By Erik Porse and Stephanie Pincetl

In October 2022, water agencies in Southern California with Colorado River water rights announced plans to reduce water diversions. The agencies offered voluntary conservation of 400,000 acre-feet per year through 2026. This annual total is nearly 10% of the state’s total annual usage rights for the Colorado River. The cutbacks help prepare for long-term implications of climate change for the river’s management, which are starting to be acknowledged. In urban Southern California, an important aspect of this need is reducing imported water reliance through investments in local water resources.

Diversion rights along the Colorado River are over a century old. In 1922, states signed an agreement to allocate the river’s resources. Native Americans were excluded, but some of the sovereign nations have subsequently gained water rights through legal decisions and agreements. Arizona did not ratify the compact until 1944. Within the 1922 agreement, the river’s originally estimated 16.5 million acre-feet of annual yield was divided, with 7.5 million acre-feet allocated to states in both the upper and lower basins. A paltry 1.5 million acre-feet was allocated to Mexico in the 1944 Treaty with Mexico. Subsequent research estimated the historic volume of available water on the Colorado closer to 14 million acre-feet, and from 2000-2020, average inflow volumes were 12.5 million acre-feet. These numbers leave the river dry at its ultimate destination, the Colorado River Delta in Mexico. In recent decades, no flows reached the Gulf of California until 2014, when agencies negotiated a timed release of flows from Lake Mead that wetted the Delta for a few days as part of an international agreement between the U.S. and Mexico (Minutes 319 & 323).

In 2022, the river’s available water was projected to be just 8.4 million acre-feet of inflows into Lake Powell, another year well below historical averages. Huge reservoirs along the river, namely Lake Mead and Lake Powell with over 50 million acre-feet of storage, buffer against dry periods. But today the reservoirs are nearly drained after decades of severe dryness. Existing agreements developed over 15 years outline how states in the Upper and Lower Basins must adaptively reduce demand during drought based on reservoir storage levels. Given the stark forecasts through 2024, in June 2022 the U.S. Bureau of Reclamation (USBR) issued an ultimatum to parties along the river: develop plans to reduce draws by 2 to 4 million acre-feet that address chronic overuse and align with agreed conservation plans, or be subject to federal determinations. Among the Lower Basin states, California was not required to reduce draws until Lake Mead reached 1,045 feet in elevation. As of November 30, 2022, its elevation was 1,043 feet. Recovering the basin’s reservoir levels will require continued conservation along with multiple wet years.

What would happen if Southern California lost access to Colorado River water for an extended period? Almost a decade ago, we looked at this question for urban areas in Los Angeles (L.A.) County. Cities use less than 30% of California’s 4.4 million acre-feet of Colorado River water allocations. This is mostly for the Metropolitan Water District of Southern California (MWD) and San Diego County Water Authority.  The remaining California water use from the Colorado River is for agricultural districts and Native American sovereign nations. The percentages can vary between years based on decades of complex water sharing and storage agreements. End-of-year projections for 2022 forecasted that MWD would receive 25% of California’s total Colorado River diversions. Within areas of Los Angeles, Orange, Riverside, San Bernadino, and San Diego Counties served by MWD, Colorado River water supplies are critical. MWD balances imported water supplies from the Colorado River and Northern California to maintain sufficient storage in its reservoirs, helping augment local groundwater, stormwater capture, and recycled water.

To investigate the impacts of reduced imported water supply in Los Angeles, we modeled scenarios of full (100%) to no (0%) imported water availability for L.A. County water agencies that supply 10 million people. We examined impacts on urban water operations, trees, and landscapes, along with opportunities to update existing groundwater rights allocations. At the time, we didn’t anticipate that as soon as 2022, L.A. County agencies could face little to no available new water allocations from both the State Water Project and Colorado Rivers.

Results present a window into water management futures in Los Angeles. Overall, L.A. County water agencies could likely reduce long-term water reliance on imported supplies, especially the Colorado River, to 30% of total demand. Agencies would need to boost stormwater capture and recycled water production, while also investing in many more climate-appropriate landscapes. In recent years, agencies have increased investments in these strategies. Continued conservation is also key. The modeled scenarios included water allocations for health and safety, as well as estimated irrigation requirements for the urban tree canopy (if irrigated appropriately). Total per capita use was 80-100 gallons per capita per day (gpcd) in scenarios with reduced imported water availability.

The analysis captured many current trends. For instance, water use in L.A. County has continued to decline, most recently facilitated by severe drought since 2020. Recent total urban per capita use was approximately 100 gpcd across Southern California agencies in 2019-2020. In modeled scenarios, landscape transformation was a key contributor to long-term conservation. In addition, agencies have continued to invest in stormwater capture and groundwater recharge projects, especially through the county-wide Safe Clean Water Program approved in 2018. Modeled scenarios also showed the importance of water storage, not only in MWD’s reservoirs, but also in groundwater basins. Groundwater continues to be an important source that has historically averaged 35% of supply in both L.A. County and urban Southern California more broadly. We didn’t model any new ocean desalination projects in L.A. County because no such plans were underway.

We also missed some key developments. The modeled scenarios included existing and planned recycled water projects at the time, but did not account for large scale water reuse projects by the Los Angeles Department of Water and Power (LADWP), MWD, and L.A. County Sanitation. Additionally, we modeled household water needs for health and safety as 50-55 gpcd, which was the legislative standard at the time. The current indoor standard is slated to drop to 42 gpcd through 2030. Last, we didn’t explicitly include significant policy and behavioral changes that have emerged. For instance, research supported by MWD estimated that a rebate-funded turf replacement project yields an additional two turf replacement projects not funded through incentives. So-called “non-functional” turf on commercial, industrial, and institutional properties will likely become much less common in Southern California.

Achieving equitable local water reliance will require key policy changes. Many of L.A. County’s groundwater basins were adjudicated years ago and the remaining ones have been brought under regulations through the Sustainable Groundwater Management Act or “SGMA”. In adjudicated basins, existing water rights for pumping will need to offer broader access to agencies that may be unable to fund new projects. This would further incentivize stormwater capture and would spur agencies to address current adjudications, including possible municipalization of pumping rights. New water financing strategies are also needed to pay for large water projects such as countywide water recycling. Agencies will need to increase retail rates to subsidize affordable water for low-income households (within Proposition 218 restrictions). Tradeoffs in water availability for recycled water and environmental instream flows will need to be resolved, likely through guidelines for seasonal releases.  Finally, increased and sustained funding streams for landscape transformation will be needed, supported by cooperation between water agencies and the landscaping and nursery industries. Nascent programs are already taking root.

Much work remains to adapt Southern California’s water use habits to future climate conditions. The Colorado River will remain an important source of supply for coming years, but expectations of availability will change. Southern California’s cities have significant fiscal resources and planning capacity to adapt. The hardest hit areas will likely be rural and agricultural communities in California that rely on Colorado River water. Yet, cities use food grown by farms and, over decades, water management practices between urban and rural agencies in the region have become entwined. The result is a system where everyone is affected. As we face more dry years in California and the Pacific Southwest, empathy for the livelihoods of fellow Californians is perhaps our most potent resource for climate adaptation.

Erik Porse is a Research Engineer at the Office of Water Programs at Sacramento State and an Assistant Adjunct Professor at UCLA’s Institute of the Environment and Sustainability.

Stephanie Pincetl is a Professor at UCLA’s Institute of the Environment and Sustainability and Founding Director of the California Center for Sustainable Communities at UCLA.

The results were part of projects from a multi-university team that also included Mark Gold, Diane Pataki, Terri Hogue, Katie Mika, Kim Manago, Elizaveta Litvak, Madelyn Glickfeld, and Felicia Federico.

Further Reading:

Davis, Margaret Leslie. 1993. Rivers in the desert: William Mulholland and the inventing of Los Angeles, 1st ed. HarperCollins Publishers, New York, NY.

Erie, Steven, HD Brackman. 2006. Beyond Chinatown: the Metropolitan Water District, growth, and the environment in southern California. Stanford University Press, Stanford, CA.

Felicia Federico, A. Youngdahl, S. Subramanian, C. Rauser, M. Gold. 2019 Sustainable LA Grand Challenge Environmental Report Card for Los Angeles County Water. UCLA

Galt, Joe. Sharing Colorado River Water: History, Public Policy and the Colorado River Compact. University of Arizona Water Resources Research Center. August 1997.

MWD. Integrated Water Resources Plan: 2020 Update. Technical Appendices. Los Angeles, CA.

MWD. 2020 Urban Water Management Plan. Los Angeles, CA. June 2021.

Mika, Kathryn B., E. Gallo, E. Porse, T. Hogue, S. Pincetl, and M. Gold. 2017. LA Sustainable Water Project: Los Angeles City-Wide Overview. UCLA, Los Angeles, CA

Ostrom, Elinor. 1965. Public Entrepreneurship: A Case Study in Ground Water Basin Management. Ph.D. Dissertation, University of California, Los Angeles

Ostrom, Vincent. 1962. “The political economy of water development.” The American Economic Review 52.2: 450-458.

Porse, Erik, KB Mika, E Litvak, KF Manago, K Naik, M Glickfeld, TS Hogue, M Gold, DE Pataki, and S Pincetl. 2017. “Systems Analysis and Optimization of Local Water Supplies in Los Angeles.” Journal of Water Resources Planning and Management. 143, no. 9 (2017): 04017049.

U.S. Bureau of Reclamation. Annual Operating Plan for Colorado River Reservoirs 2022. December 8, 2021.