Citation:

Beyene, Mussie T, Scott G Leibowitz, AND Michael J Pennino. Parsing Weather Variability and Wildfire Effects on the Post-Fire Changes in Daily Stream Flows: A Quantile-Based Statistical Approach and Its Application. WATER RESOURCES RESEARCH. American Geophysical Union, Washington, DC, 57(10):e2020WR028029, (2021). https://doi.org/10.1029/2020WR028029

Impact/Purpose:

In the western United States, nearly 65% 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.

Description:

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. and 0. 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 5,000% (Standard Error;  2%) at the 0. quantile and 161% ( 10%) at the 0. quantile. The fire-related increase in flow was often pronounced at the 0. 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.

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