Variable wildfire impacts on the seasonal water temperatures of western US streams: An empirical study
Citation:
Beyene, M., S. Leibowitz, M. Snyder, J. Ebersole, AND V. Almquist. Variable wildfire impacts on the seasonal water temperatures of western US streams: An empirical study. PLOS ONE . Public Library of Science, San Francisco, CA, 17(7):e0268452., (2022). https://doi.org/10.1371/journal.pone.0268452
Impact/Purpose:
Over the past thirty years, high-severity wildfires have become more pervasive across the western US. High-severity fires can burn down riparian vegetation that offers shade to streams, stabilize streambanks, and trap sediments and precipitation. Water resource managers in the region are concerned about wildfires’ effect on streams thermal regimes and their implications for salmonids. The present study assesses wildfire effects on winter and summer stream temperatures of three WUS streams, focusing on understanding the diversity in stream water temperature response across seasons and watersheds. It differs from previous studies in the following critical ways. We are studying post-fire effects not just on the summer water temperatures as most studies do, but also on winter water temperatures. Moreover, aside from the paired watershed comparison approach, we are not aware of any statistical method advanced for approximating fire effects on the thermal regime of unregulated streams. In this study, we demonstrate the capability of three different statistical methods that utilize local stream temperature data to detect and characterize wildfire contributions on the post-fire stream thermal regime while accounting for the differences in pre-and post-fire weather conditions. This study also looks at multiple facets of the post-fire changes in seasonal stream water temperature, including post-fire changes in air-water temperature response and weather and fire contributions on the post-fire changes in stream temperatures. Finally, the results of our three separate analytical approaches offer evidence that large, high-severity riparian vegetation burns can affect the winter and summer thermal regime of streams with medium-sized (78-334 km2) watersheds for at least the following three post-fire years. While this research is mainly methodological in nature, it could ultimately lead to a better understanding of how wildfires affect stream temperatures – and therefore stream biota – under a range of natural conditions.
Description:
High severity fires can burn down riparian vegetation that shade streams, stabilize streambank and trap moisture and sediment. Water resources managers are concerned about post-fire effects on streams’ thermal regime and their implications for salmonids. In this study, we assessed wildfire effects on the winter and summer water temperature indices of stream stations at Boulder Creek (Oregon), Elk Creek (Oregon), and Gibbon River (Wyoming) over the three following years. We also employed three statistical approaches (bootstrap, air-water temperature regression analysis, and random forest regression) that utilize local water temperature data to examine different facets of the post-fire change in seasonal stream temperatures while accounting for weather effects. Random forest regression results show that at Gibbon River and Boulder Creek stations, wildfires were related to a significant (p < 0.05) decrease in the average winter mean water temperature (MWT) and accumulated degree days (ADD) by 0.31-0.38 ºC and 29.1-61.9 degree days Celsius (DDC), respectively, relative to that of pre-fire. This was despite the weekly air-water temperature regression analysis indicating a steeper air-water temperature relationship at these stations post-fire. The burning of riparian vegetation at Gibbon River and Boulder Creek stations also corresponded to a 0.4-1.1 ºC increase in the average summer MST and a 32-100 DDC increase in the average summer ADD, relative to that of pre-fire. The fire-related changes in seasonal stream temperature indices across the three-stream stations appeared to be determined primarily by the site’s distance from the burn perimeter. The contrasting wildfire effect on the winter and summer stream temperatures could pose additional challenges for thermally-sensitive fishes that are already imperiled by other stressors in these streams and rivers.
URLs/Downloads:
DOI: Variable wildfire impacts on the seasonal water temperatures of western US streams: An empirical study