Posted by: Deirdre Des Jardins | April 26, 2021

The State Water Project was originally designed for a six year drought

A 1983 Bulletin by the Department of Water Resources (Bulletin 160-83) [1] documents that Oroville reservoir was designed to provide long-term carryover storage and reliable water deliveries in case of a repeat of the 1928-1934 drought. The Department of Water Resources  proposed in the same Bulletin 160-83 to change State Water Project operations to take greater risks with carryover storage to increase total water deliveries. The operational change was made on the basis of a long forgotten study which estimated that the 1928-1934 drought only had a probability of recurrence of 1 in 200-400 years. Bulletin 160-83 states:

A few major reservoirs were developed for long-term carryover storage (water stored for use over several dry years), which means that storage capacity is several times the firm annual yield. Examples of such facilities are Shasta, Oroville, Berryessa, and New Melones.  (p. 23)

Bulletin 160-83 further states:

Supply Dependability and Risk

The thrust in California water development over the past few decades has been to increase water supplies to match needs, and in many areas, to increase the dependability of supplies. Much attention has been given to this by the SWP and the CVP which were designed to withstand reoccurrence of the 1928-1934 drought. Projects, facilities, and programs of other agencies have similar built-in-risks. But uncertainty regarding the capability of increasing developed supplies over the next several decades may justify and in fact may require taking greater risks in delivering water to customers.

Selection of the 1928-1934 drought to evaluate yield was not based on the relation of drought frequency to cost of facilities. Rather, it was based on the fact that both the CVP and SWP received popular support following the 1928-1934 drought, and Californians wanted the projects to provide essentially a full supply during the entire drought, regardless of its frequency of reoccurrence. Of course, during normal and above-normal years, projects can deliver much more water than is defined as yield under this criterion  Surface water projects of other agencies use different yield-determining dry periods, but the concept is the same. This operational procedure works well where adequate water supplies are already developed to meet existing and future uses. Unfortunately, the State’s water uses are outpacing the rate at which increased supplies are being added.

Some water projects would take greater risks by delivering a higher annual supply, leaving less carryover storage in case of drought. This would allow growing needs to be met in normal years. While the final answer lies in what nature will actually provide, there is a good argument that, in the present era of uncertainty regarding future water development, given the frequency of reoccurrence of droughts, existing facilities may be operating in a more conservative manner than is necessary. The 1928-1934 dry period is estimated to have a reoccurrence of one in 200 to 400 years. However, such dry periods could occur in successive decades. Nevertheless, with such a small frequency probability, it may be that projects should take a greater risk and deliver a higher annual average supply.   (p. 255-256, underlining added)

Drought Recurrence

The estimate that the 1928-1934 drought has a recurrence of 1 in 200-400 years is not supported by the Sacramento Valley hydrology reconstructed from tree rings by David Meko. Six year droughts of similar severity occurred in the 1840s and 1780s. And four years after Bulletin 160-83, the 1987-92 drought began.

Bulletin 160-83 did not disclose what specific changes DWR made to carryover storage targets. The changes were shown in an obscure 1988 article in the academic journal Climatic Change by William E. Riebsame, entitled “Adjusting Water Resources Management to Climate Change”[2]  Riebsame cited an unpublished 1985 report by DWR, “Evaluation of the State Water Project Rule Curve Procedure,” and an unpublished report in 1988, “State Water Project Rule Curve for 1988.”  The new and old rule curves for total end of year system storage (including Oroville and San Luis Reservoir) was reproduced by Riebsame on p. 84:

Graph of storage going to dead pool

If DWR had continued operations under the 1977 rule curve, water stored in Oroville and San Luis reservoirs would be conserved to provide to provide water supply reliability. The low storage in Oroville reservoir in the 2012-2016 drought and again this year appears to be a consequence of this fundamental change in operational strategy for the State Water Project in the early 1980s.  The current SWP operational strategy maximizes deliveries, but at the cost of loss of reliability in water exports, and a loss of reliability for in-basin uses.


[1] Department of Water Resources, Bulletin 160-83. Available at

[2] Riebsame, W.E., Adjusting Water Resources Management to Climate Change, Climatic Change, 13 (1988) 69-97. Available at



  1. That’s a nugget. I worked my way thru grad school in the 70s at DWR, when we had a big drought (1977-81), so I have wondered why reliability of deliveries has declined so much in recent decades. Maybe this change is part of the explanation.

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