Accuracy of Snowpack Measurements at Watershed Scale

Sveta Berezovskaya and Doug L. Kane
Water and Environmental Reseach Center, University of Alaska, Fairbanks

An experiment in northern Alaska was carried out to evaluate the accuracy of snow water equivalent (SWE) estimations in tundra snowpack. In northern basins, water held in the snowpack contributes significantly to both seasonal and annual water balances. It is critical to realize and address the problems of measuring and processing observational snow data so that the data can be used properly to advance our understanding of hydrological systems. The standard method of obtaining SWE is by gravimetric measurement using a sample core. To capture spatial snowpack variability, a snow survey usually includes both gravimetric SWE sampling and snow depth measurements collected over a large area. This technique is referred to as “double sampling”. A combination of well-developed depth hoar at the base of tundra snowpacks and extensive surficial organic soils in permafrost regions can significantly affect snow water equivalent and snow depth sampling accuracy. The experiment in the Imnavait Creek area shows that the depth of tundra snowpack is typically overestimated, because low density organic material (overlaying impermeably frozen ground) is often incorporated into the snowpack depth. When observers probe the snow depth, it is difficult to recognize the snow-ground interface, because the organic layer offers similar resistance to the probe that the snow does. Error is larger for the sedge tussocks areas on the windward slopes with shallow snow cover and decreases toward the valley bottom due to the snow-river ice interface at the bottom of the snowpack. In April 2006, the average snow depth in the Imnavait basin was overestimated from 11 to 31%. It suggests that snow water equivalent from any type of double sampling technique tends to be overestimated. Whereas snow depths show a systematic overestimation error, SWE obtained with snow cores is underestimated. The difficulty in these interpretations is that actual, accurate SWE is unknown. We noticed that the difference between basin average snow core SWE and double sampling SWE varies from 4 to 20%. The true SWE should lie somewhere in between.

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