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Wildfires act as potent agents of weathering and erosion, triggering the mobilization of dry ravel sediment and often resulting in temporary increases in sediment transport rates and associated debris-flow hazards. Existing hillslope sediment flux models fail to adequately capture the complex dynamics between erosion and deposition, particularly after wildfires and in landscapes dominated by processes such as bioturbation, tree falls, or other disturbances. To better understand bioturbated dry ravel and subsurface soil properties, we seek to study two hillslopes affected by the 2020 Santa Clara Unit Lightning Complex Fire at the University of California Blue Oak Ranch Reserve near San Jose, CA, by employing a novel application of short-lived radionuclides to characterize dry ravel transport processes. We used gamma spectroscopy on soil cores and recently excavated material from squirrel burrows, sampled along transects from the channel to the ridge on two opposing hillslopes to determine the concentrations of short-lived meteoric radionuclide 210Pb. Our initial findings indicate that the excess 210Pb in soil core sediment varies from the hillslope ridge to toe. In the ridge soil cores, concentrations initially increase within the top 5 cm, followed by a sharp exponential decline with depth. However, the toe soil cores show a sharp exponential decrease in concentrations from the soil surface to ~35-45 cm depth. The toe cores have a concentration of ~ 80 Bq/kg near the surface, while the ridge cores have a much lower concentration of ~ 30 Bq/kg. Based on this preliminary data, we infer that deposition of lower-concentration soil excavated from squirrel burrows leads to mixing of the upper soil layers. In contrast, the well-preserved exponential decay profile and higher surface concentrations at the steeper toe locations indicate less mixing overall. These initial findings warrant further examination of sediment characteristics at various depths through continued gamma spectroscopy and comparative analysis of shallow subsurface structure from ground-penetrating radar. These observations enhance our understanding of the roles of surface gradient and bioturbation in post-fire steepland sediment dynamics. 

Abstract. Post-fire changes to the transport regime of dry ravel, which describes the gravity-driven transport of individual particles downslope, are poorly constrained but critical to understand as ravel may contribute to elevated sediment fluxes and associated debris flow activity observed post-fire in the western United States. In this study, we evaluated post-fire variability in dry ravel travel distance exceedance probabilities and disentrainment rates in the Diablo Range of central coastal California following the Santa Clara Unit Lightning Complex fire of August 2020. Between March 2021 and March 2022, we conducted repeat field experiments simulating ravel with in situ particles (3–35 mm diameter) on a range of experimental surface gradients (0.38–0.81) on both grassy south-facing slopes and oak woodland north-facing slopes. We characterized post-fire evolution in particle transport by fitting a probabilistic Lomax distribution model to the empirical travel distance exceedance probabilities for each experimental particle size, surface gradient, and time period. The resulting Lomax shape and scale parameters were used to identify the transport regime for each subset of simulated ravel, ranging from “bounded” (light-tailed or local) to “runaway” (heavy-tailed or nonlocal) motion. Our experimental results indicated that as vegetation recovered over the first 2 years post-fire, the behavior of small particles (median intermediate axis of 6 mm) became less similar across the experimental sites due to different vegetation structures, whereas medium and large particles (median intermediate axes of 13 and 28 mm, respectively) exhibited a general transition from more runaway to more bounded transport, and large particles became less sensitive to surface gradient. All particle sizes exhibited a decrease in the length scale of transport with time. Of all particle subsets, larger particles on steeper slopes were more likely to experience nonlocal transport, consistent with previous observations and theory. These findings are further corroborated by hillslope and channel deposits, which suggest that large particles were preferentially evacuated from the hillslope to the channel during or immediately after the fire. Our results indicate that nonlocal transport of in situ particles likely occurs in the experimental study catchment, and the presence of wildfire increases the likelihood of nonlocal transport, particularly on steeper slopes. 

Ostensibly, the main goal of the ISEE Professional Development Program (PDP) is to teach scientists and engineers how to be intentional, inclusive educators by experiencing and designing inquiry-based learning activities. However, the PDP program has many indirect, positive effects on its participants as well, including building community and a sense of STEM identity, fluency to understand and discuss diversity, equity, and inclusion topics, and recognizing the importance of psychological safety in learning, academia, and industry. We present four narratives from past participants with underestimated minority identities, who discuss how the PDP program had a positive impact on their growth as scientists and engineers. In each case, the PDP provided critical tools, knowledge or support that enabled their success as graduate students and into their respective career and life journeys.