As California emerges from drought, the water supply situation for Sonoma and Marin Counties is the best it has been for years. The water supply pools of both major regional reservoirs, Lake Sonoma and Lake Mendocino, are over 100 percent full. And yet the situation could still be significantly better.
Both reservoirs serve two vitally important purposes: water supply and flood protection. And these two goals conflict. If one were to operate a reservoir solely for water supply, one would always want it to be as full as possible. Conversely, if a reservoir was operated solely for flood control, it should be as empty as possible to be able to capture rain from an upcoming storm.
This tension is resolved by splitting the total capacity of the reservoirs into two “pools:” a water supply pool and a flood control pool. For Lake Sonoma, the first 245,000 acre feet of storage constitutes the water supply pool, and the remainder of the lake’s total storage of 381,000 acre feet is the flood control pool.
The tension between these two uses was illustrated by an event last year. A major storm in March 2016 caused total storage in Lake Sonoma to reach 285,000 acre feet. But as soon as the storm threat passed, 40,000 acre feet of water was released downstream to the ocean to empty the flood control pool.
How much water is 40,000 acre feet? A football field, minus the end zones, is about one acre. So, 40,000 acre feet would be a football field covered by a column of water seven and a half miles high. It is a lot of water.
Looked at another way, 40,000 acre feet is as much water as all Petaluma water customers use in five years.
The situation from last year is now repeating itself. The recent storms raised storage in Lake Sonoma to 325,000 acre feet. As you read this, the 80,000 acre feet of water above the water supply pool is being released downstream to the ocean.
In this era of climate change and uncertain water supplies, the obvious question is this: Can reservoir operations be modernized to retain more water for water supply without compromising flood control?
The answer is probably yes. Two aspects of the operations of our region’s other reservoir, Lake Mendocino, provide guidance. Lake Mendocino is a much smaller reservoir and efforts have been underway for years to stretch its water supply mission.
First, unlike Lake Sonoma, Lake Mendocino’s water supply pool varies by time of year. During the rainy season, its water supply pool is 68,400 acre feet. But during the summer months, when major storms are unlikely in California, the flood control pool shrinks while the water supply pool grows to 112,000 acre feet.
The Lake Sonoma watershed is subject to the same weather patterns and storm systems as the Lake Mendocino watershed, so Lake Sonoma’s water supply pool should also grow in the drier months when major storms are unlikely.
A second aspect of Lake Mendocino operations should also be extended to Lake Sonoma. In recent years the responsible government agencies have collaborated on a new demonstration management program for Lake Mendocino called “Forecast Informed Reservoir Operations,” or FIRO.
Under FIRO, “exceedances” above the water supply pool are allowed to remain as long as weather forecasts do not show a major storm approaching, which would require water to be released to free up capacity in the flood control pool.