Characterizing drinking-water facility exposure to historical wildfires in Northern Colorado

Wildfire increasingly threatens forested headwater watersheds that supply drinking water to downstream communities, yet wildfire exposure is often characterized at regional scales that do not identify which historical fires are most impactful to specific intake facilities. This limitation is especially important in managed water systems where inter-basin transfers (IBTs) expand source-water footprints beyond natural watershed boundaries. Here, we developed a reproducible spatial framework to quantify historical wildfire exposure for drinking-water intakes and applied it in northern Colorado as a case study. The analysis integrated historical (1984–2024) wildfire perimeters, burn severity, subwatershed boundaries, natural hydrologic flow paths, and IBT connectivity. Across the 7,308 km² study area, 1,895 km² burned (25.9% of the landscape), at least one fire occurred in 54 of 90 watershed, and approximately 31% of total burned area was classified as high severity. Across all fire events with downstream intake relevance, fires affected drinking-water systems serving an average of 344,854 people and 4 downstream intake facilities per event. Intake-level exposure depended not only on fire size, but also on burn severity and hydrologic connectivity. Incorporating IBTs expanded the effective source-water footprint and altered the relative importance of individual subwatersheds for evaluating fire impacts on downstream intake systems. These findings improve wildfire characterization in source-water exposure assessment and support development of decision-support tools (e.g., Forests to Faucets) linking forests, water, and communities.