Watershed Symposium 2019

Megafires and Hurricanes: Utah Watersheds Respond to Climate Change

Summary:
In late 2018 a wildfire burned over 100,000 acres in Utah County. Following the wildfire, a storm surge from the remnant of Hurricane Rosa caused extreme runoff and erosion. We collected water samples before, during, and after the events, quantifying carbon and nutrient concentrations and biodegradability.

Full Abstract:
Climate change in the western U.S. is causing larger wildfires and more extreme precipitation events. When these two ecological changes collide, they create massive ecosystem disturbance, affecting terrestrial and aquatic environments as well as human well-being. In October 2018, such a scenario occurred when the remnants of Hurricane Rosa dumped torrential rain on a two-week old, 610-km2 burn scar in central Utah. The wildfires, flash flooding, and debris flows triggered the evacuation of approximately 10,000 residents and created a sediment plume Utah Lake that was visible from space. We collected stream water samples from 10 watersheds during and after the storm, allowing us to quantify the interactive effects of megafire and extreme rain on aquatic biogeochemical fluxes. We analyzed samples for a broad suite of physicochemical parameters including organic matter concentration and biodegradability, water isotopes, major ions, trace metals, and nutrients. While the burned and unburned streams showed various concentration-discharge relationships, the effects of the megafire were apparent in nearly every parameter we quantified, increasing particulate loading and resulting in a substantial loss of terrestrial carbon, nitrogen, and phosphorus. Additionally, the urban footprint was a major predictor of concentration and variability of nutrient concentrations. Together, these findings suggest that the combination of increased wildfire and extreme storms could affect water supply for humans and ecosystem status of downstream rivers and lakes.