Diana Zamora-Reyes, Valerie Trouet1, and Bryan Black1
Most of the rainfall over California occurs from November to March and is highly dependent on the frequency of winter storms steered towards the state. Thus, the annual amount tends to vary every year; this was evident in the recent extremely dry-to-extremely wet cycle that caused more than $6 billion in damage throughout California from 2012-2017 through drought, wildfires and floods. The 2015 Sierra Nevada snowpack reached a 500-year low due to below average precipitation and above average temperatures, while in 2017 it was above average due to record-high precipitation totals. Is this extreme-to-extreme pattern normal, or has it occurred in the past?
First, by using 1901-2016 precipitation data, we found that Central and Southern California have a significant positive trend in variability starting in the 1950s. We then extended these results by using hundreds of years of data derived from annual tree-rings, which showed that the variability in both precipitation and streamflow over the same region has been steadily increasing since the 1950s reaching unprecedented levels at present. This has substantial implications for water resources and management, as they will become more unstable and extreme natural hazards will become more frequent in the future as suggested in previous studies.
1Bryant Bannister Laboratory of Tree-Ring Research, The University of Arizona, Tucson, AZ