3.2.3. Drought Index

California is prone to drought, with examples being the 1976–1977, 1988–1992, 2007–2009, and 2012–2013 droughts [80]. While the occurrence and lasting period of drought events are difficult to predict decades in advance, the overall tendency (i.e., trend) of drought events can shed light on long-term drought response planning activities. This section looks at projected future drought conditions (represented by the SPEI index) at one- to four-year temporal scales which are relevant to our operational planning practices. Figure 11 shows trend slopes of SPEI-12, SPEI-24, SPEI-36, and SPEI-48 calculated from projected precipitation and temperature data, along with their counterparts in the historical period. On average, all regions are expecting a decreasing trend (negative slope value). This is particularly true for dry regions including the Colorado River, South Lahontan, and Tulare Lake. All 20 projections have a decreasing tendency consistently, indicating more severe droughts for those

regions on the annual, two-year, three-year, and four-year scales. For other regions, there is no such consensus. However, the majority of projections show a decreasing trend. It should be highlighted that, for the wettest region, North Coast, most projections have a relatively milder decreasing trend compared to the historical baseline. This suggests that projected increase in precipitation over this region outweighs the effect of warming. For the coolest region, North Lahontan, the median trend slopes of projected SPEI values are generally around the historical trend slope values. This implies that projected precipitation increase in this region offsets the impact of warming. For other regions, most projections have a steeper decreasing trend compared to their historical counterparts, indicating that projected increases in precipitation are not sufficient to offset the effect of warming. Particularly, for the driest region, Colorado River, the decreasing rates of all 20 projections are higher than its baseline counterpart. This suggests that this region is the least resilient to warming and thus most prone to aridity (as represented by SPEI index) among all study regions.

**Figure 11.** Box-and-whisker plots of significant trend slopes of: (**a**) SPEI-12; (**b**) SPEI-24; (**c**) SPEI-36; and (**d**) SPEI-48 during projection period (2020–2099) (at significance level 0.05). The slope information in historical period (1951–2013) is also shown.

It is worth noting that not all trends identified in the historical and projection periods are statistically significant at a significance level of 0.05. The Central Coast region and Sacramento River region show no significant changes in SPEI values (Figure 11). Additionally, on three-year scale and four-year scale, all Central Valley regions (SAC, SJQ, and TUL) have no increasing or decreasing tendency in drought represented by SPEI during the historical period. In the projection period (Table 6), for the driest two regions, Colorado River and South Lahontan, all 20 projections show consistently significant trend in SPEI at the four time scales considered. This is no such consensus for other regions. However, the majority of projections still show significant trends. For instance, only one out of 20 (5%) projections exhibit insignificant changes in Tulare Lake region. For another relatively dry region, South Coast, all projections show significant trends in SPEI on the three-year scale (SPEI-36) and four-year scale (SPEI-48) while only one projection (5%) has insignificant trend on the annual scale (SPEI-12) and two-year scale (SPEI-24). The wettest region, North Coast, has the highest amount (25% to 30%) of projections that show no significant trend. For the second wettest region, Sacramento River, 15–20% of the projections indicate no significant trend. Overall, those projections agree more with each other on the increasing aridity in dry regions than in wet regions irrespective of the time scales investigated.


**Table 6.** Number (percent) of SPEI projections with insignificant trend <sup>1</sup> in the projection period.

<sup>1</sup> Significance level 0.05.
