Prolonged Drought in a Northern California Coastal Region Suppresses Wildfire Impacts on Hydrology

Abstract

Wildfires naturally occur in many landscapes, however they are undergoing rapid regime shifts. Despite the emphasis in the literature on the most severe hydrological responses to wildfire, there remains a knowledge gap on the thresholds of wildfire (i.e., burned area/drainage area ratio, BAR) required to initiate hydrological responses. We investigated hydrological changes in the Russian River Watershed (RRW) in California, a coastal, Mediterranean, drought‐prone, wildfire‐adapted ecosystem, following ten wildfires that burned 30% of the watershed. Our findings suggest that sub‐watersheds of the RRW have not burned beyond an intrinsic, unknown, threshold required to initiate change. Using paired watersheds, we examined spatiotemporal patterns of pre‐and‐post wildfire hydrology with a rainfall‐runoff hydrological model. Even though these successive wildfires burned 1%–50% of each sub‐watershed (1%–30% at moderate/high severity), we found little evidence of wildfire‐related shifts in hydrology. As a function of BAR, wildfire imposed limited effects on runoff ratios (runoff/precipitation) and runoff residuals (observations—model simulations). Our findings that post‐wildfire runoff enhancements asymptote beyond 30% burn indicate that when a watershed is burned beyond a certain threshold, the magnitude of the hydrologic response no longer increases. Drought and storm conditions explained much of the variability observed in streamflow, whereas wildfire explained only moderate variability in streamflow even when wildfire accounted for >45% BAR. While the BAR in the RRW was sufficiently beyond previously reported minimum disturbance thresholds (>20% burned forest), the lack of hydrological response is attributed to buffering effects of wildfire adaptation and drought factors that are unique to Mediterranean ecoregions.

Wildland–urban interface wildfire increases metal contributions to stormwater runoff in Paradise, California

“…34 Further, antecedent conditions can greatly influence water quality and watershed responses following periods of wet or dry conditions could cause attribution of water composition changes incorrectly attributed to wildfire. 61–63 While metal concentrations naturally fluctuate with hydrology, mean annual pre- and post-fire water hardness values were statistically indistinguishable in this study. In contrast, post-fire trace and major metal concentrations were found to be up to 200 times higher than the baseline data.…”

Exploring the Complex Effects of Wildfire on Stream Water Chemistry: Insights From Concentration‐Discharge Relationships

“…However, not all fire‐impacted watersheds exhibit increases in stream discharge, owing to complex interactions with landscape components (Goeking & Tarboton, 2020). Post‐wildfire hydrology can also show a strong dependency on climate (Maina & Siirila‐Woodburn, 2020; Murphy et al., 2015), with more recent work highlighting how wildfires can have spatially variable impacts on hydrological responses in ecoregions, like the Mediterranean, where little evidence of post‐fire hydrologic change was found due to drought (Newcomer et al., 2023).…”

Model and remote-sensing-guided experimental design and hypothesis generation for monitoring snow-soil–plant interactions

“…However, they have focused on estimating the spatial distributions of a single or two target variables. A recent pressing issue has been to observe and quantify interacting ecosystem or watershed responses to the disturbances resulting from climate change, such as coupled soil-plant responses to droughts or wildfires (e.g., Sloat et al, 2015;Mirus et al, 2017;Siirila-Woodburn et al, 2021;Newcomer et al, 2023), or to investigate those disturbances' key modulators, such as subsurface conditions, which are often difficult to characterize over space and time.…”