Recent decreases in snow water storage in western North America

Mountain snowpacks act as natural water towers, storing winter precipitation until summer months when downstream water demand is greatest. We introduce a Snow Storage Index (SSI), representing the temporal phase difference between daily precipitation and surface water inputs-sum of rainfall and snowmelt into terrestrial systems-weighted by relative magnitudes. Different from snow water equivalent or snow fraction, the SSI represents the degree to which the snowpack delays the timing and magnitude of surface water inputs relative to precipitation, a fundamental component of how snow water storage influences the hydrologic cycle. In western North America, annual SSI has decreased (p < 0.05) from 1950-2013 in over 25% of mountainous areas, as a result of substantially earlier snowmelt and rainfall in spring months, with additional declines in winter precipitation. The SSI and associated trends offer a new perspective on hydrologic sensitivity to climate change which have broad implications for water resources and ecosystems. The Snow Storage Index - which combines the magnitude of precipitation with the delay in water input from the timing of a melting snowpack - has decreased in western North America since 1950 due to earlier snowmelt, spring rains and reduced winter precipitation

Automated summary

Mountain snowpacks serve as natural water storage, but the Snow Storage Index (SSI) has shown a decrease in western North America since 1950 due to earlier snowmelt, spring rains, and reduced winter precipitation. The SSI measures the delay in water input from the timing of a melting snowpack, offering insights into hydrologic sensitivity to climate change and its implications for water resources and ecosystems.