Managing Water Stored for the Environment During Drought

By Sarah Null, Jeffrey Mount, Brian Gray, Michael Dettinger, Kristen Dybala, Gokce Sencan, Anna Sturrock, Barton “Buzz” Thompson, Harrison “HB” Zeff


Storing water in reservoirs is important for maintaining freshwater ecosystem health and protecting native species. Stored water also is essential for adapting to the changing climate, especially warming and drought intensification. Yet, reservoir operators often treat environmental objectives as a constraint, rather than as a priority akin to water deliveries for cities and farms. Reservoir management becomes especially challenging during severe droughts when surface water supplies are scarce, and urban and agricultural demands conflict with water supplies needed to maintain healthy waterways and wetlands. In times of drought, most freshwater ecosystems suffer.

This blog post examines 2021 water year actions by the federal Central Valley Project (CVP) and the State Water Project (SWP), which sought to maximize water deliveries while meeting environmental regulatory standards in a severe drought. Based on this experience, we offer recommendations to better protect the environment if California is faced with dry conditions in 2022 or beyond.

Priorities versus Constraints

The CVP and SWP (the “projects”) are designed principally to store and deliver freshwater to cities and farms. During droughts, the projects’ highest priority is to meet these supply obligations while complying with downstream water quality, flow, and endangered species requirements and meeting water delivery obligations to wildlife refuges. In this context, environmental standards operate as a constraint on project operations, rather than as a water supply priority. One of the most explicit examples of this is Congress’s 2016 directive that the biological opinions governing project operations must “provide the maximum quantity of water supplies practicable” to CVP and SWP contractors “without causing additional harm to the protected species.”

Project operations last spring illustrate the distinction between priority and constraint (see figure). Despite exceptionally low runoff during the winter, low reservoir storage at the end of winter, limited snowpack, and unusually warm and dry conditions, the CVP and SWP released significant quantities of water in April and May. These releases were made to meet water supply priorities for municipal and agricultural water contractors with senior water rights that predate construction of the projects.

By the end of spring, project reservoirs held insufficient water to meet water quality and flow requirements for the summer and fall. This especially harmed salmon, including winter-run Chinook that are close to extinction, because the reservoirs did not retain sufficient cold water to meet temperature standards in the upper Sacramento River. It also made it difficult for the projects to maintain water quality in the Sacramento-San Joaquin Delta for in-Delta users and Delta exports, ultimately requiring installation of a salinity barrier. The degraded water quality also is likely to take a long-lasting toll on a many native fishes.

In June, citing “critically  low  storage levels” in CVP and SWP reservoirs, the State Water Resources Control Board issued a temporary urgency change (TUC) order that relaxed environmental standards from June 1 to August 15. These changes might not have been needed if the projects had operated their reservoirs more conservatively in April and May.

No Room for Error

Delivering as much water as practicable to urban and agricultural users leaves no room to adjust for errors in forecasting or unanticipated worsening of conditions. Yet, as 2021 and previous drought years show, forecasting, modeling, and operational errors are the norm – not the exception – during droughts. These errors inevitably lead to increased harm to the environment and the likelihood of errors is increasing with a changing climate.

For example, in 2021 the projects used optimistic runoff and climate forecasts that over-estimated their ability to meet downstream temperature standards. These forecasting errors were compounded by unanticipated high diversions and in-stream losses, both above and within the Delta. The higher downstream water uses required larger reservoir releases to maintain Delta water quality and made managing water temperatures even more difficult. 

The problems of water year 2021 were not unique. Although different in detail, the same effort to deliver as much water as possible—coupled with modeling, forecasting, and operational errors—led to similar environmental problems in water years 2014 and 2015, the height of the last drought. In that drought, which was also warmer than normal, the State Water Board issued TUC orders that relaxed environmental standards.

Delayed Institutional Responses

A major challenge in drought management is the inability of project operators and regulatory agencies to respond quickly enough to head off problems. By early March, it was evident that it would be difficult for the CVP and SWP to meet both their supply commitments and their environmental obligations in 2021. This was reinforced in April when forecasted spring runoff failed to materialize. But in this drought, as in 2014 and 2015, there was a significant time lag between awareness of the problem in late winter/early spring and actions to address it, which occurred in the summer. 

As an example, a three-decade old State Water Board order (Order 90-5) requires the Bureau of Reclamation to develop a Temperature Management Plan (TMP) if there are likely to be difficulties meeting temperature standards below Shasta Reservoir. The Bureau submitted this plan in late May after extensive consultation among six state and federal agencies and their consultants. The Board approved the TMP in early June. But by that time it was too late to take corrective action by holding back more water in the reservoir, and the only remaining option was to modify water quality and flow standards. 

By late May, the State Water Board’s own models indicated the need to curtail some water rights below the reservoirs to enforce water rights priorities and protect several fish species. By June it was clear that unusually high downstream water use was going to make it difficult to meet Delta salinity standards, putting more pressure on reservoirs. 

The Board did not curtail junior water rights until June and began curtailment of most senior rights in late August. Unfortunately, these curtailments—especially those involving riparian and pre-1914 appropriative rights—came too late to reduce pressure on the reservoirs. 

Changes Needed to Manage Storage for the Environment during Drought

To better protect rivers and wetlands during drought, California must avoid the mistakes made in previous water years and droughts. This will require making environmental water supply a priority rather than a constraint, leaving a margin for errors in forecasting operations, and overcoming institutional hurdles to timely decision-making. These efforts should be accompanied by a strategy to improve water storage for ecological water uses, while reducing impacts on water deliveries to cities and farms. These reforms will be essential for adapting to protect ecosystem health in a changing climate.

Clarify Priorities

On September 21st of this year, the Director of the Department of Water Resources and the Regional Director of the Bureau of Reclamation stated in a hearing before the State Water Board that they intend to change the historical supply priorities of the projects, placing public health and safety first, environmental protection second, and water deliveries third.  

The details of this change in policy—including how it would function during drought—remain to be seen. But to be successful at reducing ecological harm during drought, it will require three additional actions.

Leave Room for Error

To make ecosystem health a priority, reservoir operators must end the practice of trying to maximize spring deliveries during droughts while hoping they will not violate flow and water quality standards later in the season. Operators also need to manage stored water more conservatively, allowing for a larger margin of error in early season runoff estimates and rethinking assumptions about climatic conditions. This would avoid reliance on TUC orders and other modifications of environmental standards to cover errors in forecasting and operation.

One approach to managing more conservatively would be to set aside ecosystem water budgets (EWBs) in reservoirs. These EWBs could function like a senior water right and be managed as a hedge against drought. Applied to reservoir operations, EWBs would guide decisions on how much water to carry over and reserve in the fall, what portion of winter and spring reservoir inflow to release, and what percentage of storage to assign for late season environmental releases.

Make Timely Decisions

Project operators, the fish and wildlife agencies, the State Water Board, and the water user community must work cooperatively to eliminate the institutional and procedural hurdles that prevent prompt actions to curb environmental harm. Start by planning for severe droughts every year, rather than reacting only when one occurs. This involves preparing annual environmental watering plans that account for current conditions and the impacts of previous years. The plans should function like a decision tree, describing the timing and type of decisions to be taken throughout the year, including course corrections if conditions change. The plans should be vetted with stakeholders and released at the beginning of every water year (October 1st).

Planning also needs regulatory support. The State Water Board can start by revising its requirements for temperature management in reservoirs to accommodate increasing drought severity, errors in forecasts and modeling, and the need to make decisions earlier in the water year. The Board also should update water quality control plans to incorporate increasing drought severity and to support timely decisionmaking, similar to ongoing efforts in the Colorado River basin. This would allow water managers to take measures that anticipate the effects of scarcity on supply and would enable the Board to make earlier decisions on curtailments.

Expand Storage Options for the Environment

The measures outlined above would entail increasing use of existing storage capacity to avoid environmental harm, especially during droughts. Additionally, expanding the volume of available storage for ecological uses will be key to adapting our water system to the changing climate. The array of Proposition 1 storage projects being considered by the California Water Commission is a start, as the public benefit of many of these projects focuses on improving drought supplies for the environment. One of these projects—Sites Reservoir—could become a model for operating storage to improve environmental management while reducing impacts on supply reliability for other water users, particularly by taking pressure off CVP and SWP reservoirs.  Underground storage in aquifers also has vast untapped potential. Some water managers already use innovative approaches to conjunctively manage water stored in reservoirs and groundwater basins to improve ecosystem health while also increasing available supplies. Examples include Yuba Water Agency’s conjunctive use program on the Lower Yuba River and groundwater banking projects in the Chino Basin and the Kern Water Bank. More efforts like these can reduce tensions between water supply and environmental management during drought and improve ecosystem outcomes.


Eight of the last ten water years have been warm and dry—part of a long-term trend of increasing drought intensity with unusually low runoff in springtime. To adapt to these changes without exposing freshwater ecosystems to increasing risk, and to avoid reliance on TUCs to ease environmental standards, new approaches to managing storage for both water supply and the environment will be needed. Water managers and regulators must set clear priorities to protect the health of rivers and wetlands, operate surface reservoirs more prudently, and respond quickly when drought is on the horizon. Despite the recent rains, we need to make these changes soon in case this drought continues into 2022 and beyond.

Aerial view of Shasta Dam, October 28, 2021. Photo credit: California Dept. of Water Resources

Sarah Null is the 2021-2022 CalTrout Ecosystem Fellow at the Public Policy Institute of California and an Associate Professor of Watershed Sciences at Utah State University.

Jeffrey Mount is a Senior Fellow at the Public Policy Institute of California’s Water Policy Center and Professor Emeritus of Earth and Planetary Sciences at the University of California, Davis.

Brian Gray is a Senior Fellow at the Public Policy Institute of California’s Water Policy Center and Professor of Law Emeritus at the University of California.

Michael Dettinger is a Visiting Researcher at Scripps Institution of Oceanography and Research Professor at Desert Research Institute. He is retired from the U.S. Geological Survey.

Kristen Dybala is a Principal Ecologist in the Pacific Coast and Central Valley Group of Point Blue Conservation Science.

Gokce Sencan is a Research Associate at the Public Policy Institute of California’s Water Policy Center.

Anna Sturrock is a UK Research and Innovation Future Leaders Fellow and Assistant Professor at the School of Life Sciences at the University of Essex.

Barton “Buzz” Thompson is the Robert E. Paradise Professor of Natural Resources Law at Stanford University and a Senior Fellow at the Stanford Woods Institute for the Environment.

Harrison “HB” Zeff is a Research Scientist of Environmental Sciences and Engineering at the University of North Carolina, Chapel Hill.

Further Reading

Abatzoglou, J. et al. “California’s Missing Forecast Flows in Spring 2021 – Challenges for seasonal flow forecasting”,, Posted on July 18, 2021.

Bardeen, S. “What It Means to Store Water for the Environment”, PPIC blog, Posted on July 26, 2021.

Grantham, T. et al. 2020 “Making the Most of Water for the Environment: A Functional Flows Approach for California’s Rivers, Public Policy of California report.

Lund, J. “The Big California Drought Stories of 2021”,, Posted on October 3, 2021.

Mount, J. et al. 2017 “Managing California’s Freshwater Ecosystems: Lessons from the 2012-2016 Drought, Public Policy of California report.

Mount, J. et al. 2018 “Managing Drought in a Changing Climate: Four Essential Reforms”, Public Policy of California report.

Moyle, P. and A. Rypel. “Drought Makes Conditions Worse for California’s Declining Native Fishes”,, Posted on June 27, 2021.

Null, S.E. and L. Prudencio. 2016 “Climate change effects on water allocations with season dependent water rights”, Science of the Total Environment 571: 943-954.

Rypel, A. et al. 2021 “A Swiss Cheese Model for Fish Conservation in California”,, Posted on January 24, 2021.

Willis, A. et al. “Dammed hot: California’s regulated streams fail cold-water ecosystems”,, Posted on August 29, 2021.

About Andrew Rypel

Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Director of the Center for Watershed Sciences.
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2 Responses to Managing Water Stored for the Environment During Drought

  1. Joseph Rizzi says:

    Louvered Fences and automatic Louvered gates on Sacramento and San Joaquin on either side of Sherman Island would keep the Delta fresh and increase our water by up to 70% per PPIC which states that 71% of fresh water in required to hold back Salt Water imports from Sea to Ports of Sac & Stockton via shipping dredged channels.

  2. This paper does an excellent job of exposing the mismanagement of public water resources by state and federal agencies in California this year. More importantly, the paper proposes scientific solutions to the errors made by these agencies in order to avoid more ecological damage being done in 2021-22 and succeeding years.

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