Near-real time monitoring of snow water equivalent, depth and density at a high-altitude site in Nepal

Seasonal snowmelt provides an important water resource to over one billion people downstream of High Mountain Asia's cryosphere. Due to the remoteness of high-altitude snowpacks, there is currently little reliable data on seasonal snow changes, negatively impacting efforts to forecast water availability in this region. Here, we use a CS725 passive gamma ray sensor and an ultrasonic snow depth sensor to present two years of high-resolution snow water equivalent (SWE), snow depth and snow density measurements from an automatic weather station installed at 5000 m a.s.l. in the Nepal Himalaya. The station requires minimal maintenance and transmits data in near-real time via satellite connection. We evaluate the accuracy of the automatic measurements using independent field observations and conduct an instrument comparison between three different methods of measuring solid precipitation. A time series of snowpack bulk density is calculated using automatic SWE and snow depth measurements and is compared to the results of an independent snow model (seNorge).

Our results demonstrate that passively measuring the attenuation of naturally emitted gamma rays by snow is an effective method to collect SWE observations in this environment. At their seasonal peak, snowpacks in this setting can store up to 200 mm w.e. and are strongly influenced by the refreezing of meltwater at the snow-soil interface. We use high resolution time-lapse camera imagery to assess instrument uncertainties and the representativeness of snow depth and SWE point measurements over larger spatial scales. Our model comparison suggests that automatic measurements of SWE and snow depth can be used to monitor the evolution of snowpack bulk density over the course of an accumulation and melt season. The station setup provides an example which could be extended into remote Himalayan environments to facilitate widespread near-real time snowpack monitoring in the future.