An Active Layer Thermal Regime at Barrow, Alaska
Frederick E. Nelson
Department of Geography, University of Delaware, Newark 19716
Kenneth M. Hinkel
Department of Geography, University of Cincinnati, Cincinnati 45221
U.S. Natural Resources Conservation Service, Lincoln, Nebraska 68508
Introduction
Ground temperature has been monitored at the Barrow CMDL facility and adjacent areas of the Barrow Environmental Observatory (BEO) continuously since 1993. Several thermistor strings extending through the active layer to the upper permafrost were monitored at intervals of 2 hours during this period. Extensive probing of the active layer was also performed several times during each summer. To ascertain long-term changes, frozen cores from the active layer and upper permafrost were obtained in 1994 and compared with data collected from the same locations in 1962. Instrumentation was expanded in 1995 to include readings of soil moisture at 2-hour intervals. This report summarizes the results obtained from these activities.
Active-Layer Dynamics
Comparison of results from active-layer determinations at Barrow for the periods 1962-1968 and 1991-1997 reveals a strong consistency in the records from different land-cover units [Nelson et al., 1998a]. The major exception involves locations that experience strong interannual variations in soil moisture content. Plotted by decade (Figure 1) the active layer shows a very strong relationship with accumulated thawing degree days. Interdecadal differences may be an artifact of changes in the stratigraphic position of segregated ice, insulation provided by organic material at the surface, soil moisture conditions, or some combination of these factors. Rather than exhibiting a simple relation with air temperature, the active-layer appears to exhibit Markovian behavior controlled by several processes in the boundary layer.
Fig. 1. Thawing degree-day accumulation versus mean active-layer thickness at a series of plots within the Barrow Environmental Observatory during the periods 1962-1968 and 1991-1997. Regression lines were computed separately for the 1960s and 1990s data [Nelson et al., 1998a].
The ground temperature records from Barrow provide strong evidence that nonconductive processes are important components of the thermal regime. The thermal magnitude of these effects was estimated by Outcalt et al. [1998]. Comparison of the empirical record with results from a numerical model, based on conductive heat transfer with fusion, indicates that evaporative cooling and upward transfer of soil moisture cools summer temperatures in the active layer by several degrees. On an annual basis, these effects cool the average active-layer temperature by 0.4oC.
Sampling and Spatial Relationships
Spatial autocorrelation analysis was performed on active-layer data obtained from a 1 km2 area of the BEO in the summers of 1995 and 1996 [Nelson et al., 1998b].
Topography, soil moisture, and active-layer thickness exhibit similar patterns, but snow-cover thickness varies at higher spatial frequencies and does not appear to affect active-layer thickness significantly over most of the area. Systematic designs involving widely separated sampling locations (e.g., 100 m) are effective on the coastal plain but are inadequate in the foothills of the Brooks Range. Because active-layer thickness is closely related to the flux of trace gases [Waelbroeck et al., 1997] this result has significant implications for studies of carbon dioxide and methane fluxes.
Soil Moisture
Soil moisture data were collected in 1996 and 1997 over the 1 km2 area used to obtain active-layer thickness [Miller et al., 1998]. Statistical analysis of these data indicate that (a) substantial differences occur in soil moisture and thaw depth in dissimilar terrain units; (b) soil moisture and thaw depth are relatively uniform within terrain units; (c) spatial patterns of thaw depth are consistent within the study area on an interannual basis; and (d) patterns of soil moisture and thaw depth do not necessarily show close spatio-temporal correspondence because they fluctuate at different temporal scales. Time series of soil moisture at point locations near the CMDL facility show large variations near the surface in response to precipitation and evaporative drying. The lower part of the active layer remains near saturation throughout the summer (Figure 2).
Fig. 2. Time series of volumetric soil moisture content (%) from a site within the Barrow Environmental Observatory. Hourly values at 10, 25, 40, and 55 cm are shown as traces. Vertical bars represent daily precipitation (cm) at Barrow. Note correspondence between near-surface soil moisture content and precipitation events [Miller et al., 1998].
Temporal Changes in Moisture Content
Comparison of moisture content obtained from the frozen active layer and upper permafrost at Barrow indicates that the latter experienced an increase in ice content between 1962 and 1994 [Hinkel et al., 1996]. This moderate increase from 57% to 62% could be an artifact of heterogeneity over short lateral distances and underscores the importance of carefully executed sampling designs.
References
Hinkel, K.M., F.E. Nelson, Y. Shur, J. Brown, and K.R. Everett, Temporal changes in moisture content of the active layer and near-surface permafrost at Barrow, Alaska: 1962-1994, Arc. Alp. Res., 28, 300-310, 1996.
Miller, L. L., K.M. Hinkel, F.E. Nelson, R.F. Paetzold, and S.I. Outcalt, Spatial and temporal patterns of soil moisture and thaw depth at Barrow, Alaska, Proc., Seventh Int. Conf. Permafrost, Centre d'Etudes Nordique, Université Laval, Québec, in press, 1998.
Nelson, F.E., S.I. Outcalt, J. Brown, and K.M. Hinkel, Spatial and temporal attributes of the active-layer thickness record, Barrow, Alaska, Proc., Seventh Int. Conf. Permafrost, Centre d'Etudes Nordique, Université Laval, Québec, in press, 1998a.
Nelson, F.E., K.M. Hinkel, N.I. Shiklomanov, G.R. Mueller, L.L. Miller, and D.A. Walker, Active-layer thickness in north-central Alaska: Systematic sampling, scale and spatial autocorrelation, J. Geophys. Res., in press, 1998b.
Outcalt, S.I., K.M. Hinkel, F.E. Nelson, and L.L. Miller, Estimating the magnitude of coupled-flow effects in the active layer and upper permafrost, Barrow, Alaska, Proc., Seventh Int. Conf. Permafrost, Centre d'Etudes Nordique, Université Laval, Québec, in press, 1998.
Waelbroeck, C., P. Monfray, W.C. Oechel, S. Hastings, and G. Vourlitis, The impact of permafrost thawing on the carbon dynamics of tundra, Geophys. Res. Lett., 24, 229-232, 1997.