Date of Award

6-2024

Degree Name

MS in Environmental Sciences and Management

Department/Program

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

Climate change and a growing wildland-urban interface are associated with an increase in the number and severity of wildfires. While severe wildfires do cause a costly and dangerous impact on the environment and the public, their after-effects comprise important subjects of study. In post-fire hydrologic studies, the reduced infiltration and plant cover that results from wildfires can commonly be associated with increased peakflows and erosion. Phenomena such as the flooding and debris flows in Montecito after the Thomas Fire in late 2017 can have important implications on infrastructure and human lives. Quantifying post-fire peak flows and flooding volumes has therefore become a special study of interest. However, the few studies that exist on post-fire hydrology across the United States have inconclusive and flawed results. Post-fire hydrological studies on coastal-redwood forests are also lacking in both number and quality.

These research gaps were addressed through a three-year, post-fire hydrological study on the Litte Creek watershed of Swanton Pacific Ranch in Davenport, CA. The study took place from the 2021 to 2024 hydrological years (HY). Hydrographs and rating curves were developed to yield results for post-fire conditions. A frequency analysis utilizing Log Pearson Type III and Weibull calculations yielded post-fire results according to 0.5-, 1-, 2-, and 5-year return intervals. A statistical model based on precipitation, antecedent precipitation index, and streamflow data modelled pre-fire values. Subsequently, the pre- and post-fire peak flows and storm volumes were compared. The post-fire results from the analysis were also compared to those estimated from the USGS Linear Regression method and the RF-5 method from Wilder et al, 2020.

It was found that a slight increase in post-fire peak flows and storm volumes occurred upon comparison to pre-fire values, primarily for the South Fork of the watershed. This observation was found to be present for flows and volumes below a 0.5-year return interval, with higher uncertainty beyond that threshold. The USGS and Wilder methods were found to v inaccurately predict the post-fire peak flows. However, calculation of USGS-Regression parameters for two areas (Main Stem and South Fork) yielded percent increases in post-fire peak flows (between 19-31%) and percent-runoff increases (between 36-59%) for peak flows between return intervals of 0.5 to 5 years. Results were found to be mostly statistically significant for all three sites (α < 0.05), except for the North-Fork site in several cases and some model intercepts across all three sites. Incision and erosion of streambeds and the recovery of vegetation since the 2020 fire may have played an underlying role in the study. Despite some errors in stage data, the small increases in post-fire parameters, and variations in burn-area conditions, the study represented a motivated attempt to use linear regression on post-fire hydrologic data and to improve upon the site’s previous study through a more comprehensive dataset. This attempt serves as one of many in studying post-fire hydrology, which serves to inform better decisions for wildfire, flood-management, and land-management agencies.

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