Lake Berryessa Rainfall vs Level

Lake Berryessa Rainfall vs Level - A Short Tutorial (9/21/22)

Peter Kilkus 

Everyone is excited by the early rainfall at Lake Berryessa. In three days, 9/18, 9/19, 9/20, the lake received 1.15” at Monticello Dam and .81” in the Berryessa Highlands. Middletown received 1.62” and other northern parts of the Lake Berryessa watershed got more than 2”. But this rainfall will essentially do nothing to raise the level of the lake.

The lake is now down to 396.49 feet - 43.51 feet below Glory Hole. The level has been dropping about an inch per day. The modest two-day rainfall we had (1.15” by 9/21 at Monticello Dam) did not have any significant impact. The level did hold steady for a day but started down again the next day. See data below. The level is now less than a foot above the lowest level, 395.8’, during our last drought.

Date, Lake Level

9/15/22, 396.76

9/16/22, 396.68

9/17/22, 396.65

9/18/22, 396.57

9/19/22, 396.57

9/20/22, 396.49

Some folks use a rule of thumb that the lake increases a foot in level for every inch of rainfall after the ground has been saturated by 2-3 inches of rain and runoff begins This easy to remember rule is not completely accurate since the relationship between the lake level and its storage capacity is not linear. The lake profile is roughly a V-shaped bowl (with peaks and valleys and inlets and large flat areas), which means that the higher the water level gets the more rain is needed to raise it further. It takes about 25% more rain to go from 430’ to 440’ than it does to go from 390’ to 400’.

Rainfall versus level for the first three months of 1998 showed that the lake rose 16 feet with 26 inches of rain – or 7.4 inches of level per inch of rain. However, the unexpectedly rapid rise of the lake in 2017 provided data that showed the rise was twice as great as the normal average – 14.5” per inch of rain.

The Lake Berryessa watershed encompasses the 576–square mile area primarily fed by Putah Creek which originates from springs on the eastside of Cobb Mountain in Lake County. Putah Creek enters Napa County about 11 miles east of Middletown. It merges with Butts Creek just before it empties into Lake Berryessa. Therefore, rainfall over the Cobb Mountain and Middletown areas provides the bulk of water entering Lake Berryessa. One reason that Lake Berryessa rose so quickly in 2017 was the very heavy rainfall that occurred on Cobb Mountain and Middletown. 

The Lake Berryessa water level is actually measured on the Monticello Dam in a “stilling well” equipped with a float tape attached to a digital rotary encoder that measures accurately to 0.01 ft. Rainfall is measured using a tipping bucket rain gauge. However, the rain gauge on the top of Monticello Dam has never been very representative of precipitation in the overall Lake Berryessa area. The gauge is working well but the location, surrounded by mountains on two sides with a strong up-draft coming up the canyon and over the dam, prevents getting reliable data. Rainfall measurements at the dam may not be representative of the area, but they are also not the best indicator of how fast the lake may rise.

I’ve been keeping level and rainfall data at Lake Berryessa for 26 years. The table below shows that we’ve had early September rainfalls in 7 of those 26 years. But early rainfall is not an indicator of a high rainfall year. For example, last year we had an early September rain and a reasonable rainfall total by the end of December, but the rest of the year was a bust. The lake only went up about 10 feet. In 2021 the lake never rose at all. It stopped at about 417.5’, stayed there for most of the winter, then started dropping again in April 2021. See Lake Level chart above.

********                       © Peter Kilkus 2021