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Investigation of the Spatial and Temporal Variations of the Seasonally Frozen Ground in the Contiguous United States

This project is funded by the Department of Energy's National Institute for Global Environmental Change


To improve our understanding of the seasonal freezing and thawing processes of soils and their interactions with hydrologic and climatic systems at local and regional scales; and provide information such as the timing, duration, thickness, and areal extent of the seasonal freeze/thaw cycle of soils for the period from 1978-present in the United States for studies of plant growth, carbon exchange, ecosystem, surface hydrology, and soil moisture, and for model improvement and validation


Tingjun Zhang is the PI; and Richard Armstrong, Christopher Oelke, and Martyn Clark are Co-PIs.

Project Summary

Based on data from 1978 to 2003, the onset date of autumn soil freeze starts in October along the Rocky Mountains and North Dakota, then gradually expands into the western U.S. and southern regions. The maximum extent of soil freeze is reached in January. Approximately 80 percent of the contiguous United States landmass experiences freezes in winter. Overall, the trend indicates a later onset date of autumn soil freeze, especially in the eastern part of the United States. The delay can be up to three weeks in some parts of the study area, a very significant change. Further validation of these results is needed, ideally using ground-based measurements.

The climatology of the last date of near-surface soil freeze starts in January in the south, and occurs later in the north. In the majority of western and northern states, the last date of soil freeze is in April, but occurs in May along the Rocky Mountain regions. The last date of soil freeze occurs later in the central Plains and eastern states, while in the western states, especially along the Rocky Mountains, the last date of surface soil freeze occurs earlier than the long-term average.

The duration of the surface soil freeze is defined as the time period from the onset date of autumn soil freeze to the last date of spring soil freeze. Overall, the average duration varies from less than a month in the south to more than eight months in some Rocky Mountain regions. Along the Rocky Mountains, duration lasts about five to six months. Over the central Plains and in the eastern part of the United States, the duration is usually three to four months, while in the northern and western states, the duration can be four to five months. For most of the country, except in the Rocky Mountain region, the duration of surface soil freeze has become longer. This is mainly due to the delay of spring thaw.

The frequency of surface soil freeze is defined as the number of days of soil freeze divided by the total number of days in a year. In the Rocky Mountain region, more than 50 percent of the time, near-surface soil is frozen. Although surface soil freeze can last as long as eight months, the frequency is just above 50 percent. This means there are days where the surface soil may not in be frozen, but is in between freeze/thaw cycles. Over most of the country, surface soil is frozen for about 20 to 40 percent of the year. In southern States, the frequency decreases accordingly. Interestingly, the frequency along the Rocky Mountains is decreasing while the rest of the country is increasing. Further study is needed to validate the discoveries in this study. Potential implication changes in frequency of soil freeze can be extremely significant in surface hydrology and carbon exchange studies.

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