Mono Lake in another drought year

When the California State Water Resources Control Board protected Mono Lake in 1994 by revising Los Angeles’ water rights in a landmark decision, it linked lake level and water exports together. The closer the lake is to its mandated ecologically sound level, the more water Los Angeles is authorized to export.

This approach, advocated by the Mono Lake Committee to protect Mono Lake and meet real water needs in the city, also works in reverse: the lower the lake, the less water can be exported.

Last year, with the level of Mono Lake falling due to drought, the Los Angeles Department of Water & Power (DWP) reduced exports by 70%, as required by the State Water Board rules. This year, after a winter of below-average (but better than last year) precipitation, the lake is not expected to rise. Committee and DWP hydrologists have studied whether the lake might actually drop to the point that zero exports would be allowed.

The trigger level for halting exports is 6377 feet above sea level. Committee and DWP staff jointly measured the lake level on April 1 and found it to be 6378.11 feet, roughly 13 inches above the trigger.

But the State Water Board requires more: Not only must the April 1 level exceed 6377, but the forecasted lake level for the entire subsequent year ending March 31, 2017 must also stay above 6377 feet.

Forecasting lake level for a year is not nearly as simple as reading the lake gauge.

March, April, and May were full of data gathering, analysis, and technical discussion. First the April 1 Mono Basin snowpack measurements came in at 84% of average. Then DWP prepared the Mono Basin runoff forecast, projecting streamflow at 74% of average. May 1 snowpack and precipitation data was then utilized to fine tune these forecasts.

The big question was how Mono Lake would respond to this level of runoff. The modeling considers many factors, including the varying surface area of the lake at different elevations, upstream operations such as reservoir storage, and of course water export to Los Angeles. Also, there are two DWP models that provide useful forecasts, and the Committee runs the newer, collaboratively developed model to validate DWP’s older model forecast.

While the technical work was underway, the Committee also engaged DWP leadership regarding the timing of planned water exports. We discovered that DWP had scheduled diversions to commence on April 1, but Committee staff and attorneys pointed out—through multiple meetings, calls, and discussions—that the lake level forecast would be unknown on April 1. It was reasonably possible that the lake would fall below 6377 feet, we argued, triggering the zero export rule, and so DWP exports should wait until the May data was in and a final consensus forecast was complete. To their credit, DWP managers agreed and, just 48 hours before diversion valves were scheduled to be turned on, DWP delayed exports until after the forecast was final.

The end result of all this forecasting work? Mono Lake is expected to remain above 6377 feet through March 31, 2017. Just barely. Snowmelt runoff is expected to lift the lake slightly to 6378.2 in July. Then the warmer, drier months of the year will drop the lake to an expected low of 6377.2 in November, and winter precipitation will lift it to 6377.6 in March of 2017.

This is a significant forecast that tells us four important things.

First, Mono Lake is forecast to stay above the critical 6377-foot threshold, which is good for the lake and the wildlife that depends on it. The salinity of the lake, productivity of brine shrimp and alkali flies, success of migratory and nesting birds, and severity of dust storms are all contingent on lake level. Significantly, the lake will be high enough to maintain a watery moat protecting the island home to the nation’s second-largest California Gull rookery.

Second, DWP will be allowed to export up to 4,500 acre-feet of water this year. That’s the same as last year, and is once again 70% less than the 16,000 acre-feet it exported annually from 1995 to 2014. These exports are equivalent to about a tenth of a foot of Mono Lake elevation and are part of the State Water Board’s wise link between protecting Mono Lake and providing for urban water needs.

Third, the lake is likely to drop below the 6377-foot mark in 2017. With the forecast putting it half a foot lower next March, Committee analysis suggests that even an average 2016–17 winter snowpack would allow the lake to fall below 6377 in 2017. That gives us a lot to consider and plan for this summer, especially the resulting threat to the gull rookery.

Fourth, the State Water Board rules are working successfully. Additional lake protections have kicked in, and the pain of the severe California drought is being shared between Mono Lake and Los Angeles. If the State Water Board had not required export reductions last year, the lake would be a quarter-foot lower today, the forecast would have it falling below 6377 this year, and the safety of the nesting islands would have been lost.

It has been, and remains, difficult to watch Mono Lake drop after so much successful effort by so many to protect this special place. But we must continue to remember that it would be much, much worse without our efforts; without the State Water Board rules, excessive diversions would have continued and the lake’s surface would be at 6350 feet above sea level today—more than 25 feet lower and far past the level of ecological collapse. Truly, our collective advocacy for Mono Lake has saved this unique place and its wildlife; now we must dig in and continue to make smart plans to survive the lingering California drought.

This post was also published as an article in the Summer 2016 Mono Lake Newsletter (pages 3 and 24).