Oral Presentation Society for Freshwater Science 2026 Annual Meeting

Fire effects on nutrient stores and transport in aridland watersheds (135338)

Heili Lowman 1 , Ashley Cale 2 , Xiaoli Dong 3 , Stevan Earl 4 , Nancy Grimm 4 , Shanthini Ode 5 , Alex Webster 6 , Ara Winter 6 , Tamara Harms 5
  1. Duke University, Durham, NC, United States
  2. University of Nevada Reno, Reno, NV, United States
  3. University of California Davis, Davis, CA, United States
  4. Arizona State University, Phoenix, AZ, United States
  5. University of California Riverside, Riverside, CA, United States
  6. University of New Mexico, Albuquerque, NM, United States

Fires in the western U.S. are increasing in both size and severity and may significantly impact streams and rivers through the addition of burned materials and the alteration of upland flowpaths. Research on the effects of fire on freshwaters has been largely focused in mesic regions, though arid regions are at risk of lagged or more prolonged responses to fire due to highly variable precipitation regimes. Flashy precipitation and discharge regimes have contributed to uncertainty in predicting the effects of fire on watershed biogeochemistry and its implications for water quality in aridlands. We propose that stream chemistry (i.e., concentration) information should be paired with discharge records to discern whether changes in water quality are due to the input of burned material or altered hydrology. To examine the effects of fire on both solute stores and transport, we assembled long-term observations of stream chemistry and discharge for >45 watersheds across arid ecoregions of the western U.S. We used multi-level Bayesian regression models to quantify changes in concentration-discharge (C-Q) relationships for three years before and after fire. We interpret changing C-Q slopes to reflect the effects of fire on solute transport routes and post-fire change in intercepts to indicate changes in solute stores. We then examined how fire size, catchment characteristics (i.e., elevation, slope), vegetation, hydrology, and adaptation to fire influence post-fire changes in C-Q relationships. Initial results suggest that C-Q slopes for nitrate (NO3-) became more positive following fire (i.e., stronger flushing), though the magnitude of effects varied significantly across ecoregions. We also found that the percent of the watershed burned had a positive effect on post-fire NO3- C-Q intercepts, suggesting an increase in NO3- availability. Examining the magnitude and controls of post-fire stream chemistry change is critical so that we may identify watersheds in arid regions that are most vulnerable to large and prolonged changes in water quality.