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Wildfires, Water, and CAWSC Science

   19 October 2017

There's no denying that California's landscape is prone to drought and warmer climates. The recent drought lasted five years (2011 – early 2017), bringing with it many environmental issues including the increased prevalence, severity, and duration of wildfires. While there used to be a distinct "wildfire season" in California, the threat of wildfire is no longer restricted to a single season, and is now a year-round hazard.

Photo of the El Dorado National Forest after the 2014 King Fire in Northern California

The 2014 King Fire burned 97,717 acres in the El Dorado National Forest and on private timberlands. The fires in these burn areas reduce the forest canopy, exposing snowpack to direct sunlight. This increases the rate of snowmelt and evaporation, impacting runoff and groundwater replenishment. Photo: Bonnie Dickson, USGS.

Wildfires and resulting burn areas can create hydrologic hazards for surrounding communities, including water-quality issues, flooding, and debris flow. Scientists at the California Water Science Center are on the job helping evaluate these situations informing California's resource managers and public of the effects wildfire has upon the state's water resources.

Water Quality and Wildfires

While most people concern themselves with the need for water for putting out fires in burning areas, USGS scientists study the quality of water in burned areas. Burn areas create a large amount of ash, which can be carried into streams and reservoirs used for public supply by runoff water, taking with it numerous contaminants. Scientists have linked the presence of multiple trace metals in post-fire stormwater. In a 2016 study, scientists discovered elevated levels of iron, lead, nickel, and zinc in the streams near Los Angeles, and determined the contaminants were resulting from the 2009 Station Fire in the nearby Angeles National Forest. The study examined the effect of the fire on trace metal contamination in nearby streams, comparing post-fire stormwater quality to criteria for aquatic life, in hopes to understand if fire contamination could harm local ecosystems.

In this study, scientists compared pre-storm and stormflow water-quality samples from streams inside and outside the burn area of the Station Fire. Ash and burned soil samples were also collected and analyzed for metal content from several locations within the perimeter of the fire. Through examining these samples, scientists determined that concentrations of heavy metals in soil, ash, and stormwater were elevated as a result of either the fire or local storms, or perhaps a combination of both.

Photo of dead rainbow trout (Oncorhynchus mykiss) in the Big Tujunga Watershed during the 2009 Station Fire, California.

Dead rainbow trout (Oncorhynchus mykiss) in the Big Tujunga Watershed during the 2009 Station Fire, California. Photo: USGS.

Understanding the effects wildfires have upon local water quality is important to helping resource managers plan for managing potential threats contaminants may have on the ecosystems and drinking water reservoirs fed by these streams. In the case of the Station Fire, the largest wildfire in southern California in 2009, decreased water quality poses potential risks to fish and frogs that live in affected watersheds, many of which were already critically endangered and close to extinction. Additionally, many of the creeks studied feed into public drinking water reservoirs, meaning resource managers need to pay particularly close attention to testing and treating water supplies before they are delivered to customers. Knowing what potential contaminants are likely to come from local fires helps conservation and treatment processes easier to plan for.

Hydrologic Monitoring Systems and Wildfire

Wildfires impact the vegetation and soil in burn areas in a few different ways. Firstly, wildfires can create a lot of dead matter, including burned plants and ash. This dead matter sits loosely on the ground, and can be easily swept away with runoff, which can rapidly turn into dangerous debris movement that may threaten downstream communities.

Wildfires also remove vegetation and soil cover that usually absorb rain and runoff water. This unbalances a natural process called evapotranspiration – the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces, and by transpiration by plants. When plants are destroyed by wildfire, they cannot help move water to the air, or absorb water into the earth. When evapotranspiration decreases, rainwater and runoff move unencumbered through burn areas, again increasing the prevalence of debris movement.

Another secondary effect of wildfire is the increased level of soil hydrophobicity, or fire-induced water repellency. When plant materials burn during a fire, they create a waxy substance that sinks into the soil and coats soil particles. This creates a barrier that causes soil to repel water. When the soil is repelling water, the amount of water absorbed into the ground decreases dramatically (depending on the thickness of the hydrophobic soil). This contributes to greater amount of runoff, which can contribute to the increased chance of flash flood occurrence.

Cuurrent California Streamflow Conditions map depicts streamflow conditions as a percentile, symbolized by colored circles.

Cuurrent California Streamflow Conditions map depicts streamflow conditions as a percentile, which is computed from the period of record for the current day of the year.

Monitoring the movement of surface water becomes essential in burn areas, where fire-related factors increase the changes of flash flooding and debris movement. CAWSC runs an extensive streamflow monitoring system, collecting data from more than 500 streamgages and 3,000 wells throughout the state. Many of these gages are Automated Local Evaluation in Real-Time (ALERT) streamgages, meaning they are designed to send warnings when water levels reach a predetermined level or change rapidly. Sudden changes generally suggest the increased risk of debris flow and flash flooding, which can pose a conceivable hazard to downstream communities. Data from ALERT gages are used by the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) to assess the need for emergency preparedness or early warning systems that can help protect life and property.

In 2015, the USGS installed 11 new ALERT streamgages in the Valley and Butte wildfire burn areas in Northern California. CAWSC chose these areas specifically to monitor and assess hazards associated with post-fire risks. According to Al Caldwell, Deputy Associate Director for Data at CAWSC, the sites "were strategically located in conjunction with the NWS to ensure that the best locations were chosen that would give emergency managers the data needed for the issuance of flood warnings. Debris flows are expected in the steep areas of these burns, and sites with high probability of landslides were also considered in the siting of these gages with the help of the USGS Landslides Hazards group." There are currently no plans to install additional gages at other burn sites in California.

When wildfires threaten life and property in California, issues like water quality, evapotranspiration, and hydrophobicity may not take precedent in most minds. To CAWSC's scientists, however, post-fire water impacts are an important topic to be studied and monitored. With field crews monitoring the state's extensive streamgage network, and scientists monitoring water quality issues in and around burn sites, California's water resource managers have accurate scientific data to help ensure the state's human and wildlife populations have safe water conditions, despite the damages of wildfires.

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