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

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, there was a significant increase in daily stream Q and SSC 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. A post-fire increase in the magnitude and frequency of peak SSC events were also recorded for stream in New Mexico. In addition, many studied streams had earlier and longer annual peak Q and SSC season. From a water supply system perspective, the results imply that wildfires correspond to increased uncertainty in source water quality and quantity.

Impact/Purpose

In the western United States, nearly 66% of the surface freshwater supply is derived from streams originating from forested watersheds, which are prone to wildfires. However, wildfire effects on the freshwater resources in the region have yet to be wholly understood nor factored into regional water resources planning and management strategies. For the first time, this paper evaluated the changes in the magnitude, frequency, and timing of annual extreme stream discharge (Q) and suspended sediment concentrations (SSC) following wildland fires, across the western USA. Emphasis was given to annual extremes as water supply storage and treatment costs, operations and infrastructures are more sensitive to changes in the statistics of extreme stream water quality and quantity. Results show that for several streams in the western United States, the post-fire increase in daily Q and SSC in the lower, middle, and upper ranges were not uniform. This result debunks the long-held assumption in wildfire studies that the post-fire changes in extreme stream water quality and quantity correspond to that of the changes in mean (median). It also implies that for water infrastructure planning and design, statistical models that can only measure the average (median) response should not be employed to estimate the change in the risk of extreme stream water quality and quantity events due to fire. This paper is also one of a handful of studies that have sought to examine the post-fire changes in stream water quality and quantity in the western United States at a continental scale. Consequently, the results of this study could be used in developing regional water resource management strategies for the western USA that mitigate the potential negative impacts of post-fire changes in stream water quantity and quality and take advantage of the beneficial impacts.

Citation

Beyene, M. AND S. Leibowitz. Changes in stream water and sediment flow regime following wildland fires in western United States. American Geophysical Union Fall Meeting, 2019, San Francisco, CA, December 09 - 13, 2019.