An Introduction to State Water Project Deliveries
May 24, 2020 at 3:55pm
Most people in California receive some of their drinking water supply from the State Water Project (SWP). The SWP also supplies water to over 10% of California’s irrigated agriculture. The SWP and its service area span much of California, delivering water to 29 wholesale contractors shown in Figure 1.
Each year, the Department of Water Resources announces SWP Table A allocations which inform water contractors’ SWP deliveries: “Table A”, “Carryover”, and “Article 21.” What are these different SWP delivery categories and how do they work?
Table A, Carryover, and Article 21 are three types of SWP deliveries described in this post. Some additional, more minor, deliveries are made as: transfer and exchange Table A, and Pool Water deliveries.
The 2020 water year is dry, but the recent May storms led to the increased 2020 SWP Allocation from 15% to 20% of SWP contractors requested “Table A” delivery amounts. Figure 2 compares the initial and final SWP allocations from 1996-2020. Some lessons from this graph include:
- 2006 was the last 100% allocation year, 14 years ago.
- Final allocations usually increase significantly from the initial allocation estimate, usually sent to SWP contractors by end of October. However, final allocations could be less than initial allocation estimates in extreme dry years.
- Drought years tend to have little or no increases from initial to final SWP allocations (such as 2007-2009 and 2012-2015).
- It is likely that 2020’s final allocation will be 20%. The last 20% final allocation year was in 2015, one of the driest years on record.
Figure 3 shows Table A, Carryover, and Article 21 deliveries from 2000-2017. Minimum, average, and Maximum Table A and carryover statistics were combined because both are categorized under Table A water while Article 21 deliveries are made above the approved Table A amounts. 2014 had the least Table A and carryover deliveries at 475 TAF while 2003 contained the most at 3202 TAF.
What is Table A?
Table A allocations represent “a portion or all of the annual Table A amount requested by SWP water contractors and approved for delivery by DWR (CA DWR 2019).” DWR and the public water agencies and local water districts developed the SWP’s long-term water supply contracts in the 1960s. Table A contract amounts originated from these long-term contracts and have been amended. The 1994 Monterey Agreements significantly revised the long-term water supply contracts. Table 1 presents each contractor’s maximum Table A contract delivery amount, adding up to 4.17 million AF, anticipated to be the SWP’s ultimate delivery capability in the 1960s (an amount rarely actually available). As a wet year example, the last column indicates how much each water contractor utilized their Max Table A amount in 2006, a 100% allocation year. San Joaquin contractors were more likely to take their full Table A and supplement water supplies with Article 21 water, described later.
What is Carryover water?
Carryover water is a portion of Table A water that contractors may save for next year’s delivery. Carryover requests allow SWP contractors to store some of their annual allocation for the next year, and not lose undelivered allocation at the end of the SWP contract year, December 31. When contractors request carryover for next year’s delivery, that water is stored in the SWP’s share of San Luis reservoir in Merced County.
However, storing carryover water in San Luis reservoir has a low operating priority and so brings a risk. SWP contractors can lose this stored carryover water when San Luis Reservoir fills. In the 2017 wet year, some contractors (Santa Barbara County , Crestline Lake Arrowhead Water Agency and San Gorgonio Pass Water Agency) needed to transfer their carryover water from San Luis to another non-SWP facility to prevent losing their carryover storage. Figure 4 shows how San Luis filled in 2017 for the first time since 2011, following the 2012-2016 drought.
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