Changes in stream water and sediment flow regime following wildland fires in western USA

Nearly 66% of the freshwater supply in the western USA (WUS) originates from forested watersheds. The recent rise in the frequency, size, and severity of wildfires across the WUS is raising concerns over fire impacts on water resources in the region, as high-severity fires in catchments can engender the release of vast quantities of water, sediments, and contaminants into downslope streams and reservoirs. From the context of water resources impact assessment, fire effects on the occurrence of extreme stream water quality and quantity merit examination, as water storage and treatment infrastructures are more sensitive to changes in extremes than to changes in the median/mean. Using stations with long-term daily data, this study investigates the post-fire changes in the daily flow of water (Q) and suspended sediments (SSC) for WUS streams, with emphasis on annual extremes. Metrics that characterize the annual frequency, magnitude, and timing of daily stream Q and SSC were developed, and seven quantiles were selected to describe the annual stream Q and SSC regime. Results show that following a fire, daily stream Q and SSC significantly increased for the majority of studied streams. However, the relative rise in daily stream Q and SSC across different quantiles was often not the same. A pronounced increase in annual low Qs and minimal changes in annual peak Qs were observed in streams in the Pacific Northwest and California, and vice versa for streams in Arizona and New Mexico. The magnitude and frequency of peak SSC events also rose for streams in New Mexico after a fire. In addition, the annual peak Q and SSC season was earlier and longer post fire for many studied streams. From a water supply system perspective, the results imply that wildfires correspond to increased uncertainty in source water quality and quantity.