Universal hydrological trends post-wildfire are obscured by local watershed variability

Changing wildfire regimes in recent decades have prompted extensive investigation into their hydrological impact. However, contradictory findings on post-fire flow regime changes have highlighted the complexity of this relationship. This study considers rich daily-resolution hydrological data on 898 burned catchments and over 8,000 unburned control catchments to choose a set of 39 paired catchment experiments, along with 500 control pair experiments. We use these 500 control catchments to evaluate the extent to which the paired watershed approach accounts for hidden co-varying factors, finding that in most instances, it does not. We assess the influence of wildfires on flow magnitude, timing, and dynamicity. We also explore the effects of wildfire on water storage, evapotranspiration (ET), and flood flows across the Western USA from 1984-2023. We find that percent changes in flow magnitude post-fire are insignificant and are uncorrelated with the percent of the catchment that burned, even when data are restricted to the critical first five years post-fire. However, linear regression analysis of 259 catchment variables found weak positive relationships between pre-fire forest cover and percent changes in flow magnitudes. Our results emphasize that the statistical uncertainty associated with even well-established methods may overshadow the magnitude of observed effects.