To burn or not to burn: An empirical assessment of the impacts of wildfires and prescribed fires on trace element concentrations in Western US streams.

The use of low-severity prescribed fires has been increasingly promoted to reduce the impacts from high-severity wildfires and maintain ecosystem resilience. However, the effects of prescribed fires on water quality have rarely been evaluated relative to the effects of wildfires. In this study, we assessed the effects of 54 wildfires and 11 prescribed fires on trace element (arsenic, selenium, and cadmium) concentrations of streams draining burned watersheds in the western US. To obtain results independent of the choice of method, we employed three independent analytical approaches to evaluate fire effects on water quality for the first three post-fire years. In general, we observed significant increases in trace element concentrations in streams burned by large, high-severity wildfires, despite substantial variability across sites. Comparatively, we did not observe increases in the spring mean concentration of arsenic, selenium, and cadmium in watersheds burned by prescribed fires. Our analysis indicated that the post-fire trace element response in streams was primarily influenced by burn area, burn severity, post-fire weather, surface lithology, watershed physiography, and land cover. This study's results demonstrate that prescribed burns could lessen the post-fire trace element loads in downstream waters if prescribed fires reduce subsequent high severity fires in the landscape.

Variable wildfire impacts on the seasonal water temperatures of western US streams: A retrospective study.

Recent increases in the burn area and severity of wildfires in the western US have raised concerns about the impact on stream water temperature-a key determinant of cold-water fish habitats. However, the effect on seasonal water temperatures of concern, including winter and summer, are not fully understood. In this study, we assessed the impact of wildfire burns at Boulder Creek (Oregon), Elk Creek (Oregon), and Gibbon River (Wyoming) watersheds on the downstream winter and summer water temperatures for the first three post-fire years. To obtain results independent of the choice of the analytical method, we evaluated the consequence of each burn using three different statistical approaches that utilize local water temperature data. Our results from the three approaches indicated that the response of water temperatures to wildfire burns varied across seasons and sites. Wildfire burns were associated with a median increase of up to 0.56°C (Standard Error; S.E. < 0.23°C) in the summer mean water temperatures (MWT) and 62 degree-day Celsius (DDC; S.E. < 20.7 DDC) in the summer accumulated degree days (ADD) for the three subsequent years across studied stream sites. Interestingly, these burns also corresponded to a median decrease of up to 0.49°C (S.E. < 0.45°C) in the winter MWT and 39 DDC (S.E. < 40.5 DDC) in the winter ADD for the same period across sites. Wildfire effects on the downstream water temperatures diminished with increasing site distance from the burn perimeter. Our analyses demonstrated that analytical methods that utilize local watershed data could be applied to evaluate fire effects on downstream water temperatures.

Wildfires can increase regulated nitrate, arsenic, and disinfection byproduct violations and concentrations in public drinking water supplies.

Wildfires are a concern for water quality in the United States, particularly in the wildland-urban interface of populous areas. Wildfires combust vegetation and surface soil organic matter, reduce plant nutrient uptake, and can alter the composition of runoff and receiving waters. At the wildland-urban interface, fires can also introduce contaminants from the combustion of man-made structures. We examine post-wildfire effects on drinking water quality by evaluating concentrations and maximum contaminant level (MCL) violations of selected contaminants regulated in the U.S. at public drinking water systems (PWSs) located downstream from wildfire events. Among contaminants regulated under the U.S. Safe Drinking Water Act, nitrate, arsenic, disinfection byproducts, and volatile organic compounds (VOCs) were analyzed in watersheds that experienced major wildfires. Surface water sourced drinking water (SWDW) nitrate violations increased by an average of 0.56 violations per PWS and concentrations increased by 0.044 mg-N/L post-wildfire. Groundwater sourced drinking water (GWDW) nitrate violations increased by 0.069 violations per PWS and concentrations increased by 0.12 mg-N/L post-wildfire. SWDW total trihalomethane (TTHM) violations increased by 0.58 violations per PWS and concentrations increased by 10.4 µg/L. SWDW total haloacetic acid (HAA5) violations increased by 0.82 violations per PWS and concentrations increased by 8.5 µg/L. Arsenic violations increased by 1.08 violations per PWS and concentrations increased by 0.92 µg/L. There was no significant effect of wildfires on average VOC violations. Nitrate violations increased in 75% of SWDW sites and 34% of GWDW sites post-wildfire, while about 71% and 50% of SWDW sites showed an increase in TTHM and HAA5 violations. Violations also increased for 35% of arsenic and 44% of VOC sites post-wildfire. These findings support the need for increased awareness about the impact of wildfires on drinking water treatment to help PWS operators adapt to the consequences of wildfires on source water quality, particularly in wildfire-prone regions.

Parsing Weather Variability and Wildfire Effects on the Post-Fire Changes in Daily Stream Flows : A Quantile-Based Statistical Approach and its Application.

Determining wildland fire impacts on streamflow can be problematic as the hydrology in burned watersheds is influenced by post-fire weather conditions. This study presents a quantile-based analytical framework for assessing fire impacts on low and peak daily flow magnitudes, while accounting for post-fire weather influences. This framework entails (a) the bootstrap method to compute the relative change in the post-fire annual flow and weather statistics, (b) Double Mass analysis to detect if post-fire baseflow and quick flow yield ratios are significantly altered, and (c) a quantile regression method to parse fire effects on flow at a specific quantile. We illustrate the applicability of this analytical framework using 44 western US streams with at least 5% of their watershed area burned. Results indicate that large, high-severity burns in upland watersheds can raise the streamflow magnitude at the 0.05 th and 0.95 th quantiles for at least the five post-fire years. Quantile regression results show that the median fire-related increase in flow for the five post-fire years can be up to 5000% (Standard Error; SE < 2%) at the 0.05 th quantile and 161% (SE < 10%) at the 0.95 th quantile. The fire-related increase in flow was often pronounced at the 0.05 th quantile for streams in the Pacific Northwest and California regions. The difference in fire effects on flow (at both quantiles) across streams was related to post-fire weather, pyrology, physiography, and land cover. The proposed analytical framework can be useful for detecting and quantifying fire effects on the low and peak stream flows in burned watersheds without overlapping disturbances.