Revisiting the recent California drought as an extreme value

Abstract

Spatially weighted averages of Palmer Drought Severity Index (PDSI) over central and southern California show that the 1 year 2014 drought was not as severe as previously reported, but it still is the most severe in the 1895–2014 instrumental record. Using the typical adjustment procedure that matches the mean and standard deviation of tree ring PDSI values to those of instrumental data shows over 10 droughts from 800 to 2006 that were more severe than the 1 year 2014 drought, with the 2014 drought having a return period of 140–180 years. Quantile mapping allows for a closer correspondence between instrumental and tree ring PDSI probability distributions and produces return periods of 700–900 years for the 1 year 2014 drought. Associated cumulative 3 and 4 year droughts, however, are estimated to be much more severe. The 2012–2014 drought is nearly a 10,000 year event, while the 2012–2015 drought has an almost incalculable return period and is completely without precedent.

Hydroclimatic Characteristics of the 2012–2015 California Drought from an Operational Perspective

“…Within the period from 2012-2015, in general, water year 2015 stood out as the driest single year; 2014-2015 was the driest two-year period; 2013-2015 tended to be the driest three-year period. Contrary to some previous studies stating that the 2012-2015 drought was unprecedented [28,30], this study illustrated that measuring by a certain number of variables (8 out of 28), the 2012-2015 drought was not without precedent in the record period.…”

“…The study extends previous studies [28,30] in the context of (1) examining typical hydroclimatic variables and operational drought assessment metrics applied in routine drought management practices; and (2) covering all hydrologic regions across the state including the Northern half. The study addresses the following science questions, which are critical for drought managers in making adaptive plans: (1) how does the 2012-2015 drought evolve in a hydroclimatic framework; (2) what are the temporal characteristics of the drought in a historical context at one-to four-year levels; and (3) what is the spatial pattern of the drought in terms of which regions have the most severe conditions and by what means?…”

“…The first four years (2012)(2013)(2014)(2015) of the drought stand out as June/July/August (June-August) Palmer Drought Severity Index (PDSI) [29], the 2012-2014 drought was the worst in the analysis period (800-2014) while 2014 was the driest year in at least the past 1200 years; (2) evaluating precipitation deficits, however, the conditions in 2014 and 2012-2014 were not unique in the analysis period. In contrast, using the same raw PDSI reconstructions but with different spatial average and bias correction procedures, Robeson [30] observed that based on June-August PDSI, the 2014 drought had a return period of 140-180 years, the 2012-2014 drought was almost a 10,000 year event, and the 2012-2015 drought was unprecedented with a nearly incalculable return period. In spite of having strikingly different findings, both studies shared a number of limitations including (1) the PDSI is determined from precipitation and temperature data.…”

Monitoring Changes in Water Use Efficiency to Understand Drought Induced Tree Mortality

“…The steady gradual increase in mortality has been concentrated in small trees [66] and until the most recent drought (2012-2015), climate conditions remained below thresholds that could cause large-scale forest die-back. In the 2012-2016 drought, the cumulative rainfall deficit was described as a one in a 1000-year event [37], and resulted in severely reduced snowpack, soil moisture, groundwater, and reservoir stocks [5]. This increase in water deficit, combined with high air temperatures and insect infestations, generated a large pulse of tree mortality in California [5].…”

“…The self-calibrated Palmer drought severity index (scPDSI; Table 1; Figure 2) was used to quantify drought severity and to calculate the frequency of severe drought (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015). The scPDSI is an improved version of the Palmer Drought Severity Index, which has been widely used to monitor drought conditions [4,[36][37][38][39][40][41][42][43]. Monthly scPDSI data was available through the Western Regional Climate Center (https://www.wrcc.dri.edu/).…”