Wildfire Effects on Soil Physical, Chemical, and Hydraulic Properties in the Santa Catalina Mountains, Arizona

Pima County Regional Flood Control District monthly brown bag webinar with speakers Christian José Barra Barrientos  and Becky Beers 

Climate-driven changes in fire activity are promoting increases in areas burned, fire frequency, and fire severity in many regions around the world. Fire can affect each ecosystem component and cause enormous economic and ecological losses. In this context, developing knowledge to improve the understanding of fire-affected areas will be critical to restoring, managing, and conserving natural resources. Among these impacts, fire effects on soils and hydrology are particularly important, as they regulate post-fire recovery and ecosystem stability. This study focused on fire effects on soils and hydrologic function, using data from different Ecosystems Response Unit (ERU) in the Santa Catalina Mountains. Measurements were made across 35 sites with different fire histories, including some that experienced zero, one, or two fires varying severity in the last two decades. The goal was to assess changes in the soil’s physical and chemical properties after wildfires and evaluate how these changes influence the hydraulic properties such as saturated hydraulic conductivity (Ksat) and sorptivity (S).

Fire-driven changes in soil hydraulic properties can affect a range of hydro-ecogeomorphic processes through their influence on rainfall-runoff partitioning. Soil nutrients, including extractable nitrate and ammonium and available phosphorus, increased in concentration with increasing fire severity the first year after the most recent fire, decreasing to values similar to unburned sites 3 years later. In contrast, Ksat and S had different responses as a function of the fire severity, time, and ERU, resulting in significant differences between ERUs 3 years after the fire. Ksat and S increased with increasing burn severity in the Ponderosa Pine Evergreen Oak and Madrean Pine Oak ERUs, while the opposite response occurred in the Mixed Conifer ERU. Models indicated that hydrophobicity was the most critical parameter that controls Ksat and S the first year after the most recent fire, but its influence decreased with additional time since the fire as soil organic matter started to recover.

Christian José Barra Barrientos is a forestry engineer who graduated from the University of Chile, Chile. For his academic training, he evaluated and described ecosystems, and his main areas of expertise are soil studies and soil-plant relationships. He developed the ability to analyze those ecosystem components and their relationships in detail through skills to gather information in the field, analyze it in the laboratory, and write essays/reports about those components. He worked as an environmental consultant in Chile for 9 years. Improving my knowledge of soil sciences and its interaction with the ecosystem inspired Christina to pursue a master’s in soil and water sciences at the UA. Where he focused on soil-plant interactions; analysis of physical, chemical, and hydraulic soil properties; and statistical analysis/modeling.

Rebecca (Becky) Beers came to the Arizona Geological Survey in March 2021after completing her bachelor’s and master’s degrees in geology at NAU, where she studied channel sedimentation patterns following large post-fire debris flows in the Pinaleño Mountains. Since joining the AZGS, she has focused her research on post-fire geomorphic hazards and landscape recovery. Becky has participated in multiple studies across Arizona and New Mexico to better understand post-fire debris flow initiation and runout paths, and her recent research focuses on investigating the effect of short-interval reburns on landscape response and recovery, as well as studying the efficacy of novel post-fire flood and debris flow mitigation measures near Flagstaff.

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