Long-term weather patterns are cyclic in nature but unpredictable in practice. The charts below shows the history of Lake Berryessa since Monticello Dam was built in 1957.
The lake has had previous droughts, one in the late 1970’s, a ten-year drought from 1986 to 1996, and the latest from 2006 to 2017.
California endured severe droughts in 1976 and 1977, which ended in 1978. The master chart below shows that Lake Berryessa levels did not even show the usual winter uptick during those years and had dropped to less than 50% of capacity by 1978.
The 1987-92 statewide drought was notable for its six-year duration and the statewide nature of its impacts. Statewide reservoir storage was about 40 percent of average by the third year of the drought, and did not return to average conditions until 1994. But Lake Berryessa had dramatically dropped to only 30% of capacity by then and did not fill up again until 1996. 1991 was the single driest year of the drought.
A 1994 study of relict tree stumps rooted in present day lakes, rivers, and marshes suggested that California sustained two epic drought periods, extending over more than three centuries. The first epic drought lasted more than two centuries before the year 1112; the second drought lasted more than 140 years before 1350. In this study the researcher used drowned tree stumps rooted in Mono Lake, Tenaya Lake, West Walker River, and Osgood Swamp in the central Sierra. A conclusion that can be drawn from these investigations is that California is subject to droughts more severe and more prolonged than anything witnessed in the historical record.
Lake Berryessa, or Solano Water Project, is the main drinking water source for more than 400,000 people so a drought can have a serious impact. SCWA member agencies and their annual Lake Berryessa allocations in are- feet are: Solano Irigation District (SID) - 141,000 AF, Fairfield – 9,200 AF, Vacaville – 5,600 AF, Suisun City – 1,600 AF, Maine Prairie – 15,000 AF, and Vallejo – 14,750 AF for a total allocation of 187,150 acre-feet.
The Solano County Water Agency (SCWA) uses a wide variety of water management tools and options to maximize resource and minimize the need to import water. The SCWA and its member agencies have comprehensive urban and agricultural water conservation programs. They also have a Drought Contingency Plan which specifies that when storage in Lake Berryessa falls below 800,000 acre-feet as measured on December 1, they will implement reasonable water conservation measures, investigate potential emergency supplies and other reasonable measures which could reduce the depletion of storage in Lake Berryessa.