Drought in California: It’s All about the Biggest Storms

Abstract:

California has been struggling through a major drought built up during the past several years, with far reaching environmental and economic consequences. Droughts are a common problem in the State’s highly variable hydroclimate, along with intervening periods in which major storms and floods are similarly problematic. Analyses of long-term historical records of precipitation, streamflow and lake levels reveals that past multi-year drought interludes have been due almost entirely (explaining 92% of variance) to periods marked by an absence of large storms—those yielding 95^th-percentile or larger daily-precipitation totals--with little contribution from fluctuations in the annual contributions from normal-to–light storms (24%). This very strong association of drought with (absence of) the largest storms is somewhat unique within the US to California, with largest-storm totals elsewhere contributing only 40-70% of year-to-year precipitation variations. Contributions to year-to-year precipitation fluctuations from the remaining (smaller) wet-days explain <31% of variance everywhere in the conterminous US. In California, the particular storm type that is missing during droughts can be determined: Historical multi-year droughts in California reflect a close (75%) relation between water-year precipitation totals and water-year numbers of atmospheric rivers making landfall in the State, in keeping with the fact that those landfalls are the predominate source of largest storms.