WILDFIRES IN WATERSHEDS: WHAT DETERMINES THE WATER-QUALITY RESPONSE? (Invited Presentation)

Wildfires in the western U.S., and many other parts of the world, have increased substantially in size, severity, and frequency in recent decades. Wildfires can drastically alter a watershed’s source material and hydrology, which can increase the risk of flooding and erosion and lead to downstream transport of sediment and other constituents. Degraded water quality after wildfire can result in higher treatment costs for drinking water providers. However, the reported nature and extent of water-quality effects after wildfire have varied widely. While some wildfires lead to massive increases in the amount of sediment, nutrients, and metals to downstream waters, other areas have seen no noticeable change. Given the wide range of potential post-wildfire effects on water supplies, water providers and land managers need better guidance on what to expect in their watersheds to enable planning for and minimizing impacts on water supplies. With the goal of providing actionable guidance, the U.S. Geological Survey Water Resources Mission Area has developed and deployed a strategy to improve monitoring, assessment, and prediction of post-wildfire water quality. Consistent measurements of water quality and stream discharge are being made in burned watersheds across five western states. These watersheds, which were all more than 20% burned between 2020 and 2022, were selected due to minimal water management or developed land cover that could mask a wildfire signal on water quality. The sites span a range of ecoregions and range in precipitation amount (530-2400 mm/y) and precipitation seasonality. In order to identify critical drivers of both short- and long-term water quality effects, we are relating post-wildfire water-quality response in these watersheds to climate, land cover, geology, catchment hydrology, and other factors. The project is also examining the state of science for modeling post-wildfire water quantity and quality and identifying opportunities for advancing our understanding and capabilities in these arenas. Given the predicted ongoing risk of wildfire to water supplies, it is imperative to develop capacity to predict, monitor and mitigate wildfire effects on our water resources.