Snow water equivalent on an Alpine glacier from continuous cosmic ray neutron sensing and numerical modelling

The assessment of snow water equivalent (SWE) is crucial for hydrological studies in glaciated catchments to quantify accumulation and ablation of a seasonal snow cover for both, the glaciated and non-glaciated terrain. Presently, the majority of the SWE assessment on glaciated terrain relies on manual measurements once or a few times per year, given the limited techniques for continuous SWE monitoring and the challenging conditions in a high mountain environment. Cosmic Ray Neutron Sensors (CRNS) offer to overcome these limitations providing sub-daily SWE estimates derived from neutron counts.
This study employs a CRNS installed on an Alpine glacier (Hintereisferner, Austria) over three years, complemented with an additional CRNS for one winter roughly 300 m lower in elevation along the glacier’s central flow line. Comparing CRNS outputs with frequent manual SWE measurements, the results demonstrate close agreement in SWE and snow density. The CRNS were found to be remarkably resilient in harsh conditions, providing nearly continuous hourly data over the measurement period. Additionally, the study evaluates at the CRNS locations the performance of two snow models, which might be considered as end members of model complexity – SNOWPACK and ΔSNOW in its latest version. While SNOWPACK, with its physically-based approach, yields the best results, ΔSNOW stands out for its simplicity, requiring only daily snow depth observations as input and performs almost as well as SNOWPACK in terms of mean absolute SWE error. 
Comparing the SWE measurements with winter precipitation from weighing gauges distributed in the catchment gives interesting details of precipitation gradients with elevation. Precipitation gradients interpolated from non-glaciated to glaciated terrain are considerably higher than on non-glaciated terrain only. On the latter, empirical bulk correction factors are frequently applied, which might fail on glaciers due to their different topographic setting. This highlights the need of separate treatment of snow on glaciers in hydrological models for correct SWE representation across the catchment.