Summer marks the traditional beginning of California’s fire season, although the warming climate has stretched the season considerably. Until the winter rains arrive, wildfires will burn forests and grasslands throughout the state. Since January 1, about 3,700 fires have consumed more than 111,000 acres of land—outpacing 2017, the most destructive and deadly fire season in state history.
While public safety and economic costs deserve and receive a great deal of attention, wildfires also have consequences for the management of water—including the amount and quality of supply, and the potential for flooding.
Fire can bring short-term water supply benefits by increasing runoff for downstream use. Most precipitation that falls on hillsides is captured by plants and soils and sent back into the atmosphere. By removing vegetation and reducing the ability of soils to hold water, most burns lead to an increase in runoff. The amount of increase depends on the type of vegetation that is burned. In Sierra conifer forests, reducing tree cover by about 40% may result in a 9% increase in water yield. Conversely, burning grasslands leads to negligible increases in water supply.
Managing forests with mechanical thinning and prescribed burning simulates the effects of low- and moderate-severity wildfire—creating opportunities to improve forest health while increasing water supply. But before concluding that forest fires and active management can reliably increase water supply, it’s worth noting that increases in water can be short-lived as vegetation rapidly grows back. And in drier forests like those of the southern Sierra Nevada, most downstream users are unlikely to see increased yields.
Wildfire can also bring some serious water management challenges.
Intense fires remove vegetation that prevents excessive erosion. Soils burned at high severity can also lose the ability to absorb water for a few years after a burn. When rain falls on these fire-ravaged slopes, the fast-moving runoff can erode soils and ash, transporting them into rivers. These flows threaten drinking-water quality and fill water supply and hydropower reservoirs with sediment. Following a series of severe wildfires in the American River headwaters, Placer County Water Agency experienced significant damage to its water supply and hydropower infrastructure, including costly impacts from increasing sediment and debris. The agency is now working with multiple partners to reduce wildfire risk above two reservoirs using prescribed burning and mechanical thinning.
The damaged soils and loss of vegetation following a severe burn can also increase the threat of downstream flooding and landslides. In some cases, these slides can become “debris flows”—fast-moving rivers of mud, rock, and other materials that can take out everything in their path. These are one of the most dangerous types of flash floods in California. The most recent and deadly example of this was in Montecito following the December 2017 Thomas Fire (the largest fire by area in state history). In early January 2018, record-high rainfall fell on the fire-scarred slopes above Montecito. The resulting debris flows killed 23 people and damaged or destroyed 400 homes.
Experts are predicting another year of intense wildfire activity. Water managers in affected regions will need to address the impacts of fire on local water supply and flood hazards, and must also account for such impacts in future planning.