Distinct Streamflow and Nutrient Export Dynamics in Wildfire-Impacted Nonperennial Streams in Central Coastal California

Abstract

Wildfires impact water quality by altering nutrient supply and hydrology with changes often perceptible during the first major storm events. However, many factors influence water quality, including wildfire characteristics, weather patterns, and watershed properties. Such factors create challenges for predicting and mitigating water quality, highlighting the ongoing need for work across diverse hydroclimatic settings. The Santa Clara Unit Lightning Complex (SCU) Fire impacted two adjacent headwater catchments in central coastal California. Previous work showed that despite similar biophysical characteristics, the two catchments are hydrologically distinct with one catchment displaying flashier streamflow and higher dissolved organic carbon (DOC) concentrations likely driven by distinct bedrock. Although prefire data are not available, here we expand on this work with detailed observations of total dissolved nitrogen, phosphate (PO₄³⁻), and DOC concentrations and loads, over the first two years following the wildfire. We observed 2–13 times higher annual solute export from the catchment with flashier streamflow behavior. We hypothesize that elevated solute export occurred due to shallow hydrologic flow paths dominating runoff generation regardless of surface-level alterations from the wildfire. Additionally, we did not observe the highest solute concentrations during the first major storms following the wildfire. Instead, solute concentrations peaked during high-intensity rainfall in year two. This work showcases the importance of the hydrogeologic setting and hydrologic routing on solute export. Additionally, these results highlight challenges in predicting water quality responses in disturbed catchments and teasing apart the role of wildfire, ongoing drought, and high-intensity precipitation in semiarid climates.