Decreasing fire season precipitation increased recent western US forest wildfire activity

Abstract

Western United States wildfire increases have been generally attributed to warming temperatures, either through effects on winter snowpack or summer evaporation. However, near-surface air temperature and evaporative demand are strongly influenced by moisture availability and these interactions and their role in regulating fire activity have never been fully explored. Here we show that previously unnoted declines in summer precipitation from 1979 to 2016 across 31-45% of the forested areas in the western United States are strongly associated with burned area variations. The number of wetting rain days (WRD; days with precipitation ≥2.54 mm) during the fire season partially regulated the temperature and subsequent vapor pressure deficit (VPD) previously implicated as a primary driver of annual wildfire area burned. We use path analysis to decompose the relative influence of declining snowpack, rising temperatures, and declining precipitation on observed fire activity increases. After accounting for interactions, the net effect of WRD anomalies on wildfire area burned was more than 2.5 times greater than the net effect of VPD, and both the WRD and VPD effects were substantially greater than the influence of winter snowpack. These results suggest that precipitation during the fire season exerts the strongest control on burned area either directly through its wetting effects or indirectly through feedbacks to VPD. If these trends persist, decreases in summer precipitation and the associated summertime aridity increases would lead to more burned area across the western United States with far-reaching ecological and socioeconomic impacts.

Keywords: climate change; hydrology; wildfire.

Fig. 1.

Forest cover and forest wildfire area burned from 1984 to 2015 in the western United States with eight NEON domains outlined in black. (Right) Total annual forested hectares burned within each NEON domain. Blue lines show the linear trend in area burned, with a solid line indicating a statistically significant trend at P < 0.10. CP, Central Plains; DSW, Desert Southwest; GB, Great Basin; NP, Northern Plains; NR, Northern Rockies; PNW, Pacific Northwest; PSW, Pacific Southwest; SR, Southern Rockies.

Fig. 2.

Linear correlations (Pearson’s r) between forest wildfire area burned (log transformed) from 1984 to 2015 and standardized May–September WRD, maximum temperature, maximum SWE, and maximum daytime VPD across eight NEON domains and for all forest areas in the western United States. An asterisk indicates statistical significance at P = 0.10. All negative values are expressed as absolute values for visual interpretation. NEON ecoregion names are identified in Fig. 1.

Fig. 3.

Linear trends in May–September precipitation (Left), the number of WRD (Center), and the mean consecutive number of dry days (Right) from Daymet (1980–2016) and GRIDMET (1979–2016) datasets. Circles (Bottom) indicate the location of weather stations used in the trend analysis. All colored areas in gridded dataset trends and station circles outlined in black indicate statistical significance at P = 0.10 using a Mann–Kendall trend test.

Fig. 4.

Linear trends in WRD (Top), log-transformed wildfire area burned (Middle), and their correlation (Bottom) from 1984 to 2015 in forested areas of the western United States.

Fig. 5.

May–September WRD anomalies correlated with maximum VPD and maximum temperature anomalies for western US forested areas from 1984–2015.

Fig. 6.

Path analysis diagram illustrating relationships among precipitation, VPD, and wildfire area burned. SWE and precipitation are exogenous variables which are allowed to influence wildfire area burned directly, and indirectly as mediated by VPD.