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Phase three: Dixie post-fire BAER soil burn severity maps released

Burned Area Emergency Response (BAER) teams are sent to federal lands following significant wildfires to identify imminent post-fire threats and give recommendations on how to reduce the potential impacts on human life and safety, property infrastructure, and critical natural and cultural resources.

These multi-disciplinary and sometimes multi-agency teams assess wildfire-impacted burned areas almost immediately after the fire threat passes. Their focus is on the direct damage caused by the fire itself, rather than from suppression, and the after-effects. Post-fire conditions of loss of vegetation and the changes in the soils may increase the flow of water and could cause potential debris and sediment flow impacts.

In August, a Forest Service (USFS) BAER team began evaluating post-fire effects of the Dixie Fire on National Forest System (NFS) lands, while a US Department of Interior (DOI) BAER team focused specifically on the portion of the Dixie Fire that burned in the Lassen Volcanic National Park (LAVO). The DOI BAER team was assembled by the National Park Service (NPS) on September 9, 2021. For information about the DOI BAER assessment, please go to the LAVO Dixie Fire webpage found at: https://www.nps.gov/lavo/learn/nature/dixie-fire.htm.

page1image1700160Due to the large size and continual active burning of the Dixie Fire, the USFS BAER team divided the burned area into three phases for their assessment and analysis. The BAER specialists recently completed their data gathering and analysis of the Dixie burned area to produce a Phase 1 soil burn severity (SBS) map on August 25—analyzing 365,678 acres, a Phase 2 SBS map on September 15— analyzing 404,473 acres, and two Phase 3 SBS maps for the west and east survey areas on October 1, 2021. The USFS BAER team analyzed a total of 207,984 acres during its Phase 3 assessment. The maps and the data display SBS categories of unburned/very low, low, moderate, and high. For the west survey area of Phase 3, approximately 62% of the 113,427 acres are either unburned/very low and/or low soil burn severity, while 36% sustained a moderate soil burn severity and only about 2% identified as high soil burn severity. The east survey area in Phase 3, approximately 44% of the total acres burn are either unburned/very low and/or low soil burn severity, while 53% sustained a moderate soil burn severity, and only about 3% identified as high soil burn severity.

Both Phase 3 SBS maps also show the acreage for each of the landowners for the total 207,984 acres in the Phase 3 assessment to be: 87,476 acres for the Plumas National Forest; 66,517 acres for the Lassen National Forest; 44,064 acres for the National Park Service; 7,305 acres of private/unknown lands; 1,821 acres for the State of California-Lands Commission; and 803 acres for the DOI Bureau of Land Management.

It is important to note that the BAER team assesses the effects to soils and not the effects to vegetation. SBS characterizes the effects at the soil surface and below-ground whereas vegetation effects are determined based on mortality and vegetation canopy changes.

Changes in soil cover, water repellency, and soil physical/biological changes determine the severity level of soil burn severity. Changes in water repellency is a much-discussed fire effect. Water repellency is a natural soil property. Fire can increase the severity and the thickness of the water repellent soil which has significant effects to post-fire water runoff.

Low SBS indicates there was only partial consumption of fine fuels while litter coverage remains relatively intact on the soil surface. Burning time at the soil surface was short, leaving root systems and root structure undamaged. Vegetative recovery time in the low category will vary based on ecological community but is expected to recover in the short-term.

Moderate SBS indicates nearly all soil cover of vegetative litter and fine fuels was consumed or converted to ash. Because soil cover is significantly reduced, accelerated water runoff is expected. Charring of the mineral soil occurs in Moderate SBS as well as shallow root burning. The extent of the burning of the leaves and needles on the trees (aka tree canopy) can be unpredictable and can range from high to relatively low mortality. Water repellency is often found at the surface and is increased for both in severity and thickness of the water repellent soils which reduces the ability of precipitation to infiltrate the soil surface.

High SBS is the result of higher intensity fire behavior or longer burning time at the soil surface. As a result of the high heat, nearly all the soil cover of vegetative litter and fuels has been consumed leaving bare soil prone to the impacts of precipitation and resulting water runoff. The surface mineral soil has been reduced to powder (single grain) and often several inches thick. This single grain soil is very easy transported or moved during rain events resulting in excessive soil erosion and sediment loading in rivers, streams, and creeks. The roots in the High SBS areas tend to be completely consumed by the resulting heat of the fire above the soil surface. Water repellency does not exist at the surface because water repellent compounds have been vaporized and tends to be found below the powdered soil surface, but the repellency thickness and more severe burning tend to be much greater than a Moderate SBS soil. Generally, there is 100% tree mortality in High SBS soils. Fire-adapted shrubs and vegetation such as bear clover, manzanita, and deer brush tend to come back with vigor because of root sprout; however, because seed sources are consumed in these High SBS areas, conifers may take many years to re-establish without tree planting.

The Dixie BAER assessment team used initial remote sensing imagery with its field validated soils data, to develop and produce the final SBS maps. The BAER team and the US Geological Survey (USGS) both use the SBS maps as an analysis tool to estimate post-fire flows, debris flow probability, erosion and sedimentation flow rates. The USGS initial preliminary hazard assessment map and report for the Dixie burned area is posted at: https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=368.

The BAER team relied on its refined soil burn severity map to produce data used in its subsequent modeling and determination of post-fire runoff and sedimentation. In specific areas that experienced moderate-to-high burn severity, there could be increased runoff from steep hillslopes and resultant increases in post-fire soil erosion and potential debris flows.

The Dixie Fire soil burn severity map can be downloaded at the “Dixie Post-Fire BAER” InciWeb site (https://inciweb.nwcg.gov/incident/7811/) as a JPEG or PDF version under the “Maps” tab.

Dixie Post-Fire BAER Assessment information is available at: https://inciweb.nwcg.gov/incident/7811/ Dixie DOI BAER Assessment information is available at: https://www.nps.gov/lavo/learn/nature/dixie-fire.htm


SPECIAL NOTE: Everyone near and downstream from the burned areas should remain alert and stay updated on weather conditions that may result in heavy rains over the burn scars. Flash flooding may occur quickly during heavy rain events-be prepared to take action. Current weather and emergency notifications can be found at the National Weather Service website: www.weather.gov/sto/.

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