Joint work with DWP investigated effects of water diversions on Mono Lake’s level
The Mono Lake Committee and the Los Angeles Department of Water & Power (DWP) have a long history of legal conflict over Mono Lake. But there’s also a lesser-known record of successful collaboration over the years. In the spring of 2023, a new collaborative effort began when Committee Executive Director Geoff McQuilkin and Board Members Martha Davis and Tom Soto met with Los Angeles Deputy Mayor of Energy & Sustainability Nancy Sutley and then-DWP General Manager Martin Adams at Los Angeles City Hall.
The Committee briefed the mayor’s office on the state of Mono Lake, notably that under the current diversion rules, the lake’s rise has repeatedly stalled at the 6,384-foot to 6,385-foot elevation over the past 25 years, eight feet below the level mandated by the California State Water Resources Control Board’s Decision 1631 in 1994. Reflecting on this problematic trend, Deputy Mayor Sutley observed that there must be more options for the future than simply “diversions-on” or “diversions-off.” Could the two parties investigate diversion scenarios that would fall somewhere in between?
A team of hydrology modeling experts from DWP, the Committee, and the California Department of Fish & Wildlife launched a collaborative modeling effort. Staff from the State Water Board were invited to observe the process. The goal of the collaboration was to develop information about alternate stream diversion scenarios by employing a Mono Basin-specific hydrology model to evaluate the effects of adjustments to the current diversion rules on lake level and achievement of the D1631 management level. From September 2023 to April 2024, an 18-person technical team came together for a dozen meetings to brainstorm criteria, discuss model runs, and dissect the results from all points of view. Additionally, a subset of the technical experts met between the larger meetings to keep progress rolling.
Constructive dialogue and willingness to study any party’s proposal led to the development of ten stream diversion scenarios for analysis. DWP initially suggested reducing exports only in wetter years, when LA’s water demand is more easily met by other sources. The Committee brought forward the idea of “bookends” to understand the full range of projected lake level attainment timeframes within the assumed hydroclimate. In this concept, Deputy Mayor Sutley’s “diversions-on” represents the current diversion rules as one bookend; the “diversions-off” bookend is stopping stream diversions altogether. By using the bookends approach, different scenarios are compared to the baseline fastest possible lake rise scenario in which all runoff reaches the lake.
The agreed-upon set of scenarios formulated stream diversion rules in a variety of ways—decreasing the total volume of export, restricting export in wetter years, increasing the thresholds at which DWP is allowed greater export, or reducing export over time. The Committee advocated for the integration of “dynamic rules” into the diversion criteria. Unlike the current rules, dynamic rules would automatically adapt to preserve lake level gains when they happen, and further restrict export allowances if an upward trend in lake level is not achieved over time. Unfortunately, the established hydrology model the group utilized was not designed to incorporate these new concepts, so dynamic rules could not be fully evaluated. The Mono Lake Committee’s Vorster Model can incorporate dynamic rules and our staff continues to investigate this concept.
The goal of the collaborative modeling effort was not to negotiate but rather to develop information and better understand the magnitude or type of adjustments required to put the lake on track to reach the management level of 6,392 feet. Takeaways from the collaboration include:
Stream diversions affect lake level. In February 2023, the State Water Board held a workshop about conditions at Mono Lake. At the time, DWP asserted that its current volume of diversions does not impact lake level and that lake fluctuations are entirely the result of hydroclimate conditions. After the collaborative modeling work, it was accepted by all parties that current diversions do have an effect on lake level and that adjusting these diversions produces different lake level outcomes over time.
Scenarios exist that will raise the lake more quickly. The collaborative modeling exercise incorporated the last 50 years of runoff and climate data, including the two significant droughts starting in 2012 and 2020. Even with these and other dry periods, there are some diversion criteria that raise the lake more quickly than others. The more effective criteria require a significant reduction in annual stream diversions. (Note that the groundwater export from the Mono Craters Tunnel cannot be adjusted so Los Angeles is guaranteed approximately 5,000 acre-feet of Mono Basin water every year under any scenario.)
Differences between diversion criteria outcomes become more pronounced during dry periods. DWP likes to point out that a couple back-to-back epic winters could bring the lake up to the management level quickly, even under the current export rules—an outcome all parties would celebrate. But it has already been 30 years, and Mono Lake can’t wait additional decades for that rare occurrence. Cycles of dry and normal runoff years extend the time it takes for the lake to achieve the management level, and year-by-year, there is a cumulative effect on lake level that differs between higher-export and lower-export scenarios. Diversion criteria that are successful even in dry climate patterns ensure resilient lake levels under any future conditions.
The collaboration wrapped up in May of this year with a presentation of a consensus report to representatives of the collaborating parties, as well as representatives from the Mono Lake Kootzaduka’a Tribe and California Trout. The information developed is already shaping the path forward and potential further collaborative work.
State Water Board staff and consultants participated in the modeling analysis and listened to the dialogue between the parties. The Board plans to hold a hearing to consider changes to stream diversion rules, which the Committee expects in 2025, and there is already evidence that productive ideas from the collaboration have informed the Board’s planning.
Throughout the process the Committee was reminded of this important point: The lake’s journey to the management level matters. Every foot of lake elevation gain improves conditions at Mono Lake. Analysis should not focus only on the length of time each diversion scenario requires to reach the 6,392-foot management level. Even when dry conditions slow the lake’s progress toward the management level, the scenarios that allow the lake to rise higher during that time mitigate air quality issues, facilitate healthier brine shrimp and alkali fly populations, and protect nesting California Gulls.
In October the consensus report was jointly presented to Deputy Mayor Sutley and was well-received, with appreciation for the group’s work and interest in continued collaboration.
This post was also published as an article in the Fall 2024 Mono Lake Newsletter. Top photo by Elin Ljung.