Abstract

Wildfires have significantly increased in number, size and severity in the western United States since 1990. High severity wildfires of conifer forests are thought to be responsible for the majority of long-term landscape erosion. As post-fire erosion occurs, mineral surfaces are exposed to the environment intensifying weathering processes, and consequently altering nutrient cycling. Furthermore, similarly to what occurs in a wildfire, catchments that experience harvesting or deforestation show shifts in net fluxes of elements such as calcium, silicon and aluminum. Additionally, the burned vegetation releases nutrients and other elements that are transported down gradient via overland flow, shallow subsurface flow, and/or groundwater flow. Ash accumulations on hillslopes may also store particulate carbon and contain elevated concentrations of elements that maybe leached into surface and ground water over extended periods of time.

In 2013, the Thompson Ridge wildfire burned headwater catchments in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve, New Mexico USA. The burn severity and area impacted were different in the three-headwater catchments. This study investigates the impact of the wildfire on solute fluxes to the surface water, including how these disturbances evolve with time, and how biogeochemical processes control post-fire solute concentrations in the surface water.

Comparison of pre- and post-fire surface water solute chemistry among the three watersheds shows increases in major cations following fire. Base cation concentrations (e.g., Ca, K and Na), increased immediately within a few weeks after the fire, likely related to leaching from combusted organic matter. These high fluxes persisted for approximately two months, and returned to pre-fire levels only for Na, while Ca and K concentrations stayed high for over two years. Additionally, Al concentrations declined significantly after the fire, a pattern previously unreported in the literature. While pre-fire Al concentrations in stream flow increased significantly during the wet seasons (snowmelt and monsoons), the post-fire observations do not show significant changes with increase in discharge.

Citation

Sanchez, R.A., Meixner, T., Mcintosh, J.C., Chorover, J. (2018): Impact of Wildfire on Solute Fluxes in Forested Catchments, Jemez River Basin, New Mexico, USA. Abstract H23L-2120 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.