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[1] The Arctic land surface water balance plays an important role in regulating the planetary heat balance and global ocean circulation. Lakes and wetlands are common features in the low-gradient Putuligayuk River watershed in northern Alaska, with important implications for the annual water balance. Evapotranspiration exceeds precipitation over the summer, and there is a gradual reduction in wetland extent. Total inundated area derived from RADARSAT ScanSAR synthetic aperture radar images throughout 1999 and 2000 varied from 15 to 67 percent of the 471 km 2 watershed. The hydrological system becomes disconnected within 2 weeks of snowmelt, and overland flow largely ceases. End-of-winter snow water equivalent and discharge during the melt period for 1999, 2000, and 2001 were used to estimate that between 30 and 37 mm (24-42 percent) of snow meltwater serves to recharge the evaporation deficit of the previous summer. The percent of snowmelt entering storage is dependent on the available surface storage.

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Rainfall-generated floods in the Arctic are rare and seldom documented. The authors were fortunate in July 1999 to monitor such a flood on the Upper Kuparuk River in response to a 50-h duration rainfall event that produced a watershed average in excess of 80 mm. Atmospheric conditions prevailed that allowed moist air to move northward over areas of little or no vertical relief from the North Pacific Ocean to the Arctic Ocean. Cyclogenesis occurred along the quasi-stationary front separating maritime and continental air masses along the arctic coast. This low-pressure system propagated southward (inland) over the 142-km 2 headwater basin of the Kuparuk River in the northern foothills of the Brooks Range; a treeless area underlain by continuous permafrost. This research catchment was instrumented with a stream gauging station, two major and six minor meteorological stations, for a total of eight shielded rain gauges. The peak instantaneous flow was estimated at 100 m 3 s Ϫ1 and was about 3 times greater than any previously measured flood peak. Historically in the Arctic, annual peak floods occur following snowmelt when the snowpack that has accumulated for 8-9 months typically melts in 7-14 days. The shallow active layer, that surficial layer that freezes and thaws each year over the continuous permafrost, has limited subsurface storage when only thawed to a depth of 40 cm (at the time of the flood). Typically for this area, the ratio of runoff volume to snowmelt volume is near 0.67 or greater and the ratio for cumulative summer runoff and rainfall averages around 0.5 or greater. For the storm discussed here the runoff ratio was 0.73. These high runoff ratios are due to the role of permafrost limiting the potential subsurface storage and the steep slopes of this headwater basin.

Evapotranspiration (ET) vies with runoff as the primary mechanism for water loss from a watershed underlain by permafrost, yet past attempts to predict ET have proven to be less than completely successful in the Arctic. Imnavait Creek, a small 2.2 km2 watershed underlain by continuous permafrost has been studied for 4 years. Evapotranspiration on a watershed scale has been calculated from water balance studies. These results are compared with point measurements of pan evaporation and daily estimates of ET by the energy balance and Priestley-Taylor methods. Since it is difficult to determine the daily change in soil moisture, the energy balance approach appears to be the best method to determine daily ET. The water balance approach is the best method to determine total ET over the course of the summer because it is possible to delete the soil moisture term due to an insignificant change annually in this watershed. Priestley-Taylor gave adequate estimates of ET with only limited data. After a pan coefficient is determined, the evaporation pan functions well over extended time periods but is less accurate for shorter periods. Evapotranspiration is greatest in early summer, immediately following the spring snowmelt, during the period of maximum incoming radiation but not necessarily maximum air or soil temperatures. The cumulative potential evaporation is greater than the cumulative summer precipitation. The source of moisture for ET in early summer is from snowmelt or moisture stored in the active layer.