emissions. Simultaneously, vegetation performs photosynthesis longer, resulting in increased sequestering of atmospheric carbon. Regional and global climate affect one another through land-atmosphere carbon feedbacks. The strength, and contribution, of this study lies in high quality fine-scale in situ datasets day-of-year occurrences for soil state transitions and vegetation growth phenophase activity at site-specific locations. Findings include (1) a more thermally variable active layer in dry tundra compared to wet tundra; (2) active layer isothermal conditions are established rapidly in the fall ( ~ 2 days) but not in the spring (~ 11 days); (3) boreal willow shrubs (Salix Spp.) have the shortest exiting dormancy period (11 days) yet the longest active above ground stem growth (61 days); (4) moist bog type environments were shown to be the optimal spruce (Picea Spp.) growth environments (active trunk growth duration of ~ 55 days); and (5) AMSR satellite data were shown to preemptively estimate land surface condition change compared to in-situ measurements during the spring transition for both tundra types while lagging during the fall transition and freeze-up periods. These results elucidate the need for further field work campaigns collecting active soil layer measurements in order to precisely gauge the seasonally thawed and active windows for soil and vegetation. Outcomes of this research include increased availability of quantified soil and vegetation activity windows. Conclusions include prospects which are valuable for studies attempting to optimize carbon flux estimations using freeze/thaw microwave satellite datasets

Shuttle imaging radar-C science plan

The Shuttle Imaging Radar-C (SIR-C) mission will yield new and advanced scientific studies of the Earth. SIR-C will be the first instrument to simultaneously acquire images at L-band and C-band with HH, VV, HV, or VH polarizations, as well as images of the phase difference between HH and VV polarizations. These data will be digitally encoded and recorded using onboard high-density digital tape recorders and will later be digitally processed into images using the JPL Advanced Digital SAR Processor. SIR-C geologic studies include cold-region geomorphology, fluvial geomorphology, rock weathering and erosional processes, tectonics and geologic boundaries, geobotany, and radar stereogrammetry. Hydrology investigations cover arid, humid, wetland, snow-covered, and high-latitude regions. Additionally, SIR-C will provide the data to identify and map vegetation types, interpret landscape patterns and processes, assess the biophysical properties of plant canopies, and determine the degree of radar penetration of plant canopies. In oceanography, SIR-C will provide the information necessary to: forecast ocean directional wave spectra; better understand internal wave-current interactions; study the relationship of ocean-bottom features to surface expressions and the correlation of wind signatures to radar backscatter; and detect current-system boundaries, oceanic fronts, and mesoscale eddies. And, as the first spaceborne SAR with multi-frequency, multipolarization imaging capabilities, whole new areas of glaciology will be opened for study when SIR-C is flown in a polar orbit