California Drought: Is Current Rain Helping State Reservoir Water Levels?

The deluge of rain currently hitting Central and Northern California may be helping to return the water levels of many crucial reservoirs to normal.

Drought parched much of the Western U.S. this past summer. In July, one-third of all land in California, Texas, Oregon, Nevada, Utah and New Mexico was classified as experiencing extreme or exceptional drought, according to data from the U.S. Drought Monitor.

"This year has seen the highest percentage of the country, over 80 percent of the continental U.S., be classified as being in a drought since the U.S. Drought Monitor was established a little over 20 years ago," Antonia Hadjimichael, an assistant professor in geosciences at Penn State University, previously told Newsweek.

This has led to the water levels of many important dams and reservoirs dropping drastically as the water evaporated.

Want to know why heavy rainfall isn't enough to solve California's drought crisis? Check out Newsweek's in-depth explainer

"Reservoirs, mainly dams at the mountain front, provide seasonal storage for water supply, in addition to storage to reduce downstream flooding," Roger Bales, a water and climate engineer at the University of California, Merced, told Newsweek. "They store winter and spring rainfall and snowmelt [in the wet season], releasing that stored water during the summer growing season for irrigation and for municipal use [in the dry season]."

He continued: "In wetter years, such as water year 2017, they will store as much winter/spring runoff as they can for water supply, while still leaving some space for flood control in case of heavy rainfall. In drier years, that may not fill all of their water-supply storage capacity."

Such was the case in water year 2022 (October 1, 2021, to September 30, 2022), Bales said, "resulting in very reduced deliveries of irrigation and municipal water during the dry season."

Lake Mead, a reservoir of the Colorado River on the border of Arizona and Nevada, provides water to 25 million people in California, Nevada and Arizona. In 2022, Lake Mead dropped to the lowest water levels seen since it was created in 1935. As of July 18, it was filled to just 27 percent of its full water capacity.

However, the heavy rainfall currently battering California may help offset the effects of the dry 2022 season.

Parts of California have seen unprecedented rainfall in the past week, with 5.46 inches of rain falling in a single day in San Francisco on December 31. This marks the second wettest day in over 170 years. The most rain that fell within 24 hours in the city occurred in November 1994, when 5.54 inches were recorded.

Between December 27, 2022, and January 3, 2023, Lake Oroville, which is north of Sacramento and is the largest reservoir in California, had its water level rise from 676.54 feet above sea level to 713.56 feet. Lake Mead has not been positively affected by these storms, however, as they are fed by the Colorado River stretching to the east.

More rain is expected to come later this week with the arrival of a second atmospheric river slung off from a bomb cyclone over the Pacific. Another 3 inches of rain are expected to fall in urban San Francisco over the coming days, with between 5 and 8 inches in the valleys and mountains nearby.

Despite this deluge, more rainfall is needed to fully return California's important reservoirs to their average levels.

"So far, the rainfall in the northern Sierra Nevada, which our largest reservoirs depend on, is at about 40 percent of the annual average," Bales said. "We need more precipitation, closer to the average, to refill the reservoirs. We need two to three more big storms" in January to March.

Another factor is that in mountain headwaters, water has drained out of the subsurface during dry years.

"So there is a little less runoff the year after a dry year than after a wet year, while the headwaters replenish this subsurface water that the forests depend on to survive dry seasons and dry years. When the subsurface does not refill sufficiently, trees run out of water and die," Bales said.

While the winter brings the gradual refilling of the reservoirs, the coming summer may bring dry and arid conditions like last year's, leading to further depletion of water levels.

The drought conditions faced by California during the summer, and its effects on reservoirs, may only worsen with the effects of climate change.

United Nations estimates indicate that by 2030, global demand for fresh water will exceed the available supply by 40 percent.

"Climate change is here, and it's time to stop fiddling," Matthew Casale, environment campaigns director at Public Interest Research Groups, previously told Newsweek.

"The historical record is clear: Natural variations do not explain the rise in CO2 concentrations in the atmosphere, or the rise in global temperatures, that we have seen over the last century," said Casale. "If the world continues to emit greenhouse gasses at anywhere near the current rate, even more dramatic global warming is inevitable and the effects on all of us could be devastating."

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