The acute water shortages now hitting California have prompted many in Congress and the state Legislature to call for new surface reservoirs to reduce the impacts of future droughts.
Some have even blamed the lack of reservoir development as a primary cause of water scarcity during the current drought.
The reality is that new surface storage would have added only modestly to the state’s water supply. We’d still be in the midst of a severe drought. Building drought resilience requires a much broader set of actions, including conservation, water trading, managing groundwater and expanding nontraditional supplies like recycled wastewater and stormwater.
Following the 1987-92 drought, urban water agencies invested heavily in surface and groundwater storage. These investments have paid dividends during this drought, greatly reducing water shortages in the state’s major cities. But few investments were made to increase water reliability for agriculture and the environment, and these two sectors have been hit particularly hard.
Last November voters approved Proposition 1, which allocated $2.7 billion in bonds for water storage. The bond requires that its funds provide public benefits, including improved environmental conditions. The California Water Commission can allocate those funds to surface or groundwater storage projects.
Although planners are examining a wide range of options, most attention has been focused on two proposed surface reservoirs: Sites, on the west side of the Sacramento Valley, and Temperance Flat, upstream on the San Joaquin River.
In total, these two projects would create about 2.6 million acre-feet of new storage capacity – nearly a 7 percent increase in statewide capacity – at an estimated cost of roughly $6.5 billion.
However, increases in storage capacity do not translate to equivalent increases in supply, because reservoirs can’t always be kept full. The U.S. Bureau of Reclamation and the California Department of Water Resources estimate that these projects would increase dry-year supply for agricultural and urban uses by up to 380,000 acre-feet per year, and less as droughts wear on. Sites, the larger reservoir, would be refilled with Sacramento River water only during wet periods, so it will not yield much water during prolonged droughts.
Recent studies from UC Davis help put this in context. This year and last, Central Valley farmers will have received 15 million acre-feet less surface water than in average years. They are pumping extra groundwater to make up for the loss – a source of great concern given falling water tables and sinking lands.
What if Sites and Temperance Flat reservoirs had been built before this drought? On the optimistic assumption that they made an additional 380,000 acre-feet of water available in 2014 and 2015, surface water deliveries would have increased by 5 percent – in the grand scheme of things, a drop in the bucket.
The bottom line? New surface storage would have been helpful during this drought, but not nearly to the extent people imagine.
Rather, the real benefits of new storage are likely to come from the increased flexibility it provides to our overall water system – by increasing our ability to store water from wet years in groundwater basins, strengthening our capacity to manage floods and improving environmental conditions in our troubled watersheds. To qualify for state matching funds, the California water commissioners have said they expect storage proposals to take this system-level, multibenefit approach.
Officials are understandably attracted to big infrastructure approaches to solving water supply problems. And in some cases, big projects are necessary. But to improve our ability to weather droughts, California needs to take actions along a number of fronts. Storage should be part of that portfolio, but it’s not a panacea.