1 INTRODUCTION

In autumn 1997, a snow fence was installed east of thevillage of Barrow (71°20�N, 156°40�W). Residents inthe newer development of Browerville had complainedof deep and extensive drifting that often covered theirdoors and windows. The snow fence is situated about1 km upwind from the development and is designed tocapture snow driven by the prevailing easterly winterwinds. The wooden plank structure is about 4 m highand extends approximately 3.2 km from Middle SaltLagoon south to near Footprint Lake. It is located about200 m east of Cakeater Road (Fig. 1).

Anecdotal information suggests that a large driftformed just downwind of the snow fence in winter1997–98. The lee drift extended 100 m from the fencefor nearly its entire length and, in some places, reachedthe height of the fence. A smaller drift also developedupwind of the fence. The lee drift was still present thefollowing June, although the surrounding tundra wasfree of snow. The thick accumulation resisted meltingand the drift persisted until mid-July. In August, itappeared that the tundra beneath the melted snowdriftwas significantly wetter and greener than the surround-ing unaffected tundra, and there were indications thatground subsidence had occurred (Fig. 2).

To determine the impact of deep snow drifting onsoil temperature, thermistors were installed at depthsof 5 and 25 cm at several sites near the snow fence in

October 1998. Three sites were established along aDRIFT transect perpendicular to the snow fence anddrift crest: near the base of the snow fence (1 m down-wind) where scouring had been observed the previouswinter, 27 m downwind where drift thickness was max-imized, and 114 m downwind where a lee scour zonedeveloped. Three additional temperature monitoringsites were established along an unaffected CONTROLtransect 300 m south of the snow fence. Beginning on 10 October 1998, temperature was recorded everytwo hours and is plotted in Figure 3. Because the

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Impact of snow fence construction on tundra soil temperatures at Barrow, Alaska

K.M. HinkelDepartment of Geography, University of Cincinnati, Cincinnati

J.G. BockheimDepartment of Soil Science, University of Wisconsin, Madison

K.M. PetersonDepartment of Biological Sciences, University of Alaska, Anchorage

D.W. NortonArctic Rim Research, Fairbanks

ABSTRACT: In autumn 1997 a snow fence was constructed near Barrow. The wooden plank structure is 4 mhigh and extends 3.2 km. In October 1998, soil temperature loggers were installed at depths of 5 and 25 cm at sev-eral sites near the snow fence and in an unaffected control region. Snow thickness is measured along surveyedtransects each spring, and thaw depth is measured along the same transects in August. Typically, the drift exceeds4 m in height and extends �50 m downwind from the fence for nearly its entire length. Near-surface soil temper-ature beneath the drift averages 8°C warmer during the winter compared to the control. Soil in scour zones beginsto thaw in late May owing to the thin or absent snow cover, and experiences relatively deep summer thaw. Thawdepth beneath the drift is relatively shallow because the drift persists 4–10 weeks, thus delaying soil thaw initia-tion. There is some evidence to suggest ground subsidence of the ice-rich permafrost is occurring, but the effectsof thaw consolidation cannot be detected by surface probing. A program of high-resolution surveying, using dif-ferential GPS methods, is being implemented.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Figure 1. View of snow fence and drift in August 2001,looking south. Middle Salt Lagoon in foreground.

temperature traces for the three CONTROL sites werevery similar, the average for the three is plotted. Further,since the soil temperature at 25 cm is nearly identicalto the temperature at the 5 cm depth during winter,only the upper trace is plotted.

In mid-May 1999, snow thickness was measured at 5 m intervals along the DRIFT and CONTROL tran-sects. The drift crest exceeded 4 m in height, while theaverage snow thickness in undisturbed tundra wasabout 40 cm. In mid-August 1999, thaw depth was mea-sured at 2.5 m intervals along the same transects. Thesedata are shown in Figure 4.

2 SITE DESCRIPTION

Barrow has a cold maritime climate. Winters are long,dry, and cold, and summers are short, moist, and cool.The mean annual air temperature is �12.6°C; July isthe warmest month at 4.1°C, and February is the coldestat �27.7°C (NOAA 1996). Mean annual precipita-tion is 124 mm, 37% of which falls as rain during Julyand August.

Five major land-cover types are recognized in theBarrow region; these include dry heath, dry meadow,moist meadow, wet meadow, and emergent aquatic tun-dra vegetations (http://www.cevl.msu.edu/ael/posters/ikonoslandcover.html).

Soils of the Barrow region are underlain by per-mafrost within 0.5 m of the surface, and are classifiedas Gelisols. Three suborders occur in the region.Turbels, or soils affected by frost mixing (cryoturba-tion), comprise 77% of the soils mapped. Orthels aremineral soils unaffected by cryoturbation and accountfor 9% of the area, while organic soils (Histels) makeup about 1%. Modern beach sediments, considered amiscellaneous land type, cover 4% of the area and wateran additional 9%. Typic Aquiturbels are the dominantsoil subgroup, covering 55% of the area.

3 SIGNIFICANCE AND POTENTIAL IMPACTS

Increasing local snow cover thickness can have severaleffects. First, since snow is an effective thermal insula-tor, ground heat loss in winter is reduced and soil tem-peratures are not as cold. As a result, soil warming andthaw in summer requires less energy as the systementhalpy is higher, and more heat energy is available towarm the soil. Thus, there is an overall tendency forthe average annual soil temperature to increase. Inregions underlain by permafrost, the active layer wouldbecome deeper as the upper permafrost melts. Meltingof supersaturated permafrost results in ground subsi-dence and thermokarst.

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Figure 2. View along the leeward drift crest in August 1999showing ponds possibly caused by thaw subsidence.

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Figure 3. Soil temperature at 5 cm depth measured every2 hr beginning 10 October 1998. The trace labeled CON-TROL is the average of measurements made at the threecontrol sites.

FenceDrift Transect14 May 1999

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Figure 4. Snow thickness and thaw depth (cm) near snowfence. Open circles indicate thaw depth measured beneathstanding water; missing circles are used where deep waterprohibited measurement. East (upwind) is to the left.

Field evidence supporting this scenario was reportedby Nicholson (1978) for the Schefferville mining area(55°N) in the center of the Nouveau-Quebec-LabradorPeninsula. In an effort to cause permafrost degradationand increase the thickness of the active layer, snowfences were installed that increased snow cover thick-ness from �10 to �90 cm. Over a 5-year period, theactive layer progressively deepened; by the end of the experiment it was 2.5 times (6.5 m) deeper than thecontrol. The effect was largely attributable to warmersoil conditions in winter.

However, there is some evidence to indicate that drift-ing can also counteract soil warming (e.g. Walker et al.2001). Thick accumulations of snow persist severalweeks or months after the surrounding tundra is snowfree. Thus, soil warming can be delayed and active layerthickness reduced.

Resolving these opposing effects requires examina-tion of the direct and indirect impact of deep snowdrifting on the soil thermal regime. For example, snowaccumulation influences the transient properties of theunderlying soil. The melting snow promotes waterponding at the ground surface and enhances meltwaterinfiltration into the ground. Previous research (Hinkelet al. 1993, 1997, 2001, Kane et al. 2001) has shownthat meltwater percolating into the frozen groundthrough pores and contraction microcracks causesrapid warming of the near-surface soil. In addition, asair is displaced by water in soil pores, the soil thermalconductivity increases and the potential downward soilheat flux is enhanced (Farouki 1981).

To date, there is no general agreement on the long-term influence of deep snow drifting on the groundthermal regime. Our experience at sites on the NorthSlope of Alaska suggests that deep drifting tends topromote melting of the upper permafrost in regionswhere the substrate is ice rich, resulting in ground sub-sidence and thermokarst. The snow fence at Barrowprovides an opportunity to monitor the soil thermalimpact from the inception of the disturbance. Since thesilty substrate around Barrow is extremely ice-rich,exceeding 70% in the upper 2 m (Brown & Johnson1965), the ground is highly susceptible to thaw subsi-dence. Furthermore, a change in the soil moistureregime and plant community are likely outcomes overa fairly short time period (Scott & Rouse 1995).

4 SOIL TEMPERATURE RECORDS

The average temperature trace at the 5 cm depth forthe three CONTROL plots is shown in Figure 3. Mini-mum temperatures of around �20°C occur in February,and thaw begins in mid-June. Significant damping ofthe thermal signal is observed, but the effects of syn-optic events are apparent.

Soil temperature beneath the DRIFT crest (27 m)reached a minimum of �8°C; this occurred in earlyJune. The minima is followed by a very rapid increasein soil temperature beginning 10 June, from �6.27to �1.56°C (�4.71°C) in less than two days. This rateof change far exceeds the average, and likely reflectsthe thermal impact of snow meltwater infiltration. Soiltemperature gradually warmed toward 0°C through-out the summer, but soil thaw did not begin until thedrift ablated in mid-July. Synoptic effects are absentfrom the winter record.

Conversely, the fence and lee scour zones experi-enced temperatures colder than �30°C in February.Furthermore, the high-frequency variations in the tem-perature traces indicate little thermal damping.Separation of the scour traces in early winter is due tothe difference in the snow thickness at these sites. Windscouring and soil exposure results in very similar tem-perature traces after March. Thaw began at the snowfence scour zone around 1 June; the record for the leescour zone (114 m) ends at this time due to vandalism.

The thermal impact of the snow can be quantified bysubtracting the mean daily temperature (MDT) at the 5 cm level at each DRIFT site from the MDT of the CONTROL; positive values indicate that the soilbeneath the drift is warmer. Figure 5 suggests that drift-ing began near the end of November when soil temper-ature beneath the drift is substantially warmer (�10°C)than the control; this is confirmed by National WeatherService data indicating a blizzard at this time. Over theperiod October–June, the soil temperature beneath thedrift was, on average, 8°C warmer than the soil alongthe control.

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Soil warmer than control

Soil colder than control

Figure 5. Temperature residuals using mean daily temper-ature (MDT) of the CONTROL at the 5 cm level; positivevalues indicate that the soil beneath the drift is warmer.

Scouring beneath the fence resulted in substantiallycolder (�3.4°C) soil temperatures, which persistedthroughout the winter. The lee scour zone demonstratesdrifting effects, following by scouring in late April.

5 PATTERNS OF GROUND THAW

The pattern of thaw depth collected in mid-August (Fig.4) indicates relatively shallow thaw beneath the snowdrift and enhanced thaw in the fence and lee scourzones. This implies that heavy drifting stabilizes per-mafrost by reducing the length of the thaw season, whilescouring promotes deeper thaw and permafrost melting.

As shown in Figure 2, the area affected by driftingappears very wet. Pools were too deep in several placesto collect thaw depth measurements. In some locations,standing water may be due to destruction of the protec-tive organic mat during fence construction. However,given the preferential occurrence of saturated soil con-ditions beneath the drift crest along most of the fencelength, it is possible that permafrost melting and subsi-dence has occurred. Since surface probing cannot detectsubsidence effects, accurate surveying using differentialGPS in subsequent summers will be used to monitorground subsidence in the control plots.

6 ONGOING STUDIES

In summer 1999, additional data loggers were deployedto collect soil temperature measurements at depths of 5,30, and 50 cm from twelve sites affected by drifting.Data loggers, powered by long-life batteries, were placedin waterproof containers and buried to prevent furtherdamage by vandalism. Measurements of soil moisturewere also collected to determine if the soil is becomingwetter. An extensive program of soil and vegetationsampling and mapping was conducted near the fenceand in the control region to monitor the alteration ofsoil properties and plant communities in response tochanges in the soil temperature and moisture regimes(Scott & Rouse 1995).

Soil temperature measurements have been collectedcontinuously since that time, although the temperaturedata has not been downloaded since logger installation.Early snowfall during field visits effectively concealedlogger location in autumn 2000. The snow drift thatdeveloped in winter 1999–2000 was similar to that ofthe previous winter. However, winter 2000–2001 wascharacterized by heavy snow and strong winds, and thedrift that formed has covered logger containers sincethat time.

By April 2001, the drift exceeded 4 m in height andhad completely buried the snow fence along most of

its length. Compared to previous years, it was sub-stantially wider both upwind and downwind from thefence. The leeward drift contained about 60% moresnow (by volume) than previous years – although thesnowcover on the tundra was thinner (32 cm) than pre-viously measured at the control plots (Fig. 1). Replicatemeasurements collected in June 2001 indicated littledrift ablation (�50 cm) over the previous two months.The summer of 2001 was significantly cooler thannormal; the average temperature for the period June-August was 0.3°C compared to 3.2°C in 2000. Bymid-August, it became apparent that the drift remnantwould persist throughout the summer. Infiltration ofsnow meltwater, ponding atop the frozen ground, andsubsequent recrystallization produced a slab of iceaveraging 0.5 m in thickness and about 50 m in width.Thus, it was not possible to recover the temperaturedata from most loggers. It will be interesting to see ifthe drift melts in subsequent years, or if changes to themicroclimate result in the net accumulation of massover time.

There is some visual evidence that ground subsi-dence has occurred in recent years, and particularly in response to the formation of the large drift in2000–2001. The vertical wooden planks originallyextended downward nearly to the tundra surface. Bysummer 2001, there was a gap below the planks ofabout 40 cm along most of the fence length. Similarly,the 25 cm diameter vertical fence support pipes showevidence of corrosion extending 40 cm above the cur-rent soil level, presumably due to contact with wetsoil. It is therefore surmised that the ground has sub-sided about 40 cm since fence installation. Comparisonwith photographs taken soon after fence installationsupports this interpretation.

7 CONCLUSIONS

The drift developed primarily during blizzard eventsand exceeded 4 m in height, while the average snowthickness in undisturbed tundra was about 40 cm. Soiltemperatures beneath the drift averaged about 8°Cwarmer during the winter compared to the control,while soil temperature in the scour zones were about3°C colder. The scour zones began to thaw in late Mayowing to the thin or absent snow cover, and experiencedrelatively deep summer thaw. Thaw depth beneath thedrift was relatively shallow because the drift persisteduntil mid July. However, measuring thaw depth by sur-face probing can yield misleading results since thismethod cannot detect the effects of thaw consolida-tion in ice-rich permafrost. High precision surveying,using differential GPS methods, is currently under-way to monitor ground subsidence in the area imme-diately affected by drifting and scoring.

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ACKNOWLEDGEMENTS

This research was supported by grants from theNational Science Foundation to KMH (OPP-9529783,9732051, and 0094769). Any opinions, findings, con-clusions, or recommendations expressed in the mate-rial are those of the authors and do not necessarilyreflect the views of the National Science Foundation.We are grateful to the Ukpeagvik Inupiat Corporationfor access to the Barrow Environmental Observatory,and the Barrow Arctic Science Consortium for admin-istrative and logistic assistance.

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