Fire, Flood, and Drought: Extreme Climate Events Alter Flow Paths and Stream Chemistry

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 5-year postwildfire 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-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. This resulted in elevated stream concentrations of constituents elevated in ash-Ca, K, Mg, alkalinity, and dissolved organic carbon-along with sediment and nitrate. Subsurface flow paths were bypassed, leading to low concentrations of Na and SiO2, which are bedrock derived and not elevated in ash. During drought conditions, when stream discharge was <20% of 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 were not. Previous disturbances, including historical mining, also affect stream chemistry, and water-quality impairment can be exacerbated by extreme climate events. Plain Language Summary Where does water go after it falls as rain or snow in a watershed, and what happens to its chemistry as it travels to the stream? How do different weather events, including extreme climate events, affect these processes? Answering these questions is critical to understanding flood risk and protecting water delivery and water quality. The relationship between stream discharge (the volume of water in a stream conveyed during a specified time period) and concentrations of sediment, dissolved particles, and other substances in water has been used to address these questions and provide information on specific watershed processes. Studies of concentration-discharge relationships, and subsequent interpretations, have typically used fixed-interval (often weekly to monthly) sampling of largely undisturbed watersheds. However, much of the water conveyed from a watershed, along with much of the sediment and dissolved constituents, are delivered during storms and thus are not captured in such sampling regimes. In addition, much of the earth's surface is disturbed in some way (such as by urbanization, mining, agriculture, and forestry). We demonstrate that extreme climate events-wildfires, drought, and extreme amounts of precipitation-can change concentration-discharge relationships by altering water flow paths and stream water quality.