Trends in Snowpack Across California

This study conducts a comprehensive trend assessment of snowpack conditions across California, a state where snowpack is critical in terms of storing water from winter precipitation and releasing it in late spring and early summer when demands of water are typically the highest. The Mann-Kendall and Theil-Sen trend tests using trendfree pre-whitening and bootstrap approaches are applied to the historical monthly (February 1st to May 1st) snow water equivalent (SWE) measurements recorded at 260 snow courses in the state. Most sites have a record longer than 50 years. The longest record is 104 years (1911-2014). The focus is on the April 1st SWE and the peak snow mass date (PSMD). For April 1st SWE, the study indicates that only 6% (5% downward, 1% upward) of the snow courses have statistically significant trends. At a regional scale, only the Feather River watershed shows a downward trend. For PSMD, eight sites tend to peak earlier (downward trends) while only one site peaks later (upward trend). Overall, state-wide trends in snowpack are not observed, yet there are significant trends at limited sites and regions in light of the warming climate.

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