Drought and the California Delta—A Matter of Extremes

California is in an extreme drought as a result of low precipitation in water year 2012, record low precipitation in 2013, and the remarkably dry first few months of 2014. We typically receive our largest precipitation totals in Decembers and Januaries (which provide about 36% of the precipitation in the Delta’s catchment), and when those months are as dry as they were this year, subsequent months have to be unseasonably wet to avoid drought. Most of the current winter, especially December 2013–January 2014, was dry because a persistent ridge of high (atmospheric) pressures set up offshore, diverting storms away from California and into Alaska. That same diversion gave the Delta’s catchment its warmest winter in 120 years, as well as bringing all that cold weather to the eastern US. While the February 2014 storms provided minor relief and momentarily slowed the spiral into deeper drought, the drought continues apace. Nonetheless, as harsh as it is, the current drought is not unprecedented in the Delta’s history, and even less so in its prehistory. In this essay, we consider the ways that droughts in California arise from a few missing storms and from long-term variations and changes in climate, in order to identify drought-science needs for Delta management.