Compaction and cover effects on runoff and erosion in post-fire salvage logged areas in the Valley wildfire, California

Runoff and erosion can increase after wildfires, but little is known about the effects of wildfire plus post-fire salvage logging, or mitigating these effects. Past research has identified soil compaction and reduced surface cover as controls on runoff and erosion, but the relative contributions of these changes are not clear. Two years after high severity burning by the 2015 Valley Fire in California, replicated rainfall simulations were carried out in four soil conditions across compaction and cover factors: uncompacted/compacted by logging machinery and bare soil/60% wood slash-cover. Runoff after 71 mm of rainfall totaled 27 mm in the uncompacted bare plots and 39 mm in the compacted bare plots. Runoff in the slash-covered plots decreased by 50% and 33% as compared to the uncompacted and compacted bare plots, respectively, although none of the differences in runoff were significant. Rainsplash averaged 30 g for the bare plots, regardless of compaction, and decreased significantly by 70% on slash-covered plots. Sediment yield totaled 460 and 818 g m-2 for the uncompacted and compacted bare plots, respectively, and slash significantly reduced these amounts by 72% and 69%, respectively. Our results showed that post-fire soil erosion in high severity burned unlogged areas was still very high two years after the wildfire. The combination of wildfire and salvage logging doubled soil erosion by increases in both runoff amount and sediment concentration. Antecedent soil moisture (dry or wet) was the dominant factor for runoff, while surface cover was the dominant factor for erosion and sediment delivery. Covering the soil with slash reduced both runoff and erosion, suggesting this treatment would reduce long‐term sediment delivery from burned areas and skid trails. Saturated hydraulic conductivity (Ks) and interrill erodibility (Ki) calculated from these simulations confirmed previous research and will support modeling efforts related to wildfire and post-fire salvage logging.