State reservoirs fell sharply in July heat
California’s scorching July heat not only shattered temperature records, it also removed billions of gallons of water from the state’s largest reservoirs.
From July 1-18, about 4.1 billion gallons of water evaporated from California’s biggest reservoir, Shasta Lake. This loss highlights challenges water managers face not just this summer, but in a hotter, drier future with a thirstier atmosphere.
“If we have less coming in and more going out, we’re going to be struggling to manage the system moving forward if there’s more demand,” said Christopher Pearson, an associate research hydrologist with the Desert Research Institute. “The system gets out of whack, just like your checkbook does.”
Hundreds of millions of gallons — equivalent to several hundred acre-feet — evaporated each day from Shasta Lake during the historic July heat wave. Temperatures in nearby Redding reached at least 110 for six days in a row in early July. The reservoir, which has a total capacity of 4.55 million acre-feet, is the centerpiece of the federally managed Central Valley Project.
During hotter days, the air is less humid above, Pearson explained, while there’s increasing humidity at the surface as a reservoir heats up. This produces a gradient that drives upward flux of water vapor. Wind can further increase flux, leading to even more evaporation.
Another factor is the amount of water in a reservoir, since it determines the surface area exposed to air.
“The bigger the lake, the more evaporation you have,” said Don Bader, area manager for Shasta Dam, working for the Bureau of Reclamation. “Last year we started out full. This year we started out full. … Previous years the lake was not near as full, and the evaporation then was a lot less.”
During the same July time period in 2023, about 4 billion gallons of water evaporated from Shasta Lake. By comparison, in 2022, just 2 billion gallons evaporated.
The amount of evaporation this July corresponds with about 3% to 4% of the water that flows through Shasta Dam.
“If you don’t account for it, you’re going to be short,” Bader said. That’s why the Bureau of Reclamation factors evaporation into operational and allocation plans.
Billions of gallons of water have also evaporated at other major water supply reservoirs across California, including Lake Oroville, the state’s secondlargest reservoir at 3.42 million acre-feet.
“For a reservoir of Lake Oroville’s size and surface area, higher evaporation amounts during heat waves are expected,” said Tracy Hinojosa, the Department of Water Resources’ State Water Project water operations manager, by email. “But evaporation amounts remain a small portion of the reservoir’s overall storage.”
For example, an estimated 551 acre-feet of water — about 180 million gallons — evaporated from Lake Oroville
on July 12, which corresponds with just 0.018% of the reservoir’s 3.07 million acre-foot storage that day, Hinojosa explained.
More water evaporated from Trinity Lake and New Melones Lake this July than during the same period in the previous five years: 1.6 billion gallons and 1.8 billion gallons, respectively.
San Luis Reservoir bucked the trend, with 2.4 billion gallons of evaporation in July this year compared with 3 billion gallons in 2023. The reason likely comes down to volume: in 2023, the reservoir was very full, while construction this year has limited the amount of water being stored, Bader explained.
There are limitations to the data. Evaporation at reservoirs like Shasta Lake isn’t measured directly from the entire reservoir. Instead, staff measure the amount of water that evaporates from a pan that’s about 2 feet wide and extrapolate to what’s happening across the lake.
This standard approach doesn’t fully represent the behavior of a deep, dynamic reservoir, said Pearson with the Desert Research Institute. Scientists are developing methods, such as using modeling techniques, for tracking evaporation rates more accurately.
Major reservoir levels are currently at 116% of average, according to the Department of Water Resources, but other signs have indicated a return of drought conditions.
Despite the healthy levels, Pearson said all the heat stored in Shasta Lake during the prolonged stretch of high temperatures could have long-lasting effects: “Even though the early July estimates for Shasta don’t look that extreme, one concern is that the early July heat wave may lead to increased evaporation throughout the rest of summer due to increased water temperatures.”