1 Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, QC Canada.
2 Department of Civil & Environmental Engineering, University of California Irvine, Irvine, CA USA.
3 United Nations University Institute for Water, Environment and Health, Richmond Hill, ON Canada.

Snow water is key to water supply in cold regions and beyond. Here we introduce Snow Water Availability that quantifies water stored in the snow-covered portion of an area. By integrating a plausible set of available gridded datasets for snow depth, density, and cover fraction, we form four estimates of Snow Water Availability at the 25 × 25 km² across Canada and Alaska. We show that annual long-term mean of Snow Water Availability over the domain was 996 ± 170 km³ during 2000-2019. While annual Snow Water Availability increased from 799 ± 121 km³ in 2000-2009 to 1208 ± 231 km³ in 2010-2019, significant losses (p-value = 0.05) were observed in ~3% of the domain, mainly in North American Cordillera, headwaters to major rivers in western Canada. These losses alongside insignificant decreases across southern Canada can threaten water supply in a quarter of the country, where ~86% of its population reside.