Statistical Analysis of Sierra Nevada Snowpack Accumulation Trends
Tammy Johnson1, Jeff Dozier2, Joel Michaelsen3, and Peter Fohl4
Most of California's water resources accumulate within the snowpack on mountains until it melts,
usually in the spring. Several investigations on the subsequent streamflows indicate that the amount
and timing of this fresh water supply is changing. This study uses a statistical model that links snow
water equivalent (SWE) measurements over a 60-year time-series to clarify the spatial characteris-
tics of snow accumulation trends in the Sierra Nevada.
Data difficulties include inconsistent monthly sampling, added and removed stations, and possibly a
few moved or otherwise altered snow courses. To determine the effects of a monthly and irregular
sampling schedule, we analyzed daily snow sensor data spanning 28 years. Furthermore, we em-
ployed a statistical test to check for possibly discontinuous snow course stations.
Results are presented for seasonal maximum and monthly changes by river basin groupings and
range-wide elevation bins. Time-series regressions on station data from individual river basins be-
low 2400 meters consistently indicate less maximum SWE or no change. Basins above 2400 meters
indicate greater variability, with most showing increasing maximum SWE trends and earlier melt.
Snowmelt timing changes were more obvious in the lower elevations. All 15 river basins below 2400
meters estimated earlier maximum SWE timing, with 9 trends supported by 95% confidence.
We found a strong linear elevational component to monthly SWE accumulation trends. Below 2400
meters, a range-wide average indicates that 14% less SWE is accumulating and it is melting a week
earlier per 50 years. Five of the 10 lowest elevation bins' trends are supported with 95% statistical
confidence. Higher elevations exhibit greater variability, with stations averaging 8% more SWE per
50 years, while snowmelt timing tends to occur earlier.
Monthly analyses show that higher elevation changes are due to 18% increased snow accumulation
in February, as measured on March 1. This extra SWE persists through the April 1 measurements but
by May 1 higher elevation SWE levels are decreasing by 13% per 50 years. Lower elevation SWE
levels are decreasing each month: down 7% on February 1, 11% on March 1, 19% on April 1, and by
33% on May 1 per 50 years. This could be the result of warmer air masses having higher moisture
contents. These observations support doubled-CO2 model predictions for precipitation and tempera-
ture patterns in the Sierra Nevada.
Department of Geography, University of California, Santa Barbara, California 93106-4060, USA
Donald Bren School of Environmental Science and Management, University of California, Santa Barbara,
California 93106, USA
3 Department of Geography, University of California, Santa Barbara, California 93106-4060, USA
4 National Center for Geographic Information and Analysis, University of California, Santa Barbara, California