2020 Lake level forecast report: Mono Lake to drop over a foot this runoff year

We wouldn’t blame you if you missed the April 1 start of the 2020–2021 runoff year this year. Since it’s such an important date for Mono Lake, we made sure to read the lake level gauge with Los Angeles Department of Water & Power staff (physically-distanced, of course), and the lake measured 6382.6 feet above sea level.

On April 1, 2020 Los Angeles Department of Water & Power and Mono Lake Committee staff safely read the lake level gauge. The result: 6382.6 feet above sea level. Photo by Bartshe Miller.

With this number in hand, both the Mono Lake Committee and DWP look at snowpack numbers, similar past years, and a veritable river of other hydrological statistical data, and each come up with a lake level forecast for the runoff year.

While DWP’s forecast is for only a 0.6-foot drop in lake level, there is evidence that its runoff forecast based on the April snowpack is too high, making the lake level projection too optimistic. Unfortunately, the Mono Lake Committee’s 2020 lake level forecast is for a 1.2-foot net fall in lake level, which means that on April 1, 2021 we expect Mono Lake to be 1.2 feet lower than it was on April 1, 2020. There is a 0.3-foot range in the forecast based on the assumed runoff, and there is another 0.3-foot range based on which hydrologic model is used. Beyond that, the unknown weather of next winter introduces a wide range of uncertainty.

The official term for the year-type is “Dry-Normal I” based on DWP’s 71% of average runoff forecast. The year-type category determines the minimum flows released from DWP’s dams to the creeks and Mono Lake. A forecast lower than 68.5% would have made 2020 a “Dry” year and required lower streamflows, while a forecast higher than 75% would have made 2020 a “Dry-Normal II” year, with higher minimum flows.

The April 1 lake level also sets the maximum permitted surface water export from the Mono Basin—16,000 acre-feet this year because Mono Lake was higher than 6380. The chart below shows that the lake has been in this range for most of the time since Decision 1631 was issued by the State Water Board in 1994. That decision also calls for a hearing if Mono Lake fails to reach 6392 feet above sea level—something we are expecting and preparing for, since the lake clearly is no longer making progress in its mandated rise to 6392 feet.

Top photo by Bartshe Miller.