The Supreme Court yesterday unanimously refused to give the Pyramid Lake Pauite Indians more water from the Truckee River in Nevada, even though the government now concedes that it inadequately represented the Indians when the river's water was allocated.

The court refused to reopen a case settled in 1944 that measured out water from the Truckee among the Indians, a federal irrigation project and thousands of small farmers. In that case, the federal government represented the Indians and the irrigation project.

The court ruled the the government can represent the Indians and the Newlands Reclamation Project, both of which were seeking water from the Truckee in western Nevada.

Justice William H. Rehnquist, writing for the court, said that "It is simply unrealistic to suggest that the government may not perform its obligation to represent Indian tribes in litigation when Congress has obliged it to represent other interests as well."

In the case decided yesterday, the federal government took the Indians' side, conceding that it had had inadequately represented the Pauites' interest in the final allocation of water rights in 1944. The Indians had sought to increase slightly the amount of water flowing into Pyramid Lake so that its fish would not die out.

The court rejected the Justice Department's argument that it could reallocate water rights between the Indians and the irrigation project as it saw fit, because it represented both parties to begin with.

"We conclude that the government is completely mistaken if it believes that the water rights conferred to it by the 1944 decree . . . were like so many bushels of wheat, to be bartered, sold or shifted about as the government might see fit."

Justice William J. Brennan Jr., in a concurring opinion, said the Indians can sue the federal government "for the breach of duty that the United States has admitted."

The Pauites won a damage suit against the federal government in 1951 because they had not received water to which they were entitled in order to maintain the lake and their fishery. In return for $8 million, the tribe at that time excused the government from further liability for the error.

Yesterday's decision is the second time this term that Indians have lost an effort to reopen long-sealed water allocation agreements. In March, the court ruled 5 to 3 that it was too late to correct a mistake made by government lawyers representing Indians that denied them water to which they were entitled.