Contrasting extreme runoff events in areas of continuous permafrost, Arctic Alaska

摘要

Spring snowmelt floods in the Arctic are common and can be expected every year, mainly because of the extensive snow cover that ablates relatively quickly. However, documentation of extreme flows (both low and high) in the Arctic is lacking in part because extreme flows are relatively rare and gauging sites are very sparse, with most of short duration. In the nested Kuparuk River research watersheds on the North Slope of Alaska, two large summer floods have been observed (July 1999 and August 2002) in the headwaters; these high flows are contrasted to the low flows (drought conditions) observed in the summers of 2005 and 2007. It is clear that the continuous permafrost and the limited near-surface storage in the shallow active layer are responsible for both the high and low flow responses. Or, stated another way, the active layer is a poor buffer to both floods and droughts, when contrasting summer floods with snowmelt floods, it is clear from flood frequency analyses that the smaller, high-gradient headwater basins will be dominated by summer floods while those watersheds draining the low gradient coastal plain will be dominated by snowmelt floods. The two summer floods in the headwaters had flows that were three to four times greater than the largest measured snowmelt flood, while on the coastal plain the 2002 summer storm for the whole of the Kuparuk River only produced the maximum summer runoff of record that was about 1/4 of the maximum snowmelt flood. So, on the coastal plain and even for the Greater Kuparuk River that drains across the coastal plain, snowmelt floods dominate. Drought conditions prevail in summers when the limited surface water storage in the active layer and surface water bodies is depleted because evapotranspiration exceeds precipitation.