The Lake Berryessa water supply is derived from the 568 square mile drainage basin above the dam. The elevation of the basin ranges from 182 feet at the dam to 4,722 feet at the upper end of Putah Creek with most of the basin lying below 1,500 feet. There are four principal creeks that flow into Lake Berryessa: Capell Creek, Pope Creek, Eticuera Creek, and Putah Creek, the main drainage of the basin. The climate of the basin is mild and composed of two seasons, a warm dry season from May through October and a cool wet season from November through April. Most of the precipitation occurs as rain during the cool wet season with only minor amounts of snow on the upper portions of the basin .
In the Spring of 2007 the Solano County Water Agency (SCWA) performed a survey of the underwater landscape of Lake Berryessa. They wanted to accurately determine the capacity of the lake using the latest technology. During the study a a new Area-Capacity curve was developed that was slightly lower than the previous curve. This new lake level versus capacity was officially adopted by Reclamation and SCWA in 2009.
The new curve has a lake capacity of 1,551,292 AF at a level of 440 feet - 50,708 AF less than the 1,602,000 acre-feet (AF) previously calculated.The average annual inflow to the reservoir is 369,000 AF and the annual firm yield is 201,000 AF. An additional release of 22,000 AF is required annually to meet prior downstream water rights along Putah Creek. An upstream reservation of 33,000 AF was established by the State Water Resource Control Board to provide water for future development of the area above Monticello Dam. Reclamation has appropriated 7,500 AF of the 33,000 AF to provide for future development around the reservoir.
The reservoir water level may fluctuate from 455 feet to a minimum elevation of 253 feet. A water level of 309 feet is considered dead storage elevation. During the severe drought of 1977 the level decreased to 388 feet.
The latest Probable Maximum Flood (PMF) approved August 28, 1984 (and signed in January, 1986) has a peak inflow of 275,000 cfs, a 2-day volume of 586 ,000 acre-feet, and a 10 day volume of 873,200 acre-feet, and is preceded by the 100 year flood.
How fast the lake rises is dependent on rainfall over the whole watershed. The lake is 23 miles long, 3 miles wide, with 165 miles of shoreline and is fed by the headwaters to the 576 square mile Putah Creek watershed. Rainfall levels vary significantly by location. Moskowite Corners is usually about 10% - 20% higher in rain totals than the nearby (7 miles) Berryessa Highlands. Middletown rainfall is a better comparison since it is really the headwaters to Putah Creek and flows directly into the lake.
Average annual rainfalls:
Monticello Dam (97-08): 28”
Markley Cove: 25.7”
Middletown (1938-1995): 48.6”
Winters: 22.8”