AS SECRETARY of the Interior for the past 16 months, William P. Clark worked with considerable skill to turn down the heat in that big building. He ended the daily fireworks displays and, in general, got the place back to work. His predecessor, James G. Watt, with his delight in outrageous ideological gestures, had started more fights with fewer tangible results than anyone in the administration. Judge Clark discreetly ended the fights and reopened diplomatic negotiations with most of the department's former adversaries. He returned the department to its job as steward of vast reaches of this country's land and water.

True, he was assisted by luck. When the administration first came to office, energy prices had been rising fast for two years. Oil and coal companies were ringed around the department, baying for access to mining and drilling sites. But when he arrived, prices were falling and the enthusiasm for expensive exploration was greatly diminished. He had the advantage of being able to work in relatively quiet times.

Secretary Clark pushed the White House hard for more money for the national parks, a difficult thing to do when the current was running the other way. That startled people who thought that, as a Reagan administration insider, he would try to apply the rule of the market to everything in sight, including the hiking trails and camp grounds. But it was a useful reminder that the conservation movement in this country originated chiefly with conservatives, and that the relationship between those two words is not a coincidence.

The next secretary's main job will not be to generate any sweeping new policy, but rather to maintain the quality of the work force that serves this gigantic department. Mr. Reagan was not the first presidential candidate to run against the federal government and the people who comprise it. He merely represented a trend that had been increasingly pronounced for two decades. Mr. Watt was not the only Cabinet member to regard his permanent civil service with suspicion, but he carried it to a pitch of adversarial hostility. An administration that wants to increase efficiency in government needs to think carefully about the quality of the people it can attract and hold.

Judge Clark made important improvements here, but he goes home to California, putting behind him the power that flowed from being one of the president's most trusted aides (in the State Department and National Security Council as well as at Interior), having been at the department for just a brief time. Whether these improvements will prove to be more than temporary will be up to the next secretary.