From the USGS:
A new study led by U.S. Geological Survey scientists estimates that the 2020 and 2021 wildfires in northern California increased the amount of water moving through the upper Feather River watershed. With fewer plants using water after the fires, more water stayed in the soil or flowed into streams. The watershed feeds Lake Oroville, which helps supply water to more than 27 million people.
The fires that burned parts of Northern California in 2020 and 2021 took lives, destroyed buildings, and hurt air and water quality. A new study led by scientists at the U.S. Geological Survey California Water Science Center looks at another effect of these fires: how they changed the amount of water moving through the upper Feather River watershed.
This watershed drains into Lake Oroville, a key part of the State Water Project. The North Complex, Dixie, and Sugar Fires burned more than half of it.
Trees and other plants pull water from the soil and release it into the air. After the fires, there were fewer plants using water, so more water stayed in the soil or flowed into streams. The scientists measured this change by running a computer model with and without the fires’ effects on plants.
In water year 2023 (October 2022 through September 2023), the model showed about 1 million more acre-feet of water available for streamflow and groundwater because of the fires. That is almost a third of Lake Oroville’s capacity. The increase continued the next year, at about 690,000 acre-feet. Water year 2023 was also very wet, which makes it hard to separate the effects of the fires from the effects of the extra rain and snow.
“Wildfires cause real harm, and we aren’t saying they’re good for California. Our study shows that when a fire changes a forest, it can also change how much water reaches our streams and reservoirs. Understanding that change can help water managers turn a negative into a positive,” said Diana Zamora-Reyes, a U.S. Geological Survey scientist and the study’s lead author.
The study suggests that forest thinning and controlled burns may help make the water supply more reliable as rain and snow decline.
RESEARCH ARTICLE: The flip side of destructive wildfires: a water supply buffer to climate change
ABSTRACT: Wet to dry climate extremes in California have directly or indirectly impacted droughts, wildfires, and floods, resulting in costly damage. Reservoirs in California were built to store snowpack into the summer and provide multiple benefits in the state: they offer municipal and agricultural water resources during the dry season and prevent flooding downstream.
The upper Feather River watershed, which drains 9300 square kilometers (km2) into Lake Oroville, provides water to over 27 million people and 3000 km2 of farmland. The North Complex Fire in 2020 and the Dixie and Sugar Fires in 2021 burned over 50% of the watershed, and their long-term impact on the upper Feather River water supply was previously unknown.
This study quantified post-wildfire (postfire) changes in plant water use, soil moisture, and streamflow due to changes in vegetation using a physically based water balance model. Long-term trends revealed decreases in annual precipitation and mid- to high-elevation snowpack in this watershed from 1981 to 2020. Due to changes in vegetation, annual postfire streamflow at Feather River at Oroville increased by 22%, 32%, and 16% while summer baseflows increased by 5%, 12%, and 2% in water years (WYs) 2022 through 2024, respectively. Larger increases in streamflow occurred during WY 2023, an extremely wet year relative to the other years, highlighting the challenges in quantifying and managing hydrologic systems with naturally high interannual variability. Seasonal soil moisture increased by up to 34% depending on the location, predominantly in fall and summer postfire.
Despite observed historical decreases in precipitation and snowpack, changes in vegetation due to wildfires or other disturbances may increase water availability, and continued management of mixed-conifer forests (i.e. thinning and controlled burns) may help offset future reduced precipitation and snowpack and therefore increase water supply resilience.
