A federal judge is scheduled to weigh arguments Tuesday in a case pitting an endangered fish against Southern California's main water supply, and the region may be lucky to lose only a third of its water.

Environmentalists sued state and federal officials earlier this year, asking the courts to shut down the pumps that send water to Southern Californians because the pumping kills a fish called the delta smelt.

Regional water officials say that Southern California's supply of water from Northern California rainfall and snow melt could be cut by as much as a whopping 54 percent, depending upon the judge's decision that could come later this month.

Several groups have submitted proposals that would save the fish, the tiny delta smelt, by cutting back use of the pump that delivers water from the State Water Project, the same pump that sucks in and kills the fish.

Environmental groups have proposed cutting the pump's use by the up to 54 percent. Skittish water agencies and farms that rely on the water have proposed a 7 percent cutback. State and federal officials have proposed a cut up to 34 percent.

Water officials said regardless of the outcome, Southern Californians are probably going to have to tighten their belts and find new ways to cut back water use. Otherwise, residents could face mandatory water rationing for the first time since the state's last big drought in 1991.

San Diego County Water Authority leaders said recently that the proposed 54 percent cutback would make even a wet year feel very dry in Southern California.

"Half of Metropolitan's (Southern California's) supplies in a good hydrological year would be lost for species protection of the smelt," said Dennis Cushman, assistant general manager of the San Diego County Water Authority.

Metropolitan is Southern California's main water supplier. The Water Authority, which provides nearly all of local residents' water, is Metropolitan's largest customer.

Heart of the matter

Tuesday's court hearing will revolve around a long-simmering dispute over how pumps for the State Water Project and the federally operated Central Valley Project are run. Like the State Water Project, the Central Valley Project delivers water from rain- and snow-rich Northern California to the south. The federal project delivers water mainly to Central Valley farms.

The State Water Project is a 600-mile-long stretch of dams, reservoirs and pipelines that deliver water to Southern California residents. The heart of the system is the ecologically-fragile San Joaquin-Sacramento bay delta, which has been the subject of environmental debate over how to prevent killing fish and preserve habitat.

Acting on a lawsuit filed by the Natural Resources Defense Council to protect the endangered smelt, a Superior Court judge ruled in March that the state failed to get the proper permits it needed to kill fish at the Harvey O. Banks pumping station - the one that sends Southern California two-thirds of its annual water supply.

The judge ordered the state to get proper permits in 60 days or shut down the State Water Project.

The state appealed the decision, and the judge agreed to put the shutdown on hold until the appeal was heard.

The defense council, a national environmental protection group, did not rest. It asked a federal court to shut down pumps in the State Water Project and Central Valley Project immediately.

The judge, Oliver Wanger, declined to shut the pumps down, but asked all sides to submit proposals to cut back pumping in order to protect the smelt. Those are the plans that Wanger will weigh Tuesday.

Rain would help, but not much

Southern California's current water supply situation has been brought to the brink of crisis, meanwhile, by a perfect storm of drought conditions.

The Southern California region itself, particularly the Los Angeles area, is in the midst of a historic single-year drought. Meanwhile, the region's traditional sources of imported water are also drought-ridden. The Colorado River is in its eighth year of drought. And the State Water Project was only about 65 percent full this year after record-low Sierra snows.

In the past, all of the current water-supply woes could be reversed with a little help from Mother Nature in the form of more rain.

But rain and better snow packs may not help much now, Metropolitan Assistant General Manager Roger Patterson said Friday.

Patterson said rain would help recharge local reservoirs and groundwater supplies, and snow melt would increase the State Water Project's bounty.

But protecting the fish, he said, will still mean the delta's pumps will have to be shut down for periods of time while the smelt are nearby. And those shutdowns could mean the state won't have enough time - days in the year - to ship Southern California all of its water.

"We're still going to have restrictions on water supplies that you can move across the delta," he said.

- Contact staff writer Gig Conaughton at (760) 739-6696 or gconaughton@nctimes.com.