Streamflow Response to Wildfire Differs With Season and Elevation in Adjacent Headwaters of the Lower Colorado River Basin

Fires increasingly impact forested watersheds, with uncertain water resources impacts. While research has revealed higher peak flows, longer-term yields may increase or decrease following fire, and the mechanisms regulating post-fire streamflow are little explored. Hydrologic response to disturbance is poorly understood in the Lower Colorado River Basin (LCRB), where snowmelt often occurs before the growing season. Here, we quantify annual streamflow changes following what have been, before 2020, two of the largest wildfires in the modern history of the contiguous United States. We evaluate nine nested watersheds with >50 years records within the Salt River Basin to evaluate fire impact over ranges of elevation, climate, vegetation, burned area, and spatial scale. We employ double-mass comparison of paired watersheds, pre- and post-fire runoff ratio comparison, multiple linear regression of climate and fire, and time-trend analysis. Precipitation and streamflow are decoupled during dry periods; therefore we conduct separate change detection for wet and dry periods. Post-fire summer streamflow increased by 24%-38% at all elevations. While winter/spring streamflow remained constant in the highest, coldest headwaters, winter flows declined in lower-elevation headwaters. As a result, basin annual streamflow declined. These results support emerging understanding that warm semiarid watersheds respond differently to disturbance than well-studied, colder watersheds. Asynchrony between winter snowmelt and summer evaporative demand is likely important when considering long-term impacts of forest management and disturbance on water supply in the LCRB.

Automated summary

Fires have adverse effects on forested watersheds, but the impact on water resources is uncertain. This study evaluates the effects of two major wildfires in the Lower Colorado River Basin, considering various factors such as elevation, climate, vegetation, burned area, and spatial scale. The results indicate that warm semiarid watersheds respond differently to disturbance compared to colder watersheds, emphasizing the importance of considering the asynchrony between winter snowmelt and summer evaporative demand when assessing long-term impacts on water supply.