CPCRW Snowmelt 2000 Image Courtesy Bob Huebert / ARSC

CPCRW April 14, 2000

CPCRW April 29, 2000

Snow Surveys, Caribou-Poker Creeks Research Watershed Alaska William R. Bolton Spring snowmelt is usually the major hydrologic event of the year although rainfall generated events may produce the annual peak flow. Snow precipitation accounts for approximately 30-40% of the yearly total incoming precipitation, accumulating from September or October until about mid April. Significant mid-winter melt events are rare. Extensive snow surveys (19 sites) were conducted in the Caribou-Poker Research Watershed (CPCRW) from 13-16 March 2000 to measure the maximum snow water equivalent throughout the watershed. Snow measurements conducted include snow depth, snow water equivalent, and snow ablation. Snow water equivalents were measured using an Adirondack snow sampler. At each site, 10 snow water equivalent and 50 snow depths measurements were averaged using the combination technique of Rovansek et al (1993). The extensive snow surveys were then followed by periodic measurements at the CRREL meteorologic station through the completion of snowmelt. The CRREL meteorologic station is located near the confluence of Caribou and Poker Creeks, in a relatively wide and flat valley bottom.

The 2000 snowmelt was characterized by warm days and cold nights The 2000 snowmelt was characterized by warm days and cold nights. From 4-8 April and from 16-19 April 2000, temperatures remained consistently at or below 0ºC, with temperatures as cold as –20ºC. Following these cold spells, daytime temperatures of +10ºC and night temperatures of –5ºC were common. The result of these large diurnal differences is melting of the snow pack during the day and a refreezing during the night. Diurnal fluctuations, reflecting this snowmelt pattern, in the hydrograph in the C4 sub-watershed (located approximately 1 km from the CRREL meteorologic station) were also observed. Because of the relatively open area (open black spruce canopy) of the CRREL meteorologic station, complete ablation of the snowpack occurred before many other areas in CPCRW. Snowmelt is delayed along north-facing hill slopes, which are shielded from direct solar radiation, compared to the more open CRREL snow survey location. Permafrost in CPCRW is discontinuous, generally found along north facing slopes and valley bottoms. Permafrost plays an important role in the hydrology in sub-arctic watersheds. Ice-rich conditions at the permafrost table do not allow surface percolation, resulting in decreased response time to precipitation events, limited subsurface storage, and low base flows compared to permafrost-free areas.  

Near the completion of snowmelt in these protected sub-watersheds, a large rain event of 32 mm occurred (20-25 May 2000). This partial rain-on-snow precipitation event, compounded by a shallow, saturated permafrost active layer, resulted in extremely high flows in all the streams in CPCRW.

Snowpack Ablation, 2000 CPCRW, Alaska

Snowpack ablation during spring, 2000 displays remarkable variability among the many sites where snowmelt processes were monitored.

William R. Bolton, Water and Environmental Research Center, University of Alaska Acknowledgements Funding for this research was provided by the National Science Foundation Long Term Ecological Research Program, Grant Number DEB-9211769.