President Carter announced his long-awaited national water policy yesterday, asserting that for the first time the government would make conservation of water a national objective.

The policy, which won quick endorsement from a coalition of environmental groups, would tighten the criteria used to measure the value of water projects and require states for the first time to share in those costs. Administration officials said this would force states to decide which proposed federal water projects they really want.

In unveiling the policy at the White House, the president promised to use the new, more stringent criteria in proposing an unspecified number of new projects by the end of this week. He noted that these would be the first new projects proposed by the government in four years.

Water policy sparked Carter's first major battle with Congress when, only a few weeks into the new administration, the president sought to kill a number of expensive projects. Congress prevailed and preserved most of them, but that fight and general suspicion about Carter's intentions have contributed to his political problems in the West, where both water and trust in the federal government are scarce commodities.

The policy announced yesterday contained no major surprises, as administration officials had previously briefed interested groups on its contents. Under it, Carter proposed - subject to congressional approval - that the states be required to pay 10 percent of the cost of revenue-producing projects such as hydroelectric power dams, and share proportionately in the revenues from the projects. He also asked states to put up 5 percent of the cost of nonrevenue-producing projects such as flood-control dams.

Heretofore, the federal government has paid the full cost of water projects, contributing to a backlog of 783 projects, with a total completion cost of $20 billion, that are under way, and 497 projects, with an estimated total cost of $13.2 billion, that are authorized but not under construction.

The new cost-sharing formula proposed by Carter would apply only to new projects, not those under way or authorized. However, administration officials said the states would be allowed to contribute voluntarily to the cost of authorized projects and that those projects would be "moved to the head of the line" for federal funding.

The president also proposed that no state be required to contribute more than one-quarter of 1 percent of its annual general revenues to any one project.This was a major concern of the nation's governors, who argued that small states might not be able to afford even a 10 percent share of the cost of a huge water project.

The policy omitted two early objectives of the administration that were dropped several weeks ago because of political opposition.

One was to change the so-called discount rate - the formula by which the government calculates the interest cost of raising money for water projects. The rates is 3.25 percent for projects authorized before 1969. It was raised to 6.37 percent in 1974 and automatically increases one-quarter of 1 percent every year.

Critics charge that the rate is still artifically low, concealing the real cost of a project, and last year Interior Secretary Cecil D. Andrus said increasing the rate was "imperative." Yesterday Carter said the existing rate is "reasonable."

The second step discribed by the administration would have required beneficiaries of water projects - for example, farmers who receive cheap irrigation water from a dam - to pay back a large portion of the project's cost. Instead, Carter said the government will insist on recalculating and renegotiating water rates from federal projects every five years, ending the practice of signing 40-year contracts to buy water from federal projects.

The Coalition for Water Project Review, 24 environmental groups that two months ago accused the administration of "subverting" promised water policy changes, quickly endorsed the Carter plan.

"We think that the president's proposal represents a major step in the right direction," said John Burdick, coordinator of the coalition.

He said the policy contained some "disappointments," chiefly the lack of mandatory conservation measures for the use of water in agriculture, which accounts for about 80 percent of the water used in the United States.

Governors were more cautious in commenting on the policy.

Utah's Scott M. Matheson, chairman of the water policy subcommittee of the National Governor's Association, said the proposed cost-sharing formula appeared to contain the "flexibility" the governors had urged on the president.

But the Matheson said he is uneasy about the unspecified conservation measures that Carter said he would direct federal agencies to take.

I do not believe that the president understands the nature of conservation of water in the dry western states," Matheson said. "I do not think he even appreciates fully how this is the very essence of how we survive."

Other elements of the policy the president announced include:

Applying a tighter, uniform cost-and-benefit formula for all water projects and creating of a special review unit in the Water Resources Council to assure the Water Resources Council to assure the formula is followed.

Making conservation a specidic jective in measuring costs and benefits.

Asking Congress to approporiate $50 million to help fund state water plans and to implement state water-conservation programs.

The change in the cost-sharing formula and the $50 million appropriation would require congressional action. Carter said he also will ask Congress to enact legislation that would allow the states to raise the price of municipal and industrial water from federal projects as a means of encouraging conservation.