Burn area and severity modulates hydrological response to wildfire in California watersheds

Wildfire is an increasingly important disturbance in western U.S. watersheds, altering hydrologic partitioning and water resources. We evaluated how wildfire severity, watershed burned area extent, and watershed vegetation cover affect post-fire evapotranspiration and streamflow in 24 California watersheds using a paired watershed approach and hierarchical mixed effects models. Monthly gridMET precipitation, remotely sensed OpenET evapotranspiration, and in situ streamflow data spanning water years 2000–2024 in paired burned and unburned watersheds quantified fire-induced deviations from non-fire conditions and short-term ecosystem recovery. The water year immediately post-fire showed widespread reductions in evapotranspiration (−16% to −58% in 9 of 13 burned watersheds) and increases in streamflow (10% to 498% in 11 of 13 burned watersheds) relative to pre-fire conditions. Evapotranspiration reductions persisted two to three years post-fire, particularly in more extensively burned watersheds (>75%), while streamflow increases were more variable and often overlapped with unburned watershed responses. Mixed effects models showed wildfire significantly increased total streamflow without increasing its sensitivity to precipitation. Among burned watersheds, fire extent weighted severity, the product of burn severity and burned area extent, strongly controlled evapotranspiration reductions and weakened the precipitation-streamflow relationship without affecting streamflow magnitude, suggesting fire alters post-fire water partitioning and flow pathways rather than producing flashier runoff. These changes can alter inland and coastal water quality, with implications for human and ecosystem health (e.g., nutrient cycling, contaminant loads) relevant to water and hazard management decision-makers.