Background: Post-fire studies show that water repellency is limited by moisture conditions, but no existing study has examined this limiting effect at a watershed scale.

Aims: This study aimed to identify the soil moisture threshold value at which wildfire-induced hydrophobic condition transitions back to hydrophilic condition at a watershed scale.

Methods: The effect of moisture thresholds on post-fire water-repellent soil and hydrological variables including infiltration, runoff volume and peak flow are examined, using the post-wildfire hydrological model of the upper Arroyo Seco watershed, California, following the August 2009 Station Fire.

Key results: As the moisture threshold value increased from wilting point towards field capacity, the wildfire’s impact on runoff was greatest near the wilting point, and decreased sharply as the threshold increased. The percentage error in peak flow exponentially decreased as the moisture threshold increased and the corresponding Nash–Sutcliffe efficiency increased. Soil moisture threshold values >0.2 m3/m3 were significantly less sensitive to Nash–Sutcliffe efficiency, infiltration depth and percentage error in peak flow and runoff volume.

Conclusion: At the soil moisture threshold value of 0.25 m3/m3, transition from hydrophobic to hydrophilic conditions occurred. Identification of this watershed-scale soil moisture threshold value allows inclusion of the wildfire-induced hydrophobic transition back to hydrophilic condition in post-fire hydrological modelling of watersheds.