Patterns of soil temperature and moisture in the active layer and upper permafrost at Barrow, Alaska: 1993-1999

Soil temperature has been monitored continuously at hourly intervals to a depth of 1 m since 1993 at a site near Barrow, AK. Time series of soil moisture from the active layer and upper permafrost have been collected since 1996 at the same location. These records are supplemented by meteorological data from NOAA's Barrow Climate Monitoring and Diagnostics Laboratory facility and detailed description of depth-dependent soil properties at the site. Soil sensors are situated within a low-centered ice-wedge polygon characterized by meadow tundra vegetation. A thin (7 cm) organic layer grades into reworked marine silts at depth. The soil temperature and moisture are used in a site-specific, multiyear thermal analysis of the atmosphere/snow/active-layer/permafrost system. Fusion retards soil freezing during early winter as soil water is converted to ice. Soil heat transfer is dominated by conduction in winter. Infiltration of snow meltwater in spring produces a series of thermal pulses in the active layer, causing rapid warming of the upper several decimeters by about 1 degreesC. The thermal impact is limited because the soil tends to be nearly saturated at the time of freezeback. Volumetric soil water content in summer is generally around 35-40% at a depth of 15 cm, while the base of the thawed zone remains saturated near 50%. The near-surface soil exhibits drying from evapotranspiration and rewetting from precipitation events. During the period of thaw, the apparent thermal diffusivity is around 2-3 X 10(-7) m(2) s(-1) and increases with depth to reflect the greater soil water content. The maximum thaw depth at the site is typically around 35 cm. However, end-of-season thaw depth has been monitored near Barrow since 1994 and has increased between 1994 and 1998. This warming trend is also reflected in the thawing degree days calculated for the thawed soil volume. A strong correlation exists between maximum annual thaw depth and annual thawing degree days at this site over the period of record. (C) 2001 Elsevier Science B.V. All rights reserved.