https://digitalcommons.calpoly.edu/nres_rpt/33
Date of Award
9-2022
Degree Name
MS in Environmental Sciences and Management
Department
Natural Resources Management
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Christopher Surfleet
Advisor Department
Natural Resources Management
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
The size and severity of wildfires have increased in California during recent decades. This trend is highlighted through the CZU Lightning Complex fire of August 2020 which burned over 86,000 acres in the Santa Cruz mountains of California. The fire greatly impacted the Little Creek watershed, a roughly 1,300 acre watershed that exists largely within Cal Poly San Luis Obispo’s Swanton Pacific Ranch (SPR). The current trends of California’s increased wildfire regime are expected to continue, raising concerns regarding the direct and secondary effects on forest watersheds and the effectiveness of current post-fire erosion control management. Accelerated rates of erosion following wildfire have been found to occur due to the loss of vegetation cover and changes in soil physical properties. We measured hillslope erosion from ten plots at SPR using a silt fence erosion trap approach to study regional post-fire erosion dynamics in the second winter following the CZU Lightning Complex fire. Slope steepness, percent soil cover, and percent canopy coverage were found to be significant factors driving changes in post-fire hillslope erosion in a multivariate model (R2=0.88). Field-collected data from the erosion plots was used to inform spatial extrapolation of hillslope erosion and sediment delivery rates for the entire Little Creek watershed under different soil cover and precipitation scenarios using the Universal Soil Loss Equation (USLE). A watershed average hillslope erosion rate was found to be 4.23 tons/acre/year during the study period from October 2021-March 2022, a 53-fold increase when compared to pre-fire erosion rates and surpassing the watershed average soil loss tolerance factor. Annual sediment delivery to streams within the Little Creek watershed was quantified at 1.16 tons/acre from contributing hillslopes, a 58-fold increase from pre-fire sediment delivery. Using the information obtained from the results from this study, a review of scientific literature, and interviews with relevant stakeholders, we also identify current issues limiting the effectives of post-fire erosion control and provide recommended policy changes and best management practices to mitigate these problems. The results of this study provide valuable information and context regarding post-fire erosion dynamics in the Santa Cruz region and inform future managing decisions aimed mitigating accelerated rates of erosion following wildfire.