UC Davis

Climate change in California is driving an increase in the duration of the wildfire season as well as severity and size of wildfires. With the increased prevalence of mega wildfires, there is a need to understand the long-term effects of wildfire on the water quality of forested watersheds. After the 2021 Caldor Fire in the Sierra Nevada mountain range, a long-term, high-frequency surface water sampling campaign was implemented in the Cosumnes River Watershed alongside the mining of historical stream water chemistry and discharge data available through National Water Quality Monitoring Council and USGS databases. Here we develop a pipeline to integrate current measurements and historical water quality data to analyze in detail the variations in dissolved organic carbon exported from the watershed. For the purposes of this study, dissolved organic carbon was defined to be any organic matter present in stream water that could pass through a 0.45 µm PVDF filter. We hypothesized dissolved organic carbon content in the Cosumnes River would be dependent upon burn severity experienced and would reach a maximum in the first-year post fire. Our work demonstrated that storm events drive increases in DOC concentration and instantaneous loads post fire, with maximum concentrations observed in the first year. Storms in subsequent years continue to drive spikes in DOC concentrations and instantaneous loads but require higher discharges to achieve the levels observed in the first-year post fire. This analysis indicates the readily available organic matter supplied by years of drought followed by fire are depleted after the first year. Maximum DOC concentrations and instantaneous loads during high discharge storm periods are higher in the first-year post fire than in available pre-fire data. Atmospheric rivers are a strong driver of DOC transport and subsequent high concentrations. During the years of this study, snowmelt was not a driver of DOC transport. Our work suggests that the Cosumnes River Watershed is resilient in the face of wildfire, with dissolved organic carbon values returning to historical ranges in the second year after fire.