Multivariate Assessment on the Role of Climate Change in California Drought

Abstract:

The current California drought (i.e., 2011-2014) has cast a heavy burden on statewide agriculture and water resources, further exacerbated by concurrent extreme high temperatures. Furthermore, industrial-era global climate warming brings into question the role of long-term climate change on the current CA drought. One way of accounting for the combined effects of rainfall and temperature on drought is to examine soil moisture. Here we present a multivariate assessment on the role of climate change in CA drought, using two 2000-year simulations of historical precipitation and soil moisture from the CCSM4 model with preindustrial (i.e., the year of 1850) and current (i.e., the year of 2000) climate forcings, respectively. The results show that the 2011-2014 (i.e., 3-year) CA drought is not a rare event from a bivariate perspective of drought duration and severity using precipitation. For analogous 3-year events to the current CA drought, the changes in return period/frequency are indistinguishable between simulated preindustrial and current climate. The effect of climate change is not detectable on the 2011-2014 CA drought, at least not shown with CCSM4 simulations. Using a bivariate drought definition of 10-cm soil moisture and precipitation, droughts of all severities (i.e., the joint return periods of 10- to 200-year) of the 1850-vintage become more frequent in the current climate. Using the same definition of 1-m soil moisture and precipitation, moderate-severe droughts (i.e., the joint return periods of 10- to 50-year) of the 1850-vintage become less frequent, while extreme droughts (i.e., the joint return periods of 100- to 200-year) tend to become more frequent in the current climate. An implementation of offline land surface model simulations is further carried out to quantify hydrologic uncertainties.