Dynamics of Post-wildfire Wind Erosion of Soil

Dynamics of Post-wildfire Wind Erosion of Soil in Semiarid Rangelands, Idaho

 

Objectives:

This is a large and multifaceted research program with the overall goal being to determine and describe wildland fire effects on wind erosion potential of shrub steppe in southeastern Idaho, including some research areas on the INL Site. The specific objectives for the research include the following:

  • Jefferson FireTo evaluate how dust emissions, sediment supply, and erodibility varied among the strong microsite soil patterns consisting of shrub islands and relatively bare interspaces
  • To determine the relationship between variations in the soil surface morphology and vegetation in unburned and burned sagebrush steppe, and to experimentally determine how this variability affects native and exotic grasses
  • To determine the presence of burned and unburned vegetation and exposed soil susceptible to wind erosion associated with the 2010 Jefferson Fire
  • To quantify the role of wind erosion and dust emissions in post-fire environments as well as the associated potential impacts on air quality, facilitate a better understanding of the mechanics governing post-fire wind erosion and dust emissions, including a detailed linkage of erosion rates to site energy and water balance, with a focus on soil “skin” moisture and create a modeling framework capable of forecasting future post-fire wind events
  • To investigate wind erosion immediately following fire on areas previously either unburned or burned within the last 3 years
  • To determine if the aerodynamic parameters friction velocity, roughness length, and displacement height change through time following wildland fire, and to identify how these parameters relate to vegetation recovery after fire.

 

Summary:

Large pulses of wind erosion and resulting dust plumes are an increasingly important attribute of Idaho rangelands, particularly as wildfire occurrence and size increases. Fire increases erosivity by removing plant cover, but whether supply of erodible sediment and erodibility also increase has not been determined. The researchers determined that greater emission rates were due to greater sediment supply, but not greater erodibility. The results demonstrated that dust supply increases appreciably in initial post-fire years on previous shrub microsites. The abundance of shrubs is highly responsive to management practices that affect pre-fire vegetation, such as grazing-induced increases in shrubs that could render a site more vulnerable to dust emissions following fire.

 


The researchers hypothesized that coppice-interspace heterogeneity would remain after post-fire wind erosion, and that seed availability, germination, and growth were factors inhibiting plant establishment on interspaces. They surveyed coppice and interspace soils and plant communities at burned and unburned sites, and conducted a common-garden study of a native grass and an exotic grass on coppices and interspaces. Success of native grasses appears limited by a different combination of mechanisms than exotic grasses. Results indicate the importance of coppice-interspace heterogeneity for native plant communities. Exotic plants may decrease coppice-interspace heterogeneity, negatively affecting native plants.

Hyperspectral remote sensing imagery collected in August 2010, following the Jefferson Fire, were used to classify areas of unburned from burned vegetation, bare mineral soil, unaltered rock (basalt), and ash/soil mixture. Such information is valuable in differentiating fire effects to the vegetation and soil across the landscape using remote-sensing data. This remote-sensing data can be used to map areas of burned and unburned vegetation and exposed soil susceptible to wind erosion. These results can be used for calculating the total area of potential for soil erosion and deposition, and track recovery of vegetation in the years following fire.

Jefferson FireResearchers installed two air quality instrumentation towers in the downwind portion of the burned area of the Jefferson Fire in August 2010, and monitored the site for three months. Realtime concentrations of particulate matter with a diameter of less than or equal to 10 μm (PM10) were monitored at each tower location. Elevated PM10 concentrations were detected since the fire was contained, but the largest dust event to date occurred over September 4-5, 2010, during the passage of a frontal system. A dust plume originating from the burned area on September 4 is visible in satellite imagery and clearly extends 30 to 60 miles (50 to 100 km) downwind of the burned area. The frontal system had sustained winds of 40 mph during mid-day and nighttime winds around 13.5 mph. Early morning winds were from the northeast, with the stronger mid-day winds from the southwest. This appears to be the first study to report PM10 concentrations in a post-fire environment. The observed two-day dust event demonstrates that particulate emissions from burned areas can be large and potentially result in both local and downwind impacts.

The recent Middle Butte Fire burned 14,000 acres (5,698 ha) in summer 2010, including areas that had previously been either unburned or burned by the 2007 Twin Buttes Fires. Researchers investigated wind erosion immediately following the Middle Butte Fire in areas that were previously unburned or previously burned by the Twin Buttes Fire. Data were collected to calculate horizontal movement of sediment and deflation of the soil surface to determine if wind erosion differed between burned sites and re-burned sites.

To determine how stable aerodynamic properties (friction velocity, roughness length, displacement height) are over time in burned sagebrush steppe and relate these changes to the developing vegetation community, the researchers collected aerodynamic and vegetation data during spring and summer of 2008 and 2009 at the site of the 2007 Twin Buttes Fire. In 2010, they analyzed the aerodynamic data to determine friction velocity, roughness length, and displacement height over months. In 2011, they plan to analyze the vegetation data to begin making comparisons of aerodynamic properties to vegetation cover and height.