"The water came in a thirty-year cycle. There would be five to six wet and wonderful years when there might be nineteen to twenty-five inches of rain, and the land would shout with grass. Then would come six or seven pretty good years of twelve to sixteen inches of rain. And then the dry years would come ...”
— John Steinbeck, East of Eden
December 2010 produced more than 12 inches of rain at Cal Poly (home of climatology for San Luis Obispo). That was the most rain in the month of December since 1931. However, since January 2011, almost every month has experienced below-normal precipitation. In fact, so far 2013 is the third driest year on record at Cal Poly.
These years of drought combined with emergency repair work at a 4-megawatt hydroelectric power station below Nacimiento Lake Dam has produced near-record low water levels at Lake San Antonio.
Let me explain. Nacimiento Lake, and nearby Lake San Antonio, were constructed by the Monterey County Water Resources Agency and are operated as an integrated system.
Because the required repairs at the hydroelectric plant resulted in minimal releases from Nacimiento, the remainder of the required releases had to be augmented from San Antonio, thus lowering San Antonio at a much faster rate than usual.
Nacimiento Lake, which sits entirely in San Luis Obispo County, and Lake San Antonio, which is just north of the SLO County line, were constructed for flood control, as well as for water conservation and recreation.
“The reservoir system is the foundation to the Salinas Valley’s overall water situation solution,” said Robert Johnson, assistant general manager at the agency.
The lakes were built to provide flood control by storing the large impulses of runoff from winter storms, then releasing that water over the spring and summer to recharge the groundwater basin when pumping is at its maximum. The lakes, when coupled with other agency projects, help combat seawater intrusion into the Salinas Valley — which has been called the “Salad Bowl of the World.”
As of Friday, Lake San Antonio was at 7 percent of capacity, with its water elevation at 664 feet. The dead pool elevation of the lake is 645 feet. When the lake is at that level, water cannot be released via gravity flow. The previous all-time-low water elevation at the lake occurred in February 1991, when the level dropped to 657 feet, or 5 percent of capacity. The miracle rains of March of that winter quickly raised the lake’s level.
On the other hand, Nacimiento Lake is currently at 30 percent of capacity with its water elevation at 738 feet. The dead pool elevation at this lake is 670 feet. In October of 2009, the lake was only at 9 percent capacity with a level of 700 feet.
Construction of the Nacimiento Dam was completed in 1957. Since then, the reservoir has reached the elusive 100 percent capacity four times when its spillway overflowed with water in 1967, 1969, 1983 and in early 2011.
Both lakes are fed by large watersheds, but Nacimiento Lake will fill up three times faster than Lake San Antonio, Johnson said. When describing how reservoir releases were determined, Johnson used the analogy that Nacimiento was like a checking account and San Antonio was more like a savings account.
A few years ago, the Monterey County Water Resources Agency installed an inflatable rubber dam on the Nacimiento spillway. This modification allows water levels at the lake to increase to 800 feet during late fall through early spring to maintain the dam’s flood control function, while at the same time storing more water than was previously allowed.
“We did not increase the capacity of the lake, but we increased our storage flexibility,” Johnson said.
So, how much rain do we need to fill the county’s lakes and reservoirs? They fill at different rates depending on the size of the watershed that drains into them and the different amounts of precipitation that fall in our notoriously complex San Luis Obispo County microclimates. All the water managers that I spoke with say it’s nearly impossible to predict accurately.
Dean Benedix, San Luis Obispo County Utilities Department manager, told me that other lake and reservoir levels in the county were in better condition. According to data from SLOCountyWater.org, here are the other San Luis Obispo County lake and reservoir percent of capacity figures as of Friday: Lopez Lake in Arroyo Grande at 64 percent; Salinas Reservoir in Santa Margarita at 45 percent; Whale Rock Reservoir in Cayucos at 61 percent.
• • •John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at firstname.lastname@example.org.