This month’s deadly flash floods in New Mexico highlighted the risks of heavy rainfall on previously burned areas. While Ruidoso’s floods are the most recent example, colliding climate hazards threaten communities, ecosystems, and the services that these nature systems provide.
Wildfires can fundamentally change how water moves through a landscape, leading to changes in both the availability and quality of water. In areas like the western US, these changes can have meaningful impacts for the communities that rely on these resources, sometimes for years following a wildfire event. The consequences are particularly dramatic in areas burned by large, severe wildfires–the type of event that is becoming more frequent due to improper forest management and climate change.
In this blog, I’ll explore why interactions between heavy rainfall and burn scars can be so damaging, how wildfires change the availability and quality of water, and review what communities and governments are doing to proactively address and mitigate this risk.
Climate change and wildfires
While fire has been a part of many ecosystems for millennia, climate change is fundamentally changing the relationships between wildfires, communities, and ecosystems. In forests of western North America, climate change nearly doubled burned area between 1984 and 2015 by increasing the dryness of vegetation (or fuel aridity for the wonks among us). Our own research has shown that nearly 40% of the total area burned between 1984 and 2020 can be attributed to emissions traced to the largest fossil fuel producers and cement manufacturers.
The wildfires burning now aren’t the same fires that burned 30 years ago. They are burning at higher elevations, over longer fire seasons, growing with greater speed, and under more extreme fire weather conditions. They are also burning later into the night and ramping up earlier in the morning, due in part to increases in vapor pressure deficit, an increase in which has been attributed to climate change.
Fires are also burning at higher severity, which can magnify the water related consequences post-fire, particularly in ecosystems that are adapted to higher frequency, lower severity fire, as is true in many forests across western North America.
Heavy rain on burn scars can lead to flash floods, mudslides, and debris flows
Before a landscape burns, rain falls and lands in a forest canopy and on other vegetation before percolating through the soil that is stabilized by root systems. Healthy soils act as a natural water filter before water makes its way to a creek or stream or ends up in an underground aquifer.
Illustrations showing simplified examples of pre fire and post fire water dynamics. Source: UCS’ 2022 Fire and Water in the Western United States report
However, in a burned landscape, wildfire disrupts this cycle—particularly for high-severity wildfires. Soil can become hydrophobic (as in scared-of-water) or more water repellent, meaning rain does not get absorbed into the soil in the same way. This leads to increased runoff (and as we saw in Ruidoso, dangerous amounts of runoff), and can often bring lots of destabilized soil into a creek or stream with it. In extreme situations, debris flows and landslides can damage or destroy homes and property. One such example is Montecito, California in 2018 when extreme rain on the burn scar of 2017’s Thomas Fire led to multiple debris flows, burying homes and washing out roads.
Wildfires can change the availability and quality of water
Due to their impact on ecosystems, watersheds, and the built environment, wildfires can disrupt both the availability and quality of water resources for several years after a fire. This is true for both surface and groundwater, which is particularly important in places like California, where roughly 60% of water comes from surface sources and 40% comes from groundwater. In dry years, those flip with groundwater contributing as much of 60% of supply and serving as an essential source of drinking water for most Californians.
From an availability perspective, wildfires can increase streamflow by increasing runoff due to water repellent soils and decreased water uptake from vegetation. Recent research showed that streamflow increased by 30% for six years in areas across the western United States where more than 20% of a watershed burned. Without healthy soils to filter water or vegetation to absorb nutrients, this elevated streamflow can also contain and migrate higher concentrations of nitrogen and phosphorous, in some cases exceeding regulations for drinking water. The consequences of wildfire on groundwater are more complicated, reflecting the limited number of studies that address this intersection.
Wildfires are increasingly occurring in more populated urban areas, meaning cars and homes are burning in addition to vegetation and soils. From a water perspective, this means that many different types of chemicals and contaminants can be swept into drinking water supplies from heavy metals and volatile organic compounds to arsenic. Many of these substances come from ash that can be washed into water sources or spread throughout communities, like we saw following the Eaton fire earlier this year. In addition, other major fires, like the Camp fire in Paradise, California, were so hot they melted the underground distribution pipes resulting in widespread contamination. Valid concerns have emerged over short- and long-term disruptions to and contamination of tap water post-fire events.
Adapting to and mitigating climate change can help
Wildfires and extreme rainfall are both projected to increase in frequency with climate change, underscoring the need to address both climate change and the risks that wildfires pose to communities and shared water resources.
Treatments that reduce the likelihood of crown fire—one that moves into the canopy and often burns at high severity—can help to protect water resources. Treatments like prescribed fires can achieve this while maintaining soil health and vegetation with intact roots that support water filtration and absorption. These treatments mimic the intentional fires used by Indigenous communities to manage land and the low severity, high frequency wildfires that regularly burned and reburned forests in western North America.
Cities and counties can also adopt policies and take proactive steps to reduce risk. Water treatment plants, for instance, can install sediment barriers to prevent issues in reservoirs and water delivery systems. Similarly, where possible, communities can obtain water from multiple different sources, thereby limiting their reliance on a single watershed.
Most importantly, we must reduce our reliance on fossil fuels, the main driver of climate change, and limit greenhouse gas emissions. Without addressing climate change, we can’t fully address the wildfire and water issues facing the western United States.