Streamflow is Resilient to Landscape-Scale Wildfires in the Lower Colorado River Basin

Wildfires are affecting forested mountain watersheds with increasing frequency and magnitude, with uncertain water resource impacts. While research shows increased peak flow and erosion during intense summer storms, long-term water supply changes remain uncertain. Here we quantify annual streamflow following two of the largest wildfires in the modern US history: the Rodeo-Chediski Fire (2002) and Wallow Fire (2011). We focus on nine gaged watersheds with long-term records (>50 years) within the Salt River basin, the primary surface water source within the state of Arizona. These nested watersheds facilitate evaluation of fire impacts over ranges of elevation, climate, vegetation, and fire extent. Due to a non-linear relationship between precipitation and streamflow, hydrologic change detection required separation of wet and dry years. We employed four alternative empirical methods: (1) double-mass comparison of paired watersheds, (2) runoff-ratio comparison pre/post fire, (3) multiple linear regression with climate variables and annually variable fire impacts, and (4) time-trend analysis using climate-driven linear models. Wildfires had mostly negative or undetectable impacts on annual streamflow. Summer streamflow increased by 24 - 38 % in all burned watersheds, but winter flows, which dominate the annual streamflow in this snowmelt-dominated basin, declined or remained unchanged. We found different hydrologic responses in mid-elevation vs. high-elevation subregions, providing context for comparing the semiarid Lower Colorado River Basin with more well-studied, colder regions. While an overall lack of increased streamflow challenges long-held assumptions, these results support an emerging understanding that in semiarid regions, forested montane watersheds are resilient to disturbance.