Historically, with differing perspectives on the future of water management in California, one of the largest agricultural water districts and a leading environmental organization have completed a joint study that shows how both interests can benefit from operating major storage reservoirs  in conjunction with groundwater basins.

In the view of this study, healthy rivers are not just environmentally valuable, they also are central to ensuring reliable, sustainable water supplies.  That is because water supply systems that work in concert with the environment are less likely to be encumbered by court orders, water rights hearings, and other restrictions that can have drastic effects on water supplies for farming and human needs.

Beginning in 2006, the Glenn Colusa Irrigation District (GCID) and the Natural Heritage Institute (NHI) jointly embarked on an investigation of the potential benefits from changing the operations of the two largest reservoirs in California so that they can capture a larger fraction of the annual rainfall and snowmelt.   These are Shasta reservoir, the largest in the Federal Central Valley Project (CVP), and Oroville reservoir, the only water storage facility for the State Water Project. Both reservoirs are located in the Sacramento Valley, and control water flows on the Sacramento River and the Feather River, respectively. 

The study shows that water yield could be increased in these reservoirs by re-operating them to release additional water to meet irrigation demands and for ecosystem enhancement, including salmon runs.  But this creates a risk that the reservoir will not refill during the following winter and spring.  To eliminate this risk, the study looked at the possibility of drawing upon the groundwater aquifers in the Sacramento Valley to supplement the deliveries from the reservoirs. The study found that if this technique had been used during the 82 years during which records have been kept, it would have been necessary to turn to the groundwater system to assure full deliveries in only 4 of these years for the federal reservoir and 6 years for the state reservoir, and that the groundwater levels would have rebounded during the following precipitation season, or soon thereafter. 

According to Gregory Thomas, the CEO of NHI, the environmental partner, “this is surely the most rigorous study to date on the potential for optimizing the operations of existing water infrastructure in California to produce benefits for both the environment and water supply that are complementary rather than competitive.  We know from this study what will work and what may work even better by more fully integrating the management of existing reservoirs and groundwater systems and by physically interconnecting them. This work strongly suggests that alleviating the conflicts in the delta through isolated conveyance could provide substantial improvements in environmental conditions in the upstream rivers as well as within the delta itself. “

The investigation began with the expectation that surplus water generated through the re‐operation of these reservoirs could be banked in the groundwater aquifers in the Sacramento Valley, like other conjunctive use programs in the San Joaquin Valley of California, with water put into groundwater storage in wet years and extracted in dry years. However, initial assessment and site screening revealed that conditions in the Sacramento Valley are not conducive to this approach primarily because groundwater aquifers generally refill completely during each precipitation season. What emerged instead was a conjunctive management approach based on reservoir re‐operation backstopped by additional groundwater extractions to supplement reservoir storage when refill is insufficient.  The results show that this technique can provide environmental flow benefits while also improving water supply reliability, reducing flood risks, and buffering the effects of climate change.