Get the Facts About the State Water Project Allocation
The aerial view looks southeast over the California Aqueduct and agricultural fields, near Highway 166, and the John R. Teerink Wheeler Ridge Pumping Plant in Kern County, California. Photo taken May 13, 2023.
What is a State Water Project allocation?
The State Water Project (SWP) is a system of reservoirs, canals, and pumping plants that was built by the California Department of Water Resources (DWR) in the 1960s to provide water supply, flood control, wildlife habitat, and recreation. Water supplies from the SWP were always intended to fluctuate with California precipitation, which naturally swings between flood and drought. The SWP delivers water to 29 public water agencies based on long-term water supply contracts. Each year, starting in December, DWR announces to those water agencies – called SWP contractors – approximately how much water the project is likely to be capable of delivering in the coming year. Once a month for several more months, DWR assesses the water supply for “allocation” and may update the allocation if conditions warrant. The allocation is expressed as a percentage of the 29 public water agencies’ maximum water supply contract, such as 15 percent or 75 percent. DWR announces the final allocation in May or June, then make deliveries to contractors based on the final allocation and subsequent contractor demands. Current allocation amounts can be found here.
How does DWR determine the SWP allocation?
The two biggest factors DWR uses to determine and update the allocation are storage levels in Lake Oroville and San Luis Reservoir and the amount of runoff projected for the remaining year. The amount of water pumped by the SWP in the Sacramento-San Joaquin Delta is a factor in allocation announcements, but there is not a direct correlation between pumping levels and the allocation. More important is the projected runoff, which is based on the amount of precipitation that has fallen since October 1. After mid-February, the SWP allocation also takes into account Sierra Nevada snowpack. In analyzing the available water supply for the allocation, SWP engineers consider DWR’s Bulletin 120, which is updated monthly (Feb – May) and forecasts the volume of water expected to runoff from the state’s major watersheds in the spring and summer. SWP engineers also take into account the various seasonal restrictions on pumping that the SWP must follow to protect threatened and endangered species as well as the flow needed to repel salinity in waterways of the Sacramento-San Joaquin Delta. Increasingly, with climate change, DWR also must take into account and plan for the potential for extreme changes in conditions. The last several winters in California have shown that dry spells may be punctuated by big storms, and wet conditions can be interrupted by extremely dry periods. DWR determines allocations conservatively, setting and updating the allocation percentage based on what volume of water engineers are reasonably certain could be delivered even if conditions were abnormally dry the remainder of the winter.
Do all contractors get the same allocation?
Each of the 29 public water agencies that take delivery of SWP water has contracted for a different volume of water, depending upon their needs, but the SWP allocation is uniform across most contractors. In other words, the largest contractor – Metropolitan Water District of Southern California, with a maximum contracted volume of 1.9 million acre-feet of water – will get the same percentage of that volume as the smallest contractor, Littlerock Creek Irrigation District, which contracts for a total of 2,300 acre-feet. Four contractors north of the Delta: the County of Butte, Solano County WA, Napa County FC&WCD, and the City of Yuba City have special provisions in their respective water supply contracts so that in certain years they receive a slightly higher allocation of SWP water than other contractors.
What is a typical SWP allocation?
Allocations typically start low and then increase through the winter months, depending upon precipitation. For example, in 2016, the December allocation was 10 percent, and the final was 60 percent. In 2024, the allocation started at 10 percent and increased to 40 percent. Full allocation years, or 100 percent allocation, are rare with only one such allocation in the past decade in the extremely wet year of 2023. Historical allocation amounts can be found here.