CA Water: Following the Rule Curve

February storms have dispelled concerns over a dry January: [bold added]

So much rain fell in the first week of February that California’s largest reservoir, Shasta Lake, near Redding, rose 22 feet. Shasta Lake is 34 miles long. The watershed at the state’s second-largest, Lake Oroville, in Butte County, has received 24 inches of rain in the past two weeks — five times the historical average — sending the reservoir level up 23 feet from Feb. 1 to Feb. 7.

And now a new atmospheric river storm is forecast to soak the Bay Area and the rest of the state Thursday and Friday...

The rain has been so plentiful that operators of the largest reservoirs have been increasing water releases in recent days to make space for the latest storm.

The outlet valves at Shasta Dam on Tuesday,
Feb. 11 (Merc/U.S. Bureau of Reclamation)

At Shasta Lake, federal dam operators have let out 60,000 cubic feet per second since Saturday through the outlet valves at the massive Shasta Dam. That’s 450,000 gallons a second, the equivalent of 40 Olympic swimming pools every minute.

It’s a delicate balancing act, experts say. If reservoirs fill to the top too quickly in big storms, large amounts of water are released down their spillways, causing flooding to homes and communities downstream. In extreme cases, water can overtop dams, causing a risk of failure.

Operators at most large dams in California rely on a manual called a “rule curve” that is set by the U.S. Army Corps of Engineers. Evaluating decades of rainfall patterns, it recommends the highest level that each reservoir should be kept at every day, an amount that gradually increases later in the spring as winter storms dissipate.

“You are playing the odds,” [UC-Davis professor Jeff] Mount said. “What you are trying to do is balance the risk of not ending up with enough water in the spring against the risk of having too much in the winter, where you’ll get flooding.”

California is extremely lucky; says Professor Mount, "It’s unusual to get three wet years in a row.”

The men and women who operate our water infrastructure are doing an excellent job managing a complex system that is subject to both physical and manmade (make sure the fish get their share!) constraints.

Although funding was approved by the voters in 2014, it's a pity that, eleven years later, the Progressives who run the State haven't built any water storage to take advantage of the rain (the estimated completion date of the Sites reservoir is 2032).