Murphy, SheilaÌý1
1ÌýU.S. Geological Survey
Extreme climate events– such as hurricanes, droughts, extreme precipitation, and wildfires– have the potential to alter watershed processes and stream response. Yet due to the destructive and hazardous nature and unpredictability of such events, capturing their hydrochemical signal is challenging. A five-year post-wildfire study of stream chemistry in the Fourmile Creek watershed, Colorado Front Range, USA, focused on high-frequency storm sampling. During the study, the watershed was impacted by three additional extreme climate events – a drought and two periods of extreme rainfall totals. These events altered concentration-discharge relationships in ways that elucidate how hydrologic flow paths and source material availability affect stream water chemistry. Reduced infiltration after wildfire led to overland flow during thunderstorms, which conveyed ash and soil into streams, resulting in elevated stream concentrations of Ca, K, Mg, and alkalinity (which are present in wildfire ash), along with sediment, dissolved organic carbon, and nitrate. Subsurface flow paths were bypassed, leading to low stream concentrations of Na and SiO2, which are bedrock-derived (and not present in ash). During drought conditions, when stream discharge was <20% below average, concentrations of sediment, dissolved organic carbon, and Ca fell below average concentrations, but SiO2 did not. Extreme rainfall totals saturated the subsurface and led to prolonged elevated stream discharge; concentration-discharge relationships for bedrock-derived constituents (such as Ca and SiO2) were altered in that time period, while those for dissolved organic carbon was not. Previous disturbances, including historical mining, also affect stream chemistry, and water-quality impairment can be exacerbated by extreme climate events.