Water Board’s Decision 1641 adopted in wetter period

Tree in wet and dry landscape

This blog post analyzes long term changes in the Sacramento Four River runoff using a 10 year and 30 year running mean. Sacramento River runoff was lower during the Dust Bowl era drought of 1928-34, and the current 30 year mean runoff is close to the mean for the historic record. However, the current 10 year mean runoff is significantly lower than the long-term mean. The data also shows that the Water Board’s Decision 1641 was adopted during a wetter period in the historic record.

The Sacramento Four River runoff is calculated by the Department of Water Resources’ division of Cooperative Snow Surveys, and is used to determine the Water Year classification under Decision 1641. It is the sum of the unimpaired flow at the Sacramento River at Bend Bridge, Feather River inflow to Lake Oroville, Yuba River at Smartville, and American River inflow to Folsom Lake.

The mean runoff for the historic record is 17.8 MAF. The 30 year running mean for 1992-2021 is 17.6 MAF. The minimum 30 year running mean runoff was 15.3 MAF for 1918-1947.

However, when the Water Board adopted Decision 1641 in 2000, the 30 year mean runoff was 1 MAF higher than the current mean for the historic record.
Ten year mean runoff is currently 15 MAF, 2.8 MAF less than the mean for the historic record, and 5 MAF less than the ten year mean when Decision 1641 was adopted in 2000. Minimum ten year mean runoff was 12.3 MAF in 1937. This is one of the driest ten year periods in the record reconstructed from tree ring data. (DWR 2018.)
Here is the 30 year running average on some key dates:
1944 — 16.1 MAF — Shasta Dam commences operations
1960 — 17.2 MAF — Water Board Decision 990, retroactively granting permits for CVP Shasta, Delta Division.  First Coordinated Operations Agreement between United States and the State of California.
1968 — 17.9 MAF — SWP completed, Water Board Decision 1275, granting permits for SWP operations,
1978 — 18.6 MAF — Water Board adopts Decision 1485, Bay-Delta Water Quality Control Plan (WQCP) standards.
1985 — 19.4 MAF — United States and California sign updated Coordinated Operations Agreement, pledging to operate CVP and SWP to meet in-basin needs.
1995 — 18.2 MAF — Water Board adopts 1995 Bay-Delta WQCP update.
2000 — 18.8 MAF — Water Board adopts Decision 1641, mandating that CVP and SWP have interim responsibility for meeting 1995 Bay-Delta WQCP requirements,
2001 — 18.4 MAF — Water Board stays and then dismisses Phase 8 of hearing on implementing 1995 WQCP. Phase 8 would have allocated some responsibility to other water rights holders for meeting Bay-Delta WQCP requirements.
2018 — 17.3 MAF — Water Board adopts Phase 1 Bay-Delta WQCP update, United States and California sign addendum to Coordinated Operations Agreement.
2021 — 17.6 MAF
The 1928-34 drought was the design drought for the State Water Project and the Central Valley Project, and it also served as the basis for early reservoir operations planning, as we wrote about in “The State Water Project was originally designed for six year drought.” The return of 30 year mean runoff to the long-term historic mean shows the wisdom of planning for a repeat of the 1928-34 drought.
Further reading

California Department of Water Resources. 2022. Chronological Reconstructed Sacramento and San Joaquin Valley Water Year Hydrologic Classification Indices. Web page, accessed August 22, 2022.

California Department of Water Resources. 2015. California’s Most Significant Droughts: Comparing Historical and Recent Conditions. February 2015.

US Geological Survey, 2022. 2012-2016 California Drought: Historical Perspective. Web page, accessed August 22, 2022.

Related blog posts

The State Water Project was originally designed for a six year drought. April 26, 2021.

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