California’s most significant droughts: Comparing historical and recent conditions

The water years of 2012-14 stand as California’s driest three consecutive years in terms of statewide precipitation, and as of this writing in February 2015 the drought is continuing on. This report was prepared to compare the hydrology and impacts experienced during 2012-2014 with those of California’s largest historical droughts, in response to questions from local water agencies and others regarding the drought’s relative severity and the changed conditions since our prior major droughts. California’s immediately prior drought of statewide scale occurred in 2007-09; it was the first drought for which a statewide proclamation of emergency was issued. The 2012-14 period now marks the second time a statewide proclamation of emergency has been issued for drought.

California’s most significant historical statewide droughts were the six-year drought of 1929-34, the two-year drought of 1976-77, and the six-year event of 1987-92. These droughts stand out in the observed record due to their duration or severe hydrology. The 1929-34 event occurred within the climatic context of a decades-plus dry period in the 1920s-30s whose hydrology rivaled that of the most severe dry periods in more than a millennium of reconstructed Central Valley paleoclimate data. The drought’s impacts were small by present-day standards, however, since the state’s urban and agricultural development was far less than that of modern times. The 1976-77 drought, although brief in duration, was notable for the severity of its hydrology. The 1987-92 drought was California’s first extended dry period since the 1920s-30s, and provides the closest comparison for drought impacts under a present-day level of development.

The 2012-14 event set other records in addition to that of driest three-year period of statewide precipitation. The drought occurred at a time of record warmth in California, with new climate records set in 2014 for statewide average temperatures. Records for minimum annual precipitation were set in many communities in calendar year 2013. Calendar year 2014 saw record-low water allocations for State Water Project and federal Central Valley Project contractors. Reduced surface water availability triggered increased groundwater pumping, with groundwater levels in many parts of the state dropping 50 to 100 feet below their previous historical lows. These record-setting conditions speak to the need for continued improvement of our ability to respond to dry conditions. Knowledge of the impacts historically experienced in our past large droughts and the lessons learned during those events can help us be better prepared.