Forest watersheds provide over half of our national water supplies. Millions of people depend on surface freshwater supplies from fire-prone headwater forests, used for drinking, irrigation, industry, and hydropower. However wildland fires in the contiguous United States (CONUS) have increased in frequency, size, and severity, giving rise to concerns about the sustainability of clean, reliable supplies of surface freshwater. While wildfire seasons become longer, much remains unclear about the connection between wildland fire characteristics, drought, and water supplies at the national scale. The goal of this study is to generate new practical knowledge useful for decision making in minimizing wildfire impacts on water quantity and quality and enhancing watershed ecosystem resilience across the CONUS. This study focuses primarily on water quantity at a spatial scale of medium to large watersheds (>22, 000 acres or 90 km2
), approximately the size of HUC-12 watersheds. We developed a framework for evaluating wildland fire impacts on streamflow that combines double‐mass analysis with new methods (change point analysis, climate elasticity modeling, and process‐based modeling) to distinguish between multiyear fire and climate impacts. The framework captures a wide range of fire types, watersheds characteristics, and climate conditions using streamflow data, as opposed to other approaches requiring paired watersheds.
We combined river flow, climate, and wildland fire data of the past 30 years for 168 burned watersheds in the CONUS, and performed an in-depth assessment of the coupled wildland fire-surface water supply risk in 32 locations, where stream gauges were located close to the fire. These 32 locations were situated within the Osceola National Forest (NF) and Apalachicola NF in Florida, De Soto NF in Mississippi, Gila NF, Tonto NF, and Coronado NF in Arizona, Uinta NF in Utah, Okanogan NF in Washington, Sawtooth
NF, Boise NF, and Payette NF in Idaho, Siskiyou NF in Oregon, and Cleveland NF, Los Padres NF, Stanislaus NF, Lassen NF, Mendocino NF, and Klamath NF in California. This study indicates that wildfire enhanced river flow amid droughts in the Lower Colorado and Pacific Northwest regions relative to the river flow expected had the fire not occurred. However, the flow enhancement in Southern California was masked by drought. Data for the 168 burned watersheds in the CONUS show that the rule of thumb stating that at least 20% of the basal area must be removed, in order to produce any significant
change in river flow, also applies to the area affected by wildland fire. In addition, areas affected by low burn severity had no appreciable, direct effect on river flow. As a result, prescribed burns did not significantly alter river flow in the Southeast U.S. in basins larger than 10 km2 , because the area affected by the burn was typically too small (<20%), and characterized by low burn severity. We conclude that river flow after wildland fire was primarily driven by climate, with seasonal variations in precipitation playing the most dominant role in the West. Among secondary factors, wildfires with moderate to high
severity burn impacts, and topography had the greatest impact on the post-fire change in river flow in the West. Land cover, and in particular the amount of barren land and urban extent, was a secondary predictor in the East. The large regional variability of river flow responses to wildland fire has major implications for floods mitigation, watershed restoration, and for forest management policies aimed at reducing wildland fire risk and improving water supply under a changing climate. Land management options such
as prescribed burning and forest thinning to reduce fuel loads and maximize ecological benefits (i.e., water yield) must be developed to fit local conditions (i.e., climate, watershed size, and topography). Future studies are needed to examine response of ecohydrologic processes to wildland fires across multiple gradients including climate and topography to further understand and predict wildfire impacts on watersheds.