Streamflow response to wildfire differs with season and elevation in adjacent headwaters of the Lower Colorado River Basin

Authors: Biederman, Joel; ROBLES, M. - Nature Conservancy; Scott, Russell - Russ; KNOWLES, J.F. - California State University

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2022
Publication Date: 3/2/2022
Citation: Biederman, J.A., Robles, M., Scott, R.L., Knowles, J. 2022. Streamflow response to wildfire differs with season and elevation in adjacent headwaters of the Lower Colorado River Basin. Water Resources Research. 58(3). Article e2021WR030687. https://doi.org/10.1029/2021WR030687.
DOI: https://doi.org/10.1029/2021WR030687

Interpretive Summary: In the Western U.S., wildfires are impacting the forested mountains important to agricultural and urban water supply. While peak streamflow and soil erosion often increase immediately after fires, it is unknown whether water supplies are altered over multiple years. Here we quantified streamflow in eight watersheds within Arizona’s Salt River basin during 1 – 15 years following two of the largest fires in the modern history of the Western U.S. Four independent methods suggested that streamflow declined or remained the same. Summer flows were shown to increase in two of the most heavily burned watersheds. However, winter flows declined or did not change. With >80% of the annual streamflow generated during winter, winter response to vegetation change dominates annual response. We demonstrate the importance of separately analyzing wet and dry years to detect wildfire impacts on hydrology in the 21st Century, which has been warmer and drier than most of the pre-fire record.

Technical Abstract: In recent decades, forested mountain watersheds are impacted by landscape-scale wildfires of increasing frequency and magnitude, with uncertain water resource impacts. While research shows short-term increases in peak flow and soil erosion during high-intensity summer storms, comparatively little is known about streamflow response over multiple years. An outstanding question is whether short-term increases in peak flow should lead us to expect meaningful long-term increases in water supply. Here we quantify annual streamflow changes following (up to 15 yrs.) two of the largest wildfires in the modern history of the western continental U.S. We focus in detail on eight watersheds with long-term records (>50 yr.) within the Salt River basin, the primary surface water source generated within the state of Arizona. These nested watersheds facilitate evaluation of fire impacts over ranges of elevation, climate, vegetation, and fire extent (5 – 71%). We employed four alternative empirical methods: (1) double-mass comparison between burned and control watersheds, (2) runoff ratio comparison pre- and post-fire, (3) multiple linear regression with climate variables and annual satellite-based fire metrics, and (4) time-trend analysis using climate-driven linear models. We found that wildfires had mostly negative or undetectable impacts on annual streamflow. The entire Salt River basin, which was ~10% burned by each of two major fires in 2002 and 2011, showed declines of 20 – 30% in annual streamflow by runoff ratio and time-trend methods, though only the latter was significant. Summer streamflow increased in the most heavily burned watersheds, but winter flows declined or remained unchanged, controlling the annual response in this snowmelt-dominated basin. A focused analysis on one basin and its nested watersheds highlighted how detection of wildfire impacts on hydrology is challenged by the warmer, drier hydroclimate prevailing in the 21st-Century. Change detection therefore requires special consideration when applying methods of pre- and post-fire streamflow comparison.