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. 2021 Jul 21;7(30):eabh2262.
doi: 10.1126/sciadv.abh2262. Print 2021 Jul.

Ignitions explain more than temperature or precipitation in driving Santa Ana wind fires

Jon E Keeley  1   2 , Janin Guzman-Morales  3 , Alexander Gershunov  3 , Alexandra D Syphard  4 , Daniel Cayan  3 , David W Pierce  3 , Michael Flannigan  5 , Tim J Brown  6
Affiliations

Ignitions explain more than temperature or precipitation in driving Santa Ana wind fires

Jon E Keeley et al. Sci Adv. .

Abstract

Autumn and winter Santa Ana wind (SAW)-driven wildfires play a substantial role in area burned and societal losses in southern California. Temperature during the event and antecedent precipitation in the week or month prior play a minor role in determining area burned. Burning is dependent on wind intensity and number of human-ignited fires. Over 75% of all SAW events generate no fires; rather, fires during a SAW event are dependent on a fire being ignited. Models explained 40 to 50% of area burned, with number of ignitions being the strongest variable. One hundred percent of SAW fires were human caused, and in the past decade, powerline failures have been the dominant cause. Future fire losses can be reduced by greater emphasis on maintenance of utility lines and attention to planning urban growth in ways that reduce the potential for powerline ignitions.

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Figures

Fig. 1
Fig. 1. SAW monthly distribution over 71 years.
(A) Days with SAW winds, (B) average SAW wind speed per SAW event, and (C) duration of SAW events, for the first half (1948–1983) and second half (1984–2018).
Fig. 2
Fig. 2. SAW events and fire.
(A) Area burned by SAW and non-SAW fires in southern California for all years, and monthly distribution for the first half and second half of our 71-year record for (B) SAW fire frequency and (C) SAW area burned. (D) Percentage distribution for all SAW events and just those with fires over 1000 ha for Tmax during a SAW event and (E) precipitation in the week before the SAW event.
Fig. 3
Fig. 3. Burn classes.
Percentage of SAW events that fall into one of five area burned classes: 0 = no area burned, 50 = 1 to 99 ha, 500 = 100 to 999 ha, 2500 = 1000 to 4999 ha, >5000 = >5000 ha for (A) all SAW events and (B) SAW events with at least one extreme wind day. (C) Cumulative SAW index per event.
Fig. 4
Fig. 4. Weak, moderate, and extreme winds.
(A) Fire frequency, (B) total area burned, and (C) mean area burned per fire for each of the three SAW wind speed categories: Weak (Wk) = <5 m/s, Moderated (Mod) = 5 to 9.99 m/s, and Extreme = ≥10 m/s [extreme winds are presented separately for the number of extreme days per event; for the first half (1948–1983) and second half (1984–2018)]. The single large fire event in (C) was the 2017 Thomas Fire with 4 days of extreme winds.
Fig. 5
Fig. 5. Percentage of SAW events with and without fire.
Area burned classes: 0 = no area burned, 50 = 1 to 99 ha, 500 = 100 to 999 ha, 2500 = 1000 to 4999 ha, >5000 = >5000 ha for (A) all SAW events and (B) events with at least 1 day of extreme winds. (C) Area burned by ignition source for all SAW events. Ltg, lightning; Equ, equipment; Cmp, camping; Deb, debris burning; Ars, arson; Veh, vehicle; Pow, powerline related, for years 1948–1983 versus 1984–2018.
See this image and copyright information in PMC

References

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