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AbstractWater supply impairment from increased contaminant mobilization and transport after wildfire is a major concern for communities that rely on surface water from fire‐prone watersheds. In this article we present a Monte Carlo simulation method to quantify the likelihood of wildfire impairing water supplies by combining stochastic representations of annual wildfire and rainfall activity. Water quality impairment was evaluated in terms of turbidity limits for treatment by modeling wildfire burn severity, postfire erosion, sediment transport, and suspended sediment dilution in receiving waterbodies. Water supply disruption was analyzed at the system level based on the impairment status of water supply components and their contributions to system performance. We used this approach to assess wildfire‐water supply impairment and disruption risks for a system of water supply reservoirs and diversions in the Front Range Mountains of Colorado, USA. Our results indicate that wildfire may impair water quality in a concerning 15.7–19.4% of years for diversions from large watersheds. Reservoir impairment should be rare for off‐network reservoirs—ranging from at most 0.01% of years for large reservoirs to nearly 2% of years for small reservoirs. System redundancy meaningfully reduced disruption risk for alternative conveyance routes (4.3–25.0% reduction) and almost eliminated disruption risk for a pair of substitutable terminal sources (99.9% reduction). In contrast, dependency among reservoirs on a conveyance route nearly doubled risk of disruption. Our results highlight the importance of considering water system characteristics when evaluating wildfire‐water supply risks.

Wildfires in the western United States (US) are increasingly expensive, destructive, and deadly. Reducing wildfire losses is particularly challenging when fires frequently start on one land tenure and damage natural or developed assets on other ownerships. Managing wildfire risk in multijurisdictional landscapes has recently become a centerpiece of wildfire strategic planning, legislation, and risk research. However, important empirical knowledge gaps remain regarding cross-boundary fire activity in the western US. Here, we use lands administered by the US Forest Service as a study system to assess the causes, ignition locations, structure loss, and social and biophysical factors associated with cross-boundary fire activity over the past three decades. Results show that cross-boundary fires were primarily caused by humans on private lands. Cross-boundary ignitions, area burned, and structure losses were concentrated in California. Public lands managed by the US Forest Service were not the primary source of fires that destroyed the most structures. Cross-boundary fire activity peaked in moderately populated landscapes with dense road and jurisdictional boundary networks. Fire transmission is increasing, and evidence suggests it will continue to do so in the future. Effective cross-boundary fire risk management will require cross-scale risk co-governance. Focusing on minimizing damages to high-value assets may be more effective than excluding fire from multijurisdictional landscapes.