EFFECTS OF THE 2021 CALDOR FIRE (CA, USA) ON SOIL HYDRAULIC AND THERMAL PROPERTIES: A PILOT STUDY

The Caldor fire burned ~222,000 acres in the Sierra Nevada beginning Aug. 14, 2021. It was the second of two fires in recorded history to cross the Sierras and burned extensive areas with steep slopes and thin soil cover. Fire containment (Oct. 21, 2021) occurred in the wake of a record-breaking California Bomb Cyclone event that covered the region in rain and snow, increasing the potential for landslide and debris flow hazards. Much of the mountainous areas that burned in the fire perimeter were historically unburned (1878-2020), making the burned areas a prime natural laboratory to study fire-induced soil modifications. In response, we collected intact soil cores from the upper 8 cm of the land surface to study potential changes to soil hydraulic and thermal properties and to quantify effects of fire on soil structure. A total of 5 control and 8 burned samples were collected for hydraulic and thermal measurements. Hydraulic conductivity and water retention functions were measured using KSAT and HYPROP devices. Thermal conductivity was simultaneously measured using a triple probe heat pulse sensor. Aliquots of each core were subsampled for laser diffraction particle size analysis, loss on ignition, and scanning electron microscope image analyses to relate changes in soil structure with measurable soil characteristics. Preliminary results indicate that the burn caused a relative increase in silt contents due to degradation of fine soil aggregates associated with decreased soil organic matter contents in the mineral soil. Burning also caused an apparent positive shift of thermal conductivity functions, especially at low moisture contents, and an overall reduction of total porosity. Surprisingly, we found little difference in hydraulic conductivity functions of fire-affected samples compared with control samples, potentially due to the sandy textures and weak secondary structure of the soil, although we note greater variability of saturated hydraulic conductivity from burned samples. Our results demonstrate a need to study unsaturated soil characteristics to help quantify post-fire soil modifications, particularly for soils with pronounced secondary structure. Future studies should explore these trends among diverse soil types and with larger sample sets to evaluate effects of spatial variability.