Ninth International Conference on Permafrost ... - IARC Research

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Surface Offsets and N-Factors Across Altitudinal Tree Line,Wolf Creek Area, Yukon Territory, CanadaEmily A. SchultzDepartment of Geography, University of Ottawa, Ottawa, CanadaAntoni G. LewkowiczDepartment of Geography, University of Ottawa, Ottawa, CanadaIntroductionSite-specific conditions that control ground heat flowand the surface temperature regime play an important rolein determining permafrost occurrence in the discontinuouszone (Smith & Riseborough 2002). These include depth ofsnow, soil properties, and vegetation cover. As it is currentlyimpractical to fully evaluate the effects of these variables onthe energy balance in mountain basins, climate-permafrostrelations must be simplified. One method that has recentlybeen attempted is the TTOP model (Juliussen & Humlum2007), which describes the relationship between the meanannual air temperature and the temperature at the top ofpermafrost in terms of the surface and thermal offsets (Smith& Riseborough 2002). Key components of this model aren-factors, which relate air and ground climate by establishingthe ratio between air and surface freezing (winter) and thawing(summer) degree-days, thus summarizing the surface energybalance on a seasonal basis. Here we examine surface offsetsand freezing and thawing n-factor variability at a number ofsites through altitudinal tree line in the southern Yukon.Study Area and MethodsAir and ground surface temperatures were measuredhourly at 10 sites at elevations ranging from 1011–1640 ma.s.l. in and around the Wolf Creek research basin (60°32′N135°13′W) near Whitehorse. This area is located within thesporadic discontinuous permafrost zone (Heginbottom etal. 1995), but permafrost is predicted to be extensive above1400 m and continuous on mountain summits (Lewkowicz &Ednie 2004). The basin spans 3 major ecological zones thatare mainly related to elevation: boreal forest at elevationsbelow 1100 m, sub-alpine taiga/shrub tundra between 1100m and 1500 m and alpine tundra above 1500 m (Francis et al.1997). The measurement sites covered all 3 zones.Instrumentation at each site consisted of an Onset HoboPro 8 datalogger equipped with an internal and externalthermistor (accuracy of ±0.2°C). The body of each logger,containing an internal thermistor, was installed just belowthe ground surface while the external sensor was mountedwithin a solar radiation shield to measure air temperature ata height of 1.6 m.Data presented here were collected from October 2003 toAugust 2006, comprising 3 freezing seasons and 2 thawingseasons. Data, collected at site on a palsa in the middle of thebasin since April 2001, provide 2 additional freezing and 2thawing seasons for analysis.Snowpack development over the winter was trackedin 2005–06 using iButton miniature loggers, installed onstakes at 10–20 cm intervals (10, 20, 30, 40, 60 and 80 cm)at most of the logger sites. Temperatures were recorded bythe iButtons sensors at 4-hour intervals. Differences in thetemperature readings of iButtons above and below the snowwere used to determine snow depths.ResultsSurface offsetsSurface offsets varied from 0.4° to 3.6°C (Table 1).Offsets for the forested and alpine tundra sites fall within thehigh and low parts of this range, respectively, while thosemeasured at the sub-alpine taiga/shrub tundra sites coveralmost the entire range. The smallest range in values wasrecorded at the forested sites (1.5°C), slightly smaller thanat the alpine tundra sites (1.7°C), where snow accumulationis typically lowest.Table 1. Surface offsets and n-factors at the study sites.Site /Elevation Veg. Zone(m)T21011 borealT3 forestMax. SnowDepth(2005–06)70±9 cm1145 70±9 cmT41290>80 cmP91254CR11260CR21271CR31363CR41457CR51495Bowl1640sub-alpinetaiga/shrubtundraalpinetundra50±9 cm70±9 cm50±9 cm50±9 cm45±14 cmSurfaceOffset(°C)2.92.13.63.63.13.03.82.33.03.33.42.42.72.42.83.23.61.21.02.20.91.42.1nfnt0.290.49 0.920.120.36 0.920.230.35 0.830.39 -0.57 1.050.45 1.080.26 0.99- -0.490.360.560.400.310.410.73-0.550.700.520.51-0.910.92-0.801.01-0.95--0.860.87- 0.4 - -- 2.2 0.60 -273
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tN-factorsFreezing factors (nf) over the study period ranged from 0.12to 0.73 (Table 1). Figure 1 demonstrates that nf values exhibita broad relationship with both elevation and vegetation.Values for the forested sites were all 0.50. Freezing factors for the sub-alpine taiga/shrub tundrazone point to the role of variable vegetation and snow coverclose to and below tree line, with values ranging from 0.23 to0.73 at these sites. Values of nf measured at individual sitesexhibited significant interannual variability with increases ofmore than 0.20 at some sites between 2004–05 and 2005–06. Snow accumulation differences likely account for thispattern. Whitehorse recorded a maximum of 47 cm of snowon the ground in 2004–05, but a maximum of only 22 cm in2005–06 (Environment Canada 2008).Thawing factors (nt) ranged from 0.80 to 1.08, a muchsmaller range than for nf (Table 1). Values of nt calculatedfor the forested sites over the study period were both 0.92,slightly above those calculated for the tundra site (0.86 and0.87). Sites where nt was greater than 1.0 were all situatedabove 1200 m, and all are located within the sub-alpinetaiga/shrub tundra zone. Along the Mount Sima trail, there isa clear transition in nt associated with tree line, with thawingfactors at the two forested sites (T2 and T3) nearly 0.10higher than at the sub-alpine site (T4) in 2005.Discussion and ConclusionsOur results support the findings of Karunaratne and Burn(2004) and the modeling of Smith and Riseborough (2002)that snow depth is the most important variable influencing nf,because the low thermal conductivity of snow restricts heatloss from the ground surface. Nt values are higher and varyless. They may relate to land cover as suggested by Klene etal. (2001) who obtained the highest values on bare groundElevation (m)17001500130011009000.00 0.20 0.40 0.60 0.80 1.00 1.20N-factorFigure 1. Relationship between elevation, vegetation and seasonaln-factors at the study sites (2001–2006). Freezing (filled) andthawing (outlined) factors are shown for the boreal forest(diamond), sub-alpine taiga (square) and alpine tundra (triangle)vegetation zones.and wet tundra sites. Similarly, Taylor (1995) calculated thehighest values of nt at open sites and lowest at shaded sites,predominantly those located within the forest.The relationship between surface offset, n-factors,elevation, and vegetation in the study area, therefore,demonstrates that trends previously identified in relation tolatitudinal tree line also apply to the altitudinal tree line.AcknowledgmentsThis research was supported financially by the CanadianFoundation for Climate and Atmospheric Sciences, NSERC,the Northern Scientific Training Program (INAC), the GeologicalSurvey of Canada, and the Yukon Geological Survey.Logistical support was given by Water Resources Branch, YukonGovernment. Field assistance was given by Jim Coates,Phil Bonnaventure, Pauline Favero, and Martina Knopp.ReferencesEnvironment Canada. 2008. National Climate Archive[online]. http://www.climate.weatheroffice.ec.gc.ca(Accessed February 9, 2008).Francis, S., Smith, S. & Janowicz, R. 1997. Intégration desdonnées et la zonation écologique du basin versantdu ruisseau Wolf. In: J.W. Pomeroy & R.J. Granger(eds.), Wolf Creek Research Basin: Hydrology,Ecology, Environment. Saskatoon: National WaterResearch Institute, 97-104.Heginbotton, J.A., Dubreuil, M.A. & Harker, P.T. 1995.Canada-Permafrost. In National Atlas of Canada,5 th ed. Ottawa: National Atlas Information Service,Natural Resources Canada. Plate 2.1 MCR 4177,1:7,500,000.Juliussen, H. & Humlum, O. 2007. Towards a TTOP GroundTemperature Model for Mountaneous Terrain inCentral-Eastern Norway. Permafrost and PeriglacialProcesses 18: 161-184.Karunaratne, K.C. & Burn, C.R. 2004. Relations between airand surface temperature in discontinuous permafrostterrain near Mayo, Yukon Territory. Canadian Journalof Earth Sciences 41: 1437-1451.Klene, A.E., Nelson, F.E. & Shiklomanov, N.I. 2001. Then-factor in natural landscapes: variability of air andsoil-surface temperatures, Kuparuk river basin,Alaska, USA. Arctic, Antarctic and Alpine Research33: 140-148.Lewkowicz, A.G. & Ednie, M. 2004. Probability mappingof mountain permafrost using the BTS method, WolfCreek, Yukon Territory, Canada. Permafrost andPeriglacial Processes 15(1): 67-80.Smith, M.W. & Riseborough, D.W. 2002. Climate and limitsof permafrost: A zonal analysis. Permafrost andPeriglacial Processes 13: 1-15.Taylor, A.E. 1995. Field measurements of n-factors fornatural forest areas, Mackenzie Valley, NorthwestTerritories. Current Research 1995-B, GeologicalSurvey of Canada, 89-98.274
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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