POST-FIRE AEOLIAN DEPOSITION AND CARBON FLUX IN WESTERN RANGELANDS

The Soda Fire burned over 280,000 acres in southwest Idaho and southeastern Oregon in August 2015. The fire burned >25% of the Reynolds Creek Critical Zone Observatory, creating an opportunity to investigate post-fire erosional response in a sagebrush-steppe dominated watershed with complex topography. While many post-fire erosion studies focus on water-driven processes (e.g. floods and debris flows), observations of post-fire erosion from this and other rangeland systems suggests wind transport of sediment and charred material is an important agent of erosion. We measure the spatial variability, magnitude, and character of wind and water erosion through the use of 1) passive dust/colluvium traps and soil sampling 2) geochemical fingerprinting and 3) analysis of meteorological and hydrological data. Results show that while aeolian deposition of burnt organic material decreases within the first year following fire, overall mass deposition in the passive dust traps remains comparable beyond one year post-fire. The lack of shrub coverage over the burned landscape allows wind to maintain speeds above the surface threshold shear velocity, entraining material into suspension. In-channel, upper slope colluvial traps within the fire perimeter show a peak in sediment flux during winter months with fluctuating snow cover. High soil burn severity is tightly linked to pre-fire vegetation density; riparian corridors and slopes with dense sagebrush and thick soils were severely burned as compared to less densely vegetated, generally rockier, south-facing slopes. Preliminary results indicate greater volumes of material eroded from north-facing vs. south-facing hillslopes. A better understanding of the interplay between wind and water erosion over complex terrain in small watersheds will lead to better erosion and carbon flux models.