Sensitivity analysis of lake mass balance in discontinuous permafrost: the example of disappearing Twelvemile Lake, Yukon Flats, Alaska (USA)

Many lakes in northern high latitudes have undergone substantial changes in surface area over the last four decades, possibly as a result of climate warming. In the discontinuous permafrost of Yukon Flats, interior Alaska (USA), these changes have been non-uniform across adjacent watersheds, suggesting local controls on lake water budgets. Mechanisms that could explain the decreasing mass of one lake in Yukon Flats since the early 1980s, Twelvemile Lake, are identified via a scoping analysis that considers plausible changes in snowmelt mass and infiltration, permafrost distribution, and climate warming. Because predicted changes in evaporation (2  cmyr−1) are inadequate to explain the observed 17.5 cmyr−1 reduction in mass balance, other mechanisms are required. The most important potential mechanisms are found to involve: (1) changes in shallow, lateral groundwater flow to the lake possibly facilitated by vertical freeze-thaw migration of the permafrost table in gravel; (2) increased loss of lake water as downward groundwater flow through an open talik to a permeable subpermafrost flowpath; and (3) reduced snow meltwater inputs due to decreased snowpack mass and increased infiltration of snowmelt into, and subsequent evaporation from, fine-grained sediment mantling the permafrost-free lake basin.RésuméDe nombreux lacs de haute latitude Nord ont subi des changements substantiels de surface au cours des quatre dernières décades, peut être comme résultat du réchauffement climatique. Dans le permafrost discontinu de Yukon Flats, Alaska intérieur (USA), ces changements ont été non uniformes de part et d’autre de lignes de partage des eaux, ce qui suggère un contrôle local des budgets eau des lacs. Des mécanismes qui pourraient expliquer la décroissance du volume d’un lac sur Yukon Flats depuis le début des années 1980, Twelvemile Lake, ont été identifiés par une analyse étendue qui considère des changements plausibles de la masse de neige fondue et de l’infiltration, distribution du permafrost et réchauffement climatique. Parce que les changement d’évaporation prévus (2 cm/an,) sont non adéquats pour expliquer la réduction de 17.5 cm/an du bilan massique, d’autres mécanismes sont requis. Les mécanismes potentiels les plus importants trouvés incluent: (1) changements dans le flux de nappe superficiel latéral vers le lac éventuellement facilités par une migration gel-dégel de la surface du permafrost dans le gravier; (2) perte accrue de l’eau de lac par flux descendant à travers un talik ouvert vers un chenal perméable sous le permafrost; et (3) recharge réduite par eau de fusion de neige due à la décroissance de la masse du pack neigeux et infiltrations accrue de neige fondue, et évaporation subséquente depuis le manteau de sédiment à grain fin couvrant le bassin du lac libre de permafrost.ResumenMuchos lagos en las altas latitudes nórdicas han experimentado cambios sustanciales en su extensión superficial durante las últimas cuatro décadas, posiblemente como resultado del calentamiento climático. En el permafrost discontinuo de Yukon Flats, en el interior de Alaska (EEUU), estos cambios no han sido uniformes a través de cuencas adyacentes, lo que sugiere controles locales sobre los balances de agua del lago. Los mecanismos que podrían explicar la disminución de la masa en uno de los lagos en Yukon Flats desde los comienzos de 1980, el Lago Twelvemile, se identifican a través de un análisis de observación que considera cambios plausibles en la masa de nieve derretida e infiltración, en la distribución del permafrost y en el calentamiento climático. Debido a que los cambios predichos en la evaporación (2 cm yr−1) son inadecuados para explicar la reducción de 17.5 cm yr−1 observada en el balance de masa, se requieren otros mecanismos. Se encontró que los mecanismos potenciales de mayor importancia involucraban: (1) cambios en el flujo lateral de agua subterránea somera hacia el lago posiblemente facilitado por una migración vertical del nivel del permafrost en las gravas debido a procesos de congelamiento – descongelamiento; (2) incremento de la pérdida del agua del lago como flujo subterránea descendente a través de un talik abierto hacia una trayectoria de flujo en un subpermafrost permeable; y (3) entradas reducidas de agua del derretimiento de nieve debido a una reducción de la masa de la capa de nieve y un aumento de la infiltración de la nieve derretida y subsecuente evaporación hacia los sedimentos de grano fino que recubren la cuenca del lago libre de permafrost.摘要北部高纬度地区的很多湖泊在过去的四十年里经历了可能是由于气候变暖引起的实质性面积变化。在美国阿拉斯加州育空河平原的不连续永冻层,这些变化非均匀地横穿邻近流域,表明对湖水平衡的局部调控。通过考虑貌似可信的融雪水质量、入渗、永冻层分布和气候变暖的变化的域分析,识别能够解释始于80年代早期的育空河平原Twelvemile湖水量减少的机制。因为预测的蒸发量(每年2 cm)变化不足以解释观测到的质量平衡上每年17.5 cm的减少,故存在其他机制。本文发现的潜在重要机制包括:(1)砂砾石中永冻土面的垂向冻融迁移有助于流向湖泊的浅层和侧向地下水径流变化;(2)由于地下水向下流经开放的融区而到达可渗透的永冻层,湖水损失水量增加;(3)由于积雪量减少而入渗的融雪量增加,湖泊融雪量的输入项减少,且随后在细粒的沉积物覆盖的沉积盆地永冻层上发生蒸发。ResumoMuitos lagos situados nas altas latitudes do norte sofreram alterações substanciais na sua área de superfície ao longo das últimas quatro décadas, possivelmente em consequência do aquecimento climático. No permafrost descontínuo de Yukon Flats, no interior do Alasca (EUA), estas alterações têm ocorrido de forma não-uniforme entre bacias hidrográficas adjacentes, sugerindo a existência de fatores locais que controlam os balanços hídricos dos lagos. Identificam-se aqui os mecanismos que poderiam explicar o decréscimo de massa de um lago em Yukon Flats, o Lago Twelvemile, desde o início dos anos 80, através de uma análise abrangente que considera a existência de alterações plausíveis na massa do degelo e na infiltração da água, na distribuição do permafrost e o efeito do aquecimento climático. Uma vez que as mudanças previstas na evaporação (2 cm ano−1) são insuficientes para explicar a redução de 17.5 cm ano−1 observada no balanço de massa do lago, é necessário existirem outros mecanismos. Os mecanismos potenciais mais relevantes parecem envolver: (1) mudanças no escoamento subterrâneo lateral pouco profundo, facilitado possivelmente pela migração vertical da frente de congelamento no permafrost em cascalho, (2) o aumento das perdas de água do lago através da percolação subterrânea ao longo de um talik (local onde o solo não está congelado), até chegar a um caminho de fluxo no subpermafrost permeável, e (3) uma redução das entradas de água do degelo, devido à diminuição da massa acumulada de neve e por causa do aumento da infiltração da água do degelo, e consequente evaporação a partir dos sedimentos finos que existem na parte da bacia do lago livre de permafrost.

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