Wildland Fire Research: Water & Ecosystems
Wildland fires can impact water quality and forested ecosystems. Wildland fires burn vegetation that holds soil in place and retains water. Erosion and flooding can follow and cause changes to runoff, streamflow, and water quality (e.g., temperature and chemical concentrations). This can affect aquatic habitats (a special concern for sensitive species such as salmon) and, along with deposited pollutants from the smoke and damage to water infrastructure, can contaminate drinking water supplies. Wildland fire can also change the resilience of forests to other types of disturbances, such as insect infestations.
On this page:
- Drinking Water Quality
- Stream Temperatures
- Stream Flow
- Forest Resilience to Fire
- Tools and Models
- Future Directions
Drinking Water Quality
A Resource for Water Managers
As area burned by wildland fires increase, including near populous areas, what are the potential impacts on water quality?
Wildland fires impact physical, chemical, and biological water quality endpoints. Physically, streamflow, sediments and stream temperature all increased. Chemically, nutrients, ions, metals, and certain organic chemicals increased, sometimes 10–1000 times above unburned levels, and some post-fire chemicals (e.g., arsenic) in treated drinking water exceeded regulatory levels. Biologically, aquatic species commonly declined temporarily. The duration of effects was generally 5 years or less, but some effects could persist longer, especially after a severe fire and in drought conditions.
With greater fire activity, water resource managers can track these endpoints as they plan for, mitigate, and, if needed, recover from wildfire impacts to water quality. Communities who rely on a single municipal source or individual private wells could also use this information, as they could be especially impacted if those resources are damaged or contaminated.
Contaminant Levels
Pollutants mobilized during and after wildfires can threaten water quality. With an increase in wildfire activity, including in the wildland urban interface (WUI) near populous areas, more public water resource managers will need to proactively plan and prepare their water systems. What are the impacts of wildfires on contaminant levels in public drinking water systems?
EPA researchers examined the post-wildfire effects on water quality at public drinking water systems (PWSs) located downstream from wildfire events. In certain circumstances after a wildfire, concentrations of nitrates, disinfection byproducts and arsenic exceeded drinking water standards.
This study was the first to quantify the impacts of wildfires on drinking water violations and concentrations in PWSs. The information can help public water source operators, particularly in wildfire-prone regions, prepare public water systems and design post-wildfire water sampling and treatment plans.
Comparing the Impacts of Wildfires vs Prescribed Burns
Prescribed fires are a land management technique that can reduce the impacts from high-severity wildfires. As wildfires increase in size and severity, prescribed fires may be used more frequently. What are the effects of prescribed fires, relative to the effects of wildfires, on water quality?
EPA researchers assessed the effects of 54 wildfires and 11 prescribed fires on the concentrations of trace elements (arsenic, selenium, and cadmium) in western U.S. streams that drain burned watersheds. In general, large, high-severity wildfires significantly increased spring mean concentrations of trace elements. In comparison, prescribed fires rarely did.
Land managers need the ability to compare water quality effects of wildfires and prescribed fires to make informed forest management decisions. This study demonstrates that prescribed fires, when used to reduce future high severity wildfires, could also help reduce water quality impacts by decreasing the potential for concentrations of post-fire trace elements in downstream waters.
Stream Temperatures
Seasonal Changes
What are the effects of wildfires on seasonal stream temperatures, especially for cold-water fish habitat?
High-severity riparian (river vegetation) burns in the studied watersheds caused cooler winter water and warmer summer water in the downstream sites. Wildfire effects on the downstream water temperatures diminished with increasing distance from the burn perimeter.
Knowing more about the effects on stream temperatures helps water resources managers protect cold-water, temperature-sensitive fish habitats.
Summertime
Can we help regional ecosystem managers by predicting the effect of wildfires on summer stream temperatures?
For the 31 burned sites included in this study, daily summer water temperatures had an average increase (0.3 – 0.9°C) for three years post-fire, with variation across the sites. An increase was more likely with greater riparian burn area and severity, but other factors (i.e., bedrock permeability, basin area, post-fire weather, winter snow-water equivalent, pre-fire forested and barren watershed area) were also important.
Using this information, regional ecosystem managers may be able to identify streams highly sensitive to wildfire effects and could provide a preliminary stream sensitivity ranking that could be used to prioritize sites for conservation or mitigation.
Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study (2024)
Stream Flow
A Statistical Analysis
Since water in the western U.S. is scarce, and much of the available public drinking water comes from streams in forested watersheds, water managers are concerned about any disturbances, from low water flow to flash floods that carry sediment and nutrients. What are the wildfire impacts on stream flows?
Sites with small watershed burns rarely had a wildfire-related increase in annual low and peak flows. In contrast, stream sites with large, high-severity watershed burns saw an increase in annual low and annual peak flows for five years after the wildfire.
Applying this analytical framework can show wildfire-related annual low and peak flows. Knowing how wildfires change the supply of fresh water can help water supply managers protect drinking water sources.
EPA Science Matters: Wildfires: How Do They Affect Our Water Supplies?
Field Studies
Wildfires can dramatically change vegetation cover and affect hydrological processes, with changes to runoff, streamflow, and water quality. What are the impacts of wildfires on water quality, particularly in the Cascade Mountain Region of the Pacific Northwest, which is impacted by wildfires and provides water for downstream uses?
In 2020, immediately after four Oregon "megafires" (Riverside, Beachie Creek, Lionshead, and Holiday Farm) in the Cascade and Coast Range mountains, EPA initiated a field program to sample the chemical composition of streams and rivers that drain fire-impacted watersheds and compare them with similar streams not affected by the fires. The data collected as part of this study will be used to inform water models and programs that provide critical insight into mechanisms driving post-fire water quality changes.
Streams and rivers are important to downstream aquatic ecosystems, agriculture, hydropower, recreation, and municipal drinking water. When water providers and water resource managers better understand the downstream impacts of burned watersheds, they can try to minimize adverse water-quality effects, possibly by temporarily diverting compromised water or changing source water.
EPA Science Matters: EPA Researchers Investigate Impacts of Wildfires on Water Resources
Forest Resilience to Fire
A Conceptual Framework of Biological Disturbance Agents, Fuels, and Fire
Biological disturbance agents (BDAs), i.e., insects, pathogens, and parasitic plants, are a natural part of western U.S. forest ecosystems. A common thought is that tree decline and mortality due to BDAs contributes to fire risk, since, as live trees die, the dead trees become fuel. How can we better understand whether BDAs increase fire risk?
EPA researchers assessed three major groups of BDAs pervasive in western U.S. forests: 1) insects, 2) pathogens, i.e., root diseases, blights, and 3) parasitic plants. There was little evidence to support the common belief that BDAs predominantly increase the likelihood and severity of wildfire. In fact, native BDAs (such as bark beetles, rust fungi and dwarf mistletoe) can counteract negative fire outcomes by encouraging structural diversity (i.e., more variation in tree heights) and increasing landscape heterogeneity (i.e., a greater a mix of vegetation.)
This framework helps fire managers and land managers better understand the complex relationships between BDAs, fuels and fire, especially in the western U.S., where wildfire and BDA activity has increased in range, magnitude and severity under a changing climate.
An Ecological Perspective
Ponderosa pine forests are widespread in the western U.S., particularly Oregon and Washington, but more than a century of fire suppression has increased their susceptibility to the combined effects of drought and biological disturbance agents (BDAs). As the climate warms and disturbance frequency and intensity increases, can we better understand the ecology of these forests and plan to restore forest resistance to disturbances?
EPA researchers synthesized extensive information on historical conditions and dynamics to understand how resistance was maintained historically, what it means in ponderosa pine ecosystems, and how it can aid regional and local planning efforts.
Researchers clarified that prescribed fires (i.e., the process of frequent intentional ignition of controlled low-severity fires) develop and maintain resistance and are critical to maintaining the ponderosa pine forest ecosystem. Large mixed- and high-severity fires are inevitable in the next several decades, as is elevated tree mortality from drought and BDAs. Given the expanding WUI, researchers describe the challenges to sustainably managing wildland fire and applying prescribed fire while preparing communities for the impacts.
Tools and Models
HexFire
As fire frequency and severity grows, more scientists will need to incorporate fire impacts into their research. How can we provide a wildfire simulation model that can be widely used by land managers and non-fire specialists?
EPA researchers developed a wildfire forecasting model, HexFire, using EPA’s existing open-source HexSim model development platform. HexFire helps anticipate where future wildfires are likely to occur, how significant their consequences might be, and what might be done to limit these impacts.
Other wildfire simulations models are highly complex, time-consuming to learn, and geared towards fire specialists, but HexFire’s ease-of-use makes it accessible to interdisciplinary non-fire experts. Ecologists, conservation biologists, planners, and land managers can use HexFire to simulate multiple combinations of fire environments, fuel treatments, fuel breaks, and back burns and to help quantify the outcomes of the combinations on plant and animal species.
HexFire: A Flexible and Accessible Wildfire Simulator (2022)
Future Directions
Wildland fire smoke is a complex stressor that can have both short-term and long-term effects on the structure and function of ecosystems. By examining the effects of wildland fires on salmon populations in the Pacific Northwest and finding that while salmon populations are declining due to raised temperatures of wildland fires, EPA researchers have investigated cold water refuges (areas of a river that are colder in temperature than the main body of the river) and how these can ensure salmon survival. EPA researchers also analyzed the water quality in 31 streams in Oregon to provide insight into post-fire water quality changes and provided information that can be used by drinking water providers and water resource managers.
- Future research is needed to continue mapping and modeling watershed resilience to wildfire.
- By evaluating strategies that reduce adverse hydrologic and water quality effects from wildfires, we can understand how ecosystems respond to a changing climate.