Effects of wildfire on stream baseflow sources in varyingly burned forested watersheds

Wildfires are among the most transformative disturbances in forested catchments, with profound effects on post-fire water quantity and quality. The severity and extent of wildfires has grown over the past decades raising concerns over their impact on streamflow and water resources. Wildfires affect streamflow through multiple mechanisms, yet their individual effects are highly specific to pre-fire catchment characteristics. In 2023, the Lookout Creek catchment in the H.J. Andrews Experimental Forest (Oregon, USA) was affected by the Lookout Fire, which burned 70 % of the catchment at varying severity. Building on three pre-fire synoptic campaigns (2022–2023), we conducted two post-fire campaigns in 2025, collecting stable water isotope samples along twelve streams spanning unburned to severely burned watersheds. We combine end-member mixing analysis with Spatial Stream Network models to (i) quantify changes in baseflow source contributions after the fire and (ii) identify the landscape controls that govern spatial patterns in streamwater isotopes.

We model campaign-to-campaign isotopic differences to isolate fire-related shifts while accounting for network structure and flow-connected spatial dependence. Additionally, we fit models separately within each campaign to test whether the strength and direction of landscape–isotope relationships have changed through time. Explanatory factors include soil burn severity and vegetation mortality, alongside geomorphic and geologic descriptors that function as proxies for storage and connectivity.

We hypothesize that (i) burn severity and fire-induced vegetation mortality modify flow-generating processes in ways that are detectable as systematic shifts in streamwater isotope composition; and (ii) sub-catchments with greater effective storage and longer flow pathways are more resilient, exhibiting muted isotopic change due to buffering and longer lag times. The anticipated outcome is a process-based assessment of where and why baseflow sources shift after wildfire, and a set of transferable indicators to identify catchments most vulnerable to post-fire alterations in water supply.