The likely effects of climate change on local water resources in places like Sacramento are still being researched by climate and hydrologic scientists, but one thing is fairly certain: There will be less snow in the Sierra Nevada in the coming decades.

Climate model projections suggest the Sierra snowpack could dwindle to a mere 20 percent to 40 percent of its historical volume during this century. In fact, historical data show that for the last century, the April-July flows in the Sacramento River have declined steadily, apparently caused by a rise of roughly two degrees Farenheit in air temperature and a consequently thinner snowpack.

Basically, the total precipitation has not declined, but winter is bringing less snow and more rain. Whether climate change brings more or less precipitation to California, on average, more of it will fall as rain rather than snow. Consequently, less water will be available when it is most needed in the summer, because surface reservoirs will have to release more water in the winter. So the seventh-ranked economy in the world, which also provides some 50 percent of the nation's fruits and vegetables, relies on a water system that depends precariously on storage of water in a gradually diminishing snowpack.

Currently, the state has no working storage alternative that would adequately compensate for declines in the snowpack. One approach is to build more dams and raise the heights of existing dams, but there is a consensus that the problem cannot be solved solely by augmenting surface storage.

Subsurface storage is a tantalizing alternative and could be vastly increased if certain technical hurdles and limitations in our knowledge of the underworld could be addressed.

The tantalizing part: Currently there is space for storage of an additional 10 million to 50 million acre-feet of water in the Central Valley, one of the largest aquifer systems in North America. For perspective, consider that the combined capacity of our four largest reservoirs (Shasta, Oroville, Trinity and New Melones) is 13 million acre-feet. Some subsurface storage and recovery of water have already been done in the Central Valley, but the time has come to adopt a grander vision on how to better use this vast below-ground reservoir on a regional scale.

The technical hurdles part: Water percolates slowly into most aquifer systems. To capture more winter runoff and move it underground will either require new reservoirs to hold the water while it is doled out to spreading basins or a way of optimizing how water soaks into the earth. The latter has not been seriously considered but, I would argue, becomes plausible with greater knowledge of the subsurface "anatomy."

The limited knowledge part: Most aquifer systems are mostly not aquifer. For example, in the aquifer systems underlying the Central Valley, only about 20 percent to 50 percent of the volume is made up of geologic materials (sands and gravels) capable of supplying significant amounts of water to wells, while the remainder is composed of non-aquifer material (silts and clays).

These aquifer structures constitute an extensively connected network that is embedded in the non-aquifer materials. The way that water moves through such a system is not unlike how liquid moves through the human body. You have a 3D network of "avenues" (aquifers, analogous to veins and arteries) that support a relatively fast flow of liquids; the intervening regions consist of materials (in the body, tissues) that support much slower rates of movement. Liquid in the fast zones moves hundreds to thousands of times faster than in the slow zones, and there is a tremendous amount of liquid and chemical exchange occurring between the two.

Unlike the human body, however, the locations of the fast-flowing avenues and where they approach the land surface are generally not known. Fortunately, this knowledge is obtainable, and it would create a new world of possibilities.

For example, by knowing the small percentage of the landscape where the aquifers intersect the surface, one could design aquifer recharge enhancement projects to efficiently move large volumes of winter runoff into the subsurface. One possible strategy in less-developed parts of the valley is to let the river floodplains flood, thereby maximizing the chances that floodwaters soak into the "sweet spots" in the system.

The bottom line? The subsurface storage potential in the Central Valley aquifer system is exciting, and the technical hurdles are not insurmountable. The first task is to better define the subsurface anatomy. Then the means and the will to manage our subsurface water as effectively as we manage our surface reservoirs will emerge.

Graham E. Fogg is a professor of hydrogeology in the Department of Land, Air and Water Resources at the University of California, Davis.