New task for southwest water planners: handling a surplus

Accustomed to scavenging for water, southern California water officials are finding their attention redirected this spring. The Colorado River, bulging with runoff from record snowfalls, is bringing more water than the Southwest can use.

California's history has been dominated by passionate battles over the precious resource. But for now the short-term picture for water supply has turned decidedly optimistic.

Officials are scrambling for ways to capture such surpluses as the millions of acre-feet of water flowing, untapped for lack of storage space, into the Gulf of California.

Some stopgap measures - which until now have been overshadowed by more long-term, elaborate plans - are getting a fresh look.They include:

* Banking water beneath the agricultural Coachella Valley in the southernmost part of the state. This plan, expected to take effect late this year, would allow the Metropolitan Water District (MWD), which supplies much of urban southern California with water and depends on the Colorado for one-third of its supplies, to ''bank'' surplus water in wet years in basins beneath the valley. Under the plan, the district would deliver 500,000 acre-feet of water annually (during years of surplus) to Coachella Valley. That's about the equal to what Los Angeles' nearly 3 million residents use in a year. In turn, when shortages occur, the Coachella Valley's farmers could pump that water for themselves and allow the MWD to take the Colorado River water allotments usually reserved for the valley.

* ''Mining'' newly discovered underground aquifers created by seepage from Southwestern irrigation projects. This idea has not been lost on Mexicans across the border. Setting up ''drill fields'' along the border, they have quietly been ''pumping water right out from under California,'' says one California water official. Geological consultants were retained recently by the Colorado River Board to study the feasibility of ''mining'' water that has seeped through unlined irrigation canals in the Imperial Valley, just above the Mexican border.

* Water salvage projects, like a proposed deal between the Imperial Irrigation District (IID), in the Imperial Valley, and the MWD. Wary of a repeat of past water wars between central valley farmers, farther north, and thirsty Los Angeles-area urban dwellers, Imperial Valley farmers are entertaining proposals for a trade of sorts: MWD-financed repairs on inefficient IID canals and control gates in exchange for water credits equal to the amount of water saved by the repairs. The multimillion-dollar inprovements in the irrigation system could salvage an estimated 200,000 to 400,000 acre-feet of water each year, according to the MWD.

''This year we have available all the water our aqueducts can handle, and we're limited only in the ability to store it,'' explains David Kennedy, the MWD's assistant general manager.

In 1982, only 200,000 acre-feet of the river's average 14 million annual acre-foot flow of water ever reached the Gulf of Mexico, explains Myron Holburt, chief engineer of the Colorado River Board of California. By contrast, the surplus of water expected to flow into the gulf this year is about 5 million acre-feet. An acre-foot of water is the average amount used by a family of five in a year.

''It's an unforeseen event we're trying to take advantage of,'' says Mr. Kennedy, who notes that the long-term water picture still is ''not bright.'' Current water supplies are not enough to meet the projected population growth in southern California, which will lose a considerable amount of Colorado River water when Arizona's new pipeline plugs into the river in the next decade.

The concept behind new ideas for storing and salvaging water, Kennedy says, is to find ways to conserve wet-year surpluses for use during the dry periods normal for California. All of these ways, he says, are likely to ''be relatively cheaper than building new dams.''

Also, the new ideas for tapping surpluses were given an impulse by last fall's defeat of the Peripheral Canal measure, a plan that southern California water officials had strongly backed.