One of California's largest agricultural water districts and a leading environmental organization have released a joint study showing that agriculture and ecosystems could benefit from operating major storage reservoirs in conjunction with groundwater basins.
Glenn Colusa Irrigation District (GCID) and the Natural Heritage Institute (NHI) jointly commissioned a study in 2006 on the impacts of changing operations of two of California's largest reservoirs so they capture a larger amount of annual rainfall and snowmelt. The reservoirs – Shasta, the largest in the federal Central Valley Project (CVP), and Oroville, a key water storage facility in the State Water Project (SWP) – are both located in the Sacramento Valley and control water flows on the Sacramento River and the Feather River.
The study found that water yield in the reservoirs could be increased by re-operating the reservoirs to release additional water to meet irrigation demands and provide ecosystem enhancements, including benefits for salmon runs. The study, which was funded by the state Department of Water Resources and the U.S. Bureau of Reclamation, shows that without the occasional use of groundwater, such releases could create a risk that the reservoirs might not refill during the following winter and spring, particularly in dry years. That risk could be eliminated by drawing upon groundwater aquifers in the Sacramento Valley to supplement deliveries from the reservoirs, the study found.
If this technique had been used during the 82 years that records have been kept, the study found it would have been necessary to turn to the groundwater system to assure full deliveries in only four of those years for the federal reservoir and six for the state reservoir. The study also asserts that groundwater levels would have rebounded during the following precipitation season, or soon thereafter.