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Article Excerpt
ABSTRACT. This study reports on changes in the distribution, surface area, and modern water balance of lakes and ponds located in the Old Crow Basin, northern Yukon, over a 50-year period (1951 -2001), using aerial photographs, satellite imagery, a numerical lake model, and stable O-H isotope analysis. Results from the analysis of historical air photos (1951 and 1972) and a Landsat-7 Enhanced Thematic Mapper (ETM+) image (2001) show an overall decrease (-3.5%) in lake surface area between 1951 and 2001. Large lakes typically decreased in extent over the study period, whereas ponds generally increased. Between 1951 and 1972, approximately 70% of the lakes increased in extent; however, between 1972 and 2001, 45% decreased in extent. These figures are corroborated by a numerical lake water balance simulation (P-E index) and stable O-H isotope analysis indicating that most lakes experienced a water deficit over the period 1988-2001. These observed trends towards a reduction in lake surface area are mainly attributable to a warmer and drier climate. The modern decrease in lake levels corresponds well to changes in regional atmospheric teleconnection patterns (Arctic and Pacific Decadal oscillations). In 1977, the climate in the region switched from a predominantly cool and moist regime, associated with the increase in lake surface area, to a hot and dry one, thus resulting in the observed decrease in lake surface area. Although some lakes may have drained catastrophically by stream erosion or bank overflow, it is not possible to determine with certainty which lakes experienced such catastrophic drainage, since an interval of two decades separates the two air photo mosaics, and the satellite image was obtained almost 30 years after the second mosaic of air photos.
Key words: thaw lakes, lake levels, remote sensing, modeling, stable O-H isotopes, Old Crow, northern Yukon
RESUME. La presente etude fait etat des changements caracterisant la repartition, l'etendue et le bilan hydrique contemporain des lacs et des etangs situes dans le bassin Old Crow, dans le nord du Yukon, sur une periode de 50 ans (1951 -2001). L'etude s'est appuyee surdes photographies aeriennes, l 'imagerie satellitaire, un modele numerique des lacs et l'analyse des isotopes stables O-H. D'apres les resultats de l'analyse des photos aeriennes historiques (1951 et 1972) et d'une image par capteur ETM+ (Enhanced Thematic Mapper) de Landsat-7 (2001), il y a eu retrecissement general (-3,5 %) de la surface des lacs entre 1951 et 2001. D'un point de vue general, l'etendue des grands lacs a diminue au cours de la periode visee par l'etude, tandis que celle des etangs a augmente. Entre 1951 et 1972, l'etendue d'environ 70 % des lacs s'est accrue, mais entre 1972 et 2001, l'etendue de 45 % des lacs a diminue. Ces donnees ont ete corroborees au moyen de la simulation numerique du bilan hydrique des lacs (indice P-E) et de l'analyse des isotopes stables O-H, qui ont laisse entrevoir que la plupart des lacs ont enregistre un deficit en eau au cours de la periode allant de 1988 a 2001. Les tendances de reduction de la surface des lacs qui ont ete observees sont principalement attribuables a un climat plus chaud et plus sec. La diminution contemporaine du niveau des lacs correspond bien aux changements caracterisant les modeles regionaux de teleconnexion atmospherique (oscillations decadaires arctiques et pacifiques). En 1977, le climat de la region est passe d' un regime a predominance fraiche et humide (associe a I'augmentation de la surface des lacs de la region) a un regime chaud et sec, ce qui s'est traduit par la diminution de la surface des lacs qui a ete observee. Bien que certains lacs puissent avoir ete draines de maniere catastrophique en raison de I'erosion des cours d'eau ou du debordement des rives, il est impossible de determiner avec certitude quels lacs ont ete la cible d'un assechement si catastrophique puisqu'un intervalle de deux decennies separe les deux mosai'ques de photographies aeriennes, et que l'image satellitaire a ete obtenue presque une trentaine d'annees apres la deuxieme mosaique de photo aerienne.
Mots cles: lacs thermokarstiques, niveaux des lacs, teledetection, modelisation, isotopes stables O-H, Old Crow, nord du Yukon
Traduit pour la revue Arctic par Nicole Giguere.
INTRODUCTION
During the last few decades, the Canadian Arctic has undergone significant hydrological changes. Increasing air temperatures, in both summer and winter, have led to the melting of some small Arctic glaciers (Dowdeswell et al., 1997; Overpeck et al., 1997; Miller et al., 2004), a reduction in sea ice cover (Fisher et al., 2006), an increase in river discharge, and a longer open-water season for rivers and lakes (Peterson et al., 2002; Prowse and Carter, 2002; Duguay et al., 2006). These hydrological changes tend to be in the same direction; however, studies that examined modern changes in the distribution and surface area of lakes in the Arctic have shown that the direction of change is ambiguous (e.g., Frohn et al., 2005; Hinkel et al., 2005, 2007; Smith et al., 2005). Regions located in the discontinuous permafrost zone have shown a substantial decrease in lake surface area (Osterkamp et al., 2000; Yoshikawa and Hinzman, 2003; Christensen et al., 2004; Smith et al., 2005), whereas in regions with continuous permafrost, most studies have found little long-term trend in lake extent (Riordan et al., 2006). For example, Smith et al. (2005) reported an increase in the continuous permafrost of Siberia, but this study was based on only two dates (1973 and 1998) of relatively coarse resolution satellite imagery. The increase in lake surface area could be due to either a wet year in 1973 or a dry year in 1998 in that region. Smol and Douglas (2007) did document substantial lake shrinkage in the High Arctic, and Plug et al. (2008), who discerned no real trend in lake surface area in the Tuktoyaktuk Peninsula, found that lake extent was mostly related to the interannual variability in precipitation. In fact, most of the recent hydrological changes observed at high latitudes correlate better with regional atmospheric pressure differentials (such as the Arctic Oscillation, the North Atlantic Oscillation, the Pacific Decadal Oscillation, and the North Pacific indices) than with air temperature alone (Hurrell, 1995; Thompson and Wallace, 1998; Peterson et al., 2002; Bonsai et al., 2006). The reason is that atmospheric pressure differentials affect air temperatures, regional storm patterns (precipitation), prevailing winds and evaporation--in other words, the regional climate--at various temporal scales.
In the continuous permafrost zone of the western Canadian Arctic, several lacustrine plains host thousands of lakes that provide breeding habitats for aquatic animals, waterfowl, and caribou herds. Three of the largest are the Old Crow, Bluefish, and Bell-Driftwood basins located in the northern Yukon (Fig. 1). Recent aerial and ground reconnaissance surveys of these basins revealed that several large lakes (e.g., Zelma and Netro lakes) are experiencing important declines in water levels. For example, Wolfe and Turner (2008) document the rapid drainage of Zelma Lake in the Old Crow Basin over a three-week period in July 2007. The sudden drainage of the lake (a loss equivalent to 5.8 million [m.sup.3], or a 43% loss in lake surface area) was not due to warmer air temperature or increased evaporation, but rather to March-May precipitation much greater than the long-term average, which triggered the erosion of an outlet channel and subsequent drainage of the lake through that channel into the adjacent basin. Given that this reduction in lake surface area contrasts with what has been observed for lakes in other regions of the continuous permafrost zone, it is not known if this event can be extrapolated to the majority of lakes in the region, or if it represents a unique event that happened when...
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