Spreck Rosekrans is an Economic Analyst at EDF.

Over the last 20 years, water agencies have invested far more in groundwater storage projects than they have in surface storage. When discussing groundwater development, it is first and foremost essential to distinguish between drilling new wells simply to “mine” limited and unsustainable supplies and installing injectors, establishing recharge areas or other infrastructure so that aquifers can be deliberately replenished in wet years. The former is a huge problem; the latter is an opportunity and the subject of this blog.

As of 1980, large dams have collected the flow of all major streams in the Central Valley. Building additional surface storage would provide little additional supply in many watersheds as river flows, even in most wet years, would often be insufficient to fill a new reservoir. In addition, the best dam sites have already been used and additional sites would be more costly to develop.

On the other hand, new technologies have made it possible to replenish groundwater basins during wet years in many areas that were overdrafted throughout much of the 20th century. Today, as a result of these new opportunities, agricultural and urban agencies, as well as investors, have done their own analyses and developed a strong preference for groundwater over surface storage development.

A recent Los Angeles Times article described the successful groundwater program in the Inland Empire Utilities District, where Martha Davis, founder of the Mono Lake Committee, convinced General Manager Rich Atwater that concentrating on locally developed water supplies could provide improved reliability without over extracting water from sensitive remote ecosystems. In the Bay Area, the Santa Clara Valley Water District has been managing its groundwater basin for years and even the San Francisco Public Utilities Commission has initiated a groundwater program.

The largest groundwater projects, however, have been developed in the Central Valley. EDF’s “Recently Developed Water Storage Capacity in California” describes four projects in the Central Valley that collectively provide more than 4,500,000 acre-feet in new storage – far outpacing surface projects over the last 20 years.

One unique aspect of recent groundwater development is that it is often done under a “banking” arrangement for the benefit of communities that can be hundreds of miles away. Semitropic Water Storage District in Kern County, for example, stores water for urban southern California and several Bay Area water agencies. In wet years a portion of the urban agency’s surface water, usually part of its allocation from the State Water Project, is diverted to Semitropic and stored underground. In a dry year (like 2009!), some of Semitropic’s surface water allocation is diverted to the urban area to augment supply, and the Semitropic can then extract the previously stored groundwater to supplement its own supply.

Of course, banking arrangements need not be limited to groundwater, but are commonplace in surface reservoirs as well. But are there any large surface reservoirs that have space available to store additional water in wet years for those that need it in dry years? We can think of a big one, with enough space presently to hold more than the five largest Central Valley reservoirs combined.

Next: Using available surface storage