NRC Report Endorses Limiting Diversions in Dry Years, Using SWRCB's Unimpaired Flows Approach

Last week, the National Research Council panel on the Bay-Delta issued its final report, following up on the panel’s prior 2 reports (the first report concluded that the restrictions on pumping in the biological opinions were generally scientifically justified, and the second report found huge gaps in the scientific basis for the Bay Delta Conservation Plan).  Most of the media coverage of the report focused on the report’s discussion of scarcity, but little has been said about the report’s conclusions regarding flows and water diversions. 

The panel’s report acknowledged that there are multiple stressors on the Delta ecosystem and its fisheries, but concluded that it is impossible to rank which is the single most important stressor.  The panel also acknowledged that there are no silver bullets and that addressing other stressors does not mean that we can ignore the need for better flows.  These conclusions are virtually identical to those of the state and federal agencies on the pelagic organism decline, and the panel’s report once again largely vindicates the work of the biologists and other scientists working for the state and federal agencies. 

What has been less discussed, however, is that the National Research Council endorsed strong flow standards that would strictly limit diversions in dry years, and recommended that the State Water Resources Control Board develop flow standards that limit diversions to a fraction of unimpaired flows.  As the panel wrote in its Overall Conclusions (page 105):

"Thus, it appears that if the goal is to sustain an ecosystem that resembles the one that appeared to be functional up to the 1986-93 drought, exports of all types will necessarily need to be limited in dry years, to some fraction of unimpaired flows that remains to be determined. Setting this level, as well as flow constraints for wetter years, is well beyond the charge of this committee and accordingly we suggest that this is best done by the SWRCB, which is charged with protecting both water rights holders and the public trust."

Diversions from the Bay-Delta estuary have increased substantially over the past 50 years, and in recent years on average more than 50% of the water in the Sacramento / San Joaquin River watershed is diverted. 

From the Delta Vision Strategic Plan at page 37.

Yet the averages don't tell the whole story: as ACWA acknowledged recently, in dry years 70% of flows are diverted, and in wet years 30% is diverted.  These high levels of diversions in dry years are unsustainable, and are a large part of the reason why the Delta ecosystem crashed in the last decade. 

The SWRCB concluded in 2010 in its Public Trust flow proceeding that diversions should be limited to 25% of the unimpaired flows in the winter and spring, before balancing that recommendation against the effects on water supply and upstream fisheries.  While the Board cautioned that the precise flow and diversion recommendations in that report were not exact, and that new flow standards will be subject to balancing, the Board expressly concluded in 2010 that: 

  • “The State Water Board further cautions that flow and physical habitat interact in many ways, but they are not interchangeable.”
  • “The best available science suggests that current flows are insufficient to protect public trust resources.”
  • “There is sufficient scientific information to support the need for increased flows to protect public trust resources; while there is uncertainty regarding specific numeric criteria, scientific certainty is not the standard for agency decision making.”

Proposals to increase diversions in dry years are wholly inconsistent with the best available science and are the opposite of the “big gulp / little sip” approach to managing the Delta.  Last year, water exports from the Delta reached an all time high, even with the biological opinions in place.  This “big gulp” last year allowed the Kern County Water Agency to bank nearly 700,000 acre feet, and the Metropolitan Water District of Southern California to store  672,000 acre feet.  Together, the two agencies stored nearly 1.4 million acre feet of water in groundwater banks and reservoirs, which they can rely on during a in a dry year like 2012.  MWD had the highest storage levels in its history at the end of 2011, and staff recently reported to the Board that MWD may not even need to tap into storage at all this year. 

The National Research Council’s report is a wakeup call that diversions in dry years will need to be further limited in order to maintain and restore a healthy Delta ecosystem.  But California can get through the dry years thanks to a “big gulp” in wet years, and increased water use efficiency, water recycling, and other tools that make up the “Virtual River.” 

The National Research Council’s endorsement of the State Water Resources Control Board setting flow standards as a percentage of unimpaired flows approach is good news as we begin the next phase of the update to the Bay-Delta Water Quality Control Plan this month.  As California improves flow standards for the Delta, we should also ensure that investments in alternative water supply tools move forward as well.

About the Authors

Doug Obegi

Senior Attorney, Water program

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