Snow pillows: Use and verification

Snowmelt yields a substantial contribution to spring floods in Norway. The most severe floods, such as in southeastern Norway in 1995, are fed from snowmelt over extensive snow-covered high-mountain areas. To monitor the temporal evolution of the snow mass during winter and spring, a network of 23 snow pressure pillows has been established in Norway, covering 58 deg N to 71 deg N, 6 deg E to 28 deg E, and 30-1,400 m a.s.l. Hourly data are supplied twice daily to national authorities. To investigate the performance of the snow pillows, manual snow surveys, a snow accumulation-ablation model, and nearby meteorological data were analyzed for the 1998-1999 winter and spring periods. The results suggest that snow pillows and the snow model represent the accumulation period well. During the melt period, both the snow pillows and the snow model produced data in agreement with manual surveys, but at a lower accuracy than observed during the accumulation period. Problems with ice/crust layers and internal forces were pronounced during freeze-melt cycles. Reduced performance was also observed when the amount of snow was either very high or very low because of increased disturbance from wind, solar radiation, local topography, and internal forces in the snowpack. Snow melt and refreezing were simulated fairly well, but factors not accounted for in the model reduced its accuracy during the melt period. The model could be used to identify periods when the snow pillow showed artefacts related to wind transport of snow and changes in internal forces caused by freeze-melt cycles.