Interaction of wind and cold-season hydrologic processes on erosion from complex topography following wildfire in sagebrush steppe

Authors: VEGA, S.P. - University Of Idaho; Williams, Christopher - Jason; BROOKS, E.S. - University Of Idaho; Pierson Jr, Frederick; STRAND, E.K. - University Of Idaho; ROBICHAUD, P.R. - Us Forest Service (FS); BROWN, R.E. - Us Forest Service (FS); Seyfried, Mark; LOHSE, K.A. - Idaho State University; GLOSSNER, K. - Idaho State University; PIERCE, J.L. - Boise State University; ROEHNER, C. - Boise State University

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2019
Publication Date: 3/30/2020
Citation: Vega, S., Williams, C.J., Brooks, E., Pierson Jr, F.B., Strand, E., Robichaud, P., Brown, R., Seyfried, M.S., Lohse, K., Glossner, K., Pierce, J., Roehner, C. 2020. Interaction of wind and cold-season hydrologic processes on erosion from complex topography following wildfire in sagebrush steppe. Earth Surface Processes and Landforms. 45(4):841-861. https://doi.org/10.1002/esp.4778.
DOI: https://doi.org/10.1002/esp.4778

Interpretive Summary: Fire removal of vegetation and ground cover in sagebrush ecosystems commonly amplifies soil losses by wind and water. Erosion following fires on sagebrush landscapes often is the result of the interplay in sediment redistribution by wind immediately post-fire and subsequent flushing of these sediments during the snowmelt runoff period. Although such responses have been commonly observed, there have been few studies to quantify interactive effects of wind and water erosion post-fire. This study measured fire impacts on erosion from natural runoff events along hillslopes and in side channels at a snow-dominated mountainous sagebrush site over a two year period post-fire. We measured substantial levels of erosion from burned and bare hillslopes and side channels the first year post-fire and from side channels the second year post-fire. Nearly all the sediment measured during the study was associated with the interaction of wind and water erosion processes during winter. Our results demonstrate that the interaction of wind- and water-driven erosion processes is an important component for consideration in post-fire erosion assessment and prediction, and can have profound implications for soil loss from sagebrush ecosystems and other rangelands with a snow-dominated climate.

Technical Abstract: Wildfire is a natural component of sagebrush (Artemisia spp.) steppe rangelands that induces temporal shifts in plant community physiognomy, ground surface conditions, and erosion rates. Fire alteration of the vegetation structure and ground cover in these ecosystems commonly amplifies soil losses by wind- and water-driven erosion. Much of the fire-related erosion research for sagebrush steppe has focused on either erosion by wind over gentle terrain or water-driven erosion under high-intensity rainfall on complex topography. However, many sagebrush rangelands are geographically positioned in snow-dominated uplands with complex terrain in which runoff and sediment delivery occur primarily in winter months associated with cold-season hydrology. Erosion following fires on these landscapes often is the result of the interplay in sediment redistribution by wind immediately post-fire and subsequent flushing of these sediments during the cool- to cold-season snowmelt runoff period. Although such responses to fire have been commonly observed on wildands, research is limited regarding this interaction of wind- and hydrologic-driven erosion processes. In this study, we evaluated fire impacts on vegetation, ground cover, soils, and erosion across spatial scales at a snow-dominated mountainous sagebrush site over a two year period post-fire. Vegetation, ground cover, and soil conditions were assessed at various plot scales (8 m2 to 3.42 ha) through standard field measures. Erosion was quantified through a network of silt fences (n = 24) spanning hillslope and side channel or swale areas, ranging from 0.003 ha to 3.42 ha in size. Sediment delivery at the watershed scale (129 ha) was assessed by suspended sediment samples of streamflow through a drop-box v-notch weir. Wildfire consumed nearly all above-ground live vegetation at the site and resulted in more than 60% bare ground (bare soil, ash, and rock) in the immediate post-fire period. Widespread wind-driven sediment loading of swales was observed over the first month post-fire and extensive snow drifts were formed in these swales each winter season during the study. In the first year, sediment yields from north- and south-facing aspects averaged 0.99-8.62 t•ha-1 at the short-hillslope scale (~0.004 ha), 0.02-1.65 t•ha-1 at the long-hillslope scale (0.02-0.46 ha), and 0.24-0.71 t•ha-1 at the swale scale (0.65-3.42 ha), and watershed scale sediment yield was 2.47 t•ha-1. By the second year post-fire, canopy cover exceeded 120% across the site, but bare ground remained more than 60%. Sediment yield in the second year was greatly reduced across short- to long-hillslope scales (0-0.04 t•ha-1), but was similar to first year measures for swale plots (0.24-0.61 t•ha-1) and at the watershed scale (3.05 t•ha-1). Nearly all the sediment collected across all spatial scales was delivered during runoff events associated with cold-season hydrologic processes, including rain-on-snow, rain-on-frozen soils, and snowmelt runoff. Approximately 85-99% of annual sediment collected across all silt fence plots each year was from swales. The high levels of sediment delivered across hillslope- to watershed-scales in this study are attributed to preferential loading of fine sediments into swale channels by aeolian processes in the immediately post-fire period and subsequent flushing of these sediments by runoff from cold-season hydrologic processes. Our results suggest that the interaction of aeolian and cold-season hydrologic-driven erosion processes is an important component for consideration in post-fire erosion assessment and prediction and can have profound implications for soil loss from these ecosystems.