How Wildfires Affect California’s Water Supply

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.

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