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Regional Streamflow Response to Wildfire in California Watersheds

  • Author(s): Bart, Ryan Roger
  • Advisor(s): Hope, Allen
  • et al.
Abstract

As every watershed and every wildfire event is unique, streamflow response to wildfire is only representative of the specific watershed and conditions that produced the response. Most post-fire streamflow change experiments involve single watersheds, which limits extrapolation of the results beyond the particular watershed examined. A comprehensive understanding of post-fire streamflow response is needed at a regional scale to improve water resources planning and ecosystem management in California. For this dissertation, the regional effect of wildfire was examined for two different components of the streamflow hydrograph; annual streamflow yield and baseflow recession rates. Annual streamflow is a key variable for streamflow management, but high variability in post-fire annual streamflow response at the watershed scale has limited predictions of post-fire annual streamflow response at the regional scale. Baseflow recession rates are an important tool for predicting low flows, yet little is known about how baseflow recession rates respond to wildfire at either watershed or regional scales. A mixed model was introduced to regionalize post-fire streamflow change. Mixed modeling is a statistical approach used to synthesize data containing a hierarchical structure, such as streamflow data pooled from multiple watersheds experiments. A parsimonious storage-discharge model was used to provide insight into the hydrologic processes controlling baseflow recession rates. Annual streamflow significantly increased following wildfire in California at a regional scale. This response was greatest in watersheds with higher percentages of watershed area burnt and during moderately wet years. The first-order control on baseflow recession rates in California was found to be inter-seasonal changes in antecedent storage, not wildfire. Baseflow recession rates were observed to decrease by up to an order of magnitude as antecedent storage levels increased, indicating a shift in the source of recession flows from small, quickly-recharged aquifers at the beginning of the wet season to large, seasonal aquifers as the wet season progressed. Following wildfire, baseflow recession rates significantly decreased at a regional scale, suggesting that the dominant hydrologic processes affected by fire were related to post-fire reductions in above-ground vegetation (e.g. decreased interception, decreased soil evapotranspiration, decreased groundwater evapotranspiration).

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