1991 California drought

1991 California drought
Name	1991 California drought
Duration	1990–1992
Location	California, United States
Causes	Pacific Decadal Oscillation, El Niño–Southern Oscillation, Ridge of High Pressure
Affected	Central Valley, Sierra Nevada, Southern California, Sacramento Valley
Damages	Agricultural losses, reservoir drawdown, groundwater depletion

1991 California drought The 1991 California drought was a multi-year hydrological event that produced widespread precipitation deficits across California, concentrating impacts in the Central Valley (California), Sierra Nevada (United States), and Southern California. Seasonal circulation anomalies linked to the El Niño–Southern Oscillation and shifts in the Pacific Decadal Oscillation reduced snowpack and runoff, stressing water supplies for municipal, agricultural, and environmental users. Impacts intersected with competing water rights in the Sacramento–San Joaquin River Delta and prompted policy reactions from state and federal entities including the California Department of Water Resources, United States Bureau of Reclamation, and the California State Water Resources Control Board.

Background and climatic causes

Persistent precipitation deficits in 1990–1992 were associated with a positive phase of the Pacific Decadal Oscillation and an eastward-displaced El Niño, combined with an anomalous persistent subtropical ridge similar to patterns observed during the 1990s North American drought. Atmospheric teleconnections altered the Pacific storm track, reducing orographic precipitation over the Sierra Nevada (United States) and diminishing snow accumulation in basins feeding the Feather River and Tuolumne River. Sea surface temperature anomalies in the Pacific Ocean and modifications of the Aleutian Low forced fewer landfalling mid-latitude cyclones, decreasing inflows to reservoirs such as Lake Oroville, Shasta Lake, and Millerton Lake. Climate variability interacted with water management practices in the Central Valley Project and State Water Project (California), amplifying exposure to shortages.

Chronology and geographic extent

Early signs appeared in late 1989 with below-average precipitation in the Sierra Nevada (United States); deficits deepened through the 1990–1991 and 1991–1992 water years. The drought’s core affected the Sacramento Valley, San Joaquin Valley, and metropolitan regions including Los Angeles and San Diego. Snow water equivalent in major watersheds—American River, Yuba River, Kings River—fell well below historical normals by March 1991, triggering emergency operations in the Central Valley Project and the State Water Project (California). Coastal watersheds like the Santa Ana River and Los Angeles River also experienced low flows, while southern basins that normally receive orographic enhancement saw reduced recharge. Interannual variability produced localized relief in parts of northern California but widespread deficits persisted until the wet winters of the mid-1990s.

Impacts (hydrology, agriculture, economy, environment)

Hydrology: Reservoir levels at Shasta Lake, Lake Oroville, and Don Pedro Reservoir declined, reducing storage available for the Central Valley Project and municipal supplies for San Francisco and Sacramento. Reduced Sierra Nevada (United States)] snowpack diminished spring runoff into the Sacramento–San Joaquin River Delta, stressing California Aqueduct deliveries and increasing reliance on groundwater pumped from aquifers beneath the Central Valley (California).

Agriculture: The water-short conditions forced acreage fallowing across Fresno County, Kern County, and Tulare County, reducing production of Almonds, Pistachio, wine grapes, Cotton, and rice in the Central Valley. Farmers dependent on Central Valley Project and State Water Project (California) allocations shifted to groundwater, intensifying subsidence in districts managed by the Tulare Lake Basin. Commodity price effects rippled through markets monitored by the United States Department of Agriculture.

Economy: Municipalities such as Los Angeles and San Diego instituted water-use restrictions under ordinances enforced by local water agencies like the Metropolitan Water District of Southern California. Hydropower generation at Shasta Dam and Oroville Dam declined, impacting regional grids operated by entities including the California Independent System Operator. Job losses and fiscal pressures hit agri-service industries in counties like Fresno County and Kings County.

Environment: Aquatic ecosystems in the Sacramento–San Joaquin River Delta, Mono Basin, and coastal estuaries experienced lower flows and higher salinity, threatening populations of Chinook salmon, Steelhead trout, and the Delta smelt. Wetland habitats managed by agencies such as the California Department of Fish and Wildlife and the United States Fish and Wildlife Service contracted, affecting migratory bird concentrations at sites like the San Francisco Bay National Wildlife Refuge and Tule Lake National Wildlife Refuge.

Government and policy response

State and federal responses involved allocation adjustments by the United States Bureau of Reclamation and emergency declarations directed by the Governor of California. The California State Water Resources Control Board issued curtailments and imposed reporting requirements on diverters, while the California Department of Water Resources re-prioritized reservoir operations and water transfers. Legal disputes over senior and junior water rights engaged courts in Sacramento County and prompted negotiations among water districts including the Metropolitan Water District of Southern California, Contra Costa Water District, and East Bay Municipal Utility District. Federal agencies, including the United States Environmental Protection Agency and the National Oceanic and Atmospheric Administration, coordinated drought assessment and fishery protections. Stimulus for infrastructure investments influenced subsequent projects under the Central Valley Project Improvement Act and regional conjunctive-use programs developed with the United States Army Corps of Engineers.

Recovery and long-term consequences

Recovery followed with wetter winters later in the decade that refilled reservoirs and rebuilt Sierra Nevada (United States)] snowpack], but the drought influenced long-term water policy and management. Responses accelerated adoption of groundwater monitoring and allocation reforms, influencing the later Sustainable Groundwater Management Act debates and local Groundwater Sustainability Agencies in the Central Valley (California). The event highlighted vulnerability of ecosystems like the Sacramento–San Joaquin River Delta and informed later restoration efforts involving the CALFED Bay-Delta Program. Economic lessons affected crop choice and irrigation technology uptake across Fresno County, Kern County, and Madera County, including broader adoption of drip irrigation and deficit irrigation programs promoted by the United States Department of Agriculture. The drought also fed into climatological studies linking El Niño–Southern Oscillation variability and the Pacific Decadal Oscillation to California water security, shaping risk assessments used by the California Energy Commission and water managers through the 21st century.

Category:Droughts in California Category:1990s natural disasters in the United States