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Improved Understanding of Permafrost Controls on Hydrology in Interior Alaska by Integration of Ground-Based Geophysical Permafrost Characterization and Numerical Modeling
Dr. Michelle Walvoord | U.S. Geological Survey
Permafrost and seasonal ground ice are key factors that control the routing of water above and below the land surface in interior Alaska. Hence, frozen ground affects water resources, ecosystem state, landscape evolution, and soil stability. Despite its hydrologic, ecologic, and geotechnical importance, the spatial distribution of permafrost and its relation to subsurface water flow remains poorly understood for many areas of interior Alaska largely due to its remoteness and inaccessibility. This project aimed to improve the knowledge base by addressing the following objectives: (1) developing a new numerical modeling tool for simulation of groundwater/permafrost interaction, (2) demonstrating and evaluating geophysical methods for mapping permafrost distribution at several test sites, and (3) elucidating permafrost-controlled surface-water/groundwater exchanges. The last objective was accomplished using physics-based modeling to evaluate local settings that were characterized by field efforts and by evaluating larger scale interactions based on generic understanding of permafrost patterns based on geophysical and other data.
The pre-existing U.S. Geological Survey (USGS) computer simulation code SUTRA (Saturated-Unsaturated, variable-density ground-water flow with solute or energy TRAnsport) was enhanced to simulate freezing and thawing of groundwater in saturated media. An additional enhancement addressed development of model capability to simulate freezing and thawing of groundwater in variably saturated media. These developments provide a tool that enable improved understanding of the interplay between frozen ground and groundwater to be developed. Complementary ground-based geophysical techniques were used to map, between 2011 and 2014, the details of fine-scale subsurface geology and permafrost distributions at several specific study sites (10s to 100s of meters) in the Yukon Flats of east-central Alaska. A supplemental effort provided airborne geophysical characterization of large-scale permafrost distribution in interior Alaska (10s to 100s of kilometers). Comparison of results from a variety of methods enabled multi-scale interpretation of subsurface cryologic and geologic features. Results from the small- and large-scale geophysical mapping were then used as the basis for a series of hydrologic model evaluations (based on SUTRA and other methodologies) that addressed questions related to permafrost/groundwater interaction and predicted system response to past changes in climatic and hydrologic conditions and also to potential future changes in these surface conditions.
This project has resulted in both basic science and methodological advances. The groundwater modeling study of the Yukon Flats Basin of Alaska demonstrated and quantified the increase in baseflow to major rivers expected to result from permafrost degradation along a thaw sequence. The analysis showed the transition from continuous (>90%) to discontinuous (50-90%) permafrost coverage to be a critical one in terms of hydrologic change. Fully coupled fluid flow and heat transport simulations using the enhanced SUTRA model quantified rates of sub-lake talik (i.e., zone of year-round unfrozen ground that occurs in permafrost areas) development for a variety of conditions representative of those in the Yukon Flats. Lake talik evolution and cross sectional landscape simulations using SUTRA demonstrated the importance of advective heat transport in accelerating permafrost degradation compared with heat conduction alone. Simulations with an imposed surface warming trend illustrated that permafrost thaws more quickly in places where there is active groundwater flow, particularly in groundwater recharge areas. Rates of permafrost growth and thaw depend on causal and hydrologic factors. Field-based geophysical investigations and SUTRA modeling results shed new light on permafrost-lake dynamics, providing a physically based explanation for the development of new permafrost around receding lakes in the presence of climate warming. Observations of new permafrost in the margins of receding lakes were shown consistent with the insulating effects of ecological succession. SUTRA simulations indicated, however, that such permafrost will persist for no more than about 70 years, under current projections of climate warming. Field results also provided insight into the present-day distribution of permafrost within the Yukon Flats, forming a valuable baseline to which future studies can compare.
Methodological advances include demonstration of a multi-scale approach to geophysical mapping of permafrost and an integrated assessment of the capabilities and limitations of various geophysical techniques in the absence of deep subsurface information for ground-truthing. Software tools developed under the project, including SUTRA with cold region capabilities and FEMIC (Frequency-domain ElectroMagnetic Inversion Code) for analysis of large electromagnetic data sets, represent research products that will enable application of the project’s approach in other studies.
Collectively, these results have wide-ranging transferability to the characterization and system behavior of other permafrost-impacted environments.
Knowledge gained and products developed by this project provide a broad foundation for future efforts to evaluate, in detail, the potential consequences of human-induced and climate-change stresses in key areas of Alaska. Furthermore, the hydrologic modeling tools and approaches and the geophysical methods and evaluation approaches developed are readily transferrable to a wide range of issues related to permafrost characterization and dynamics affecting cold-region Department of Defense installations and operations.
A geophysical workshop offered as part of this project presented complementary approaches for subsurface characterization and monitoring in cold regions and, as a result, provided direct technology transfer.
Software for the enhanced SUTRA model and manuals describing its development and implementation will be publically available. Efforts to transition SUTRA with its capabilities for cold regions to a broad user community will continue after the project ends, in the form of regular USGS training workshops and support provided by its USGS developers. Additionally, a software package was developed for modeling and inversion of frequency-domain electromagnetic data. This will be published to enable follow-on research and wide-scale application of electromagnetic mapping of permafrost features.
All software codes produced and data collected as part of this project are archived and publically available. This open access will promote usage of the field-derived information and the software tools. The data sets will serve as an important baseline for subsequent studies.