For as long as Stuart W. Rockwell could remember, there has been a deputy chief of protocol in the State Department's Office at Protocol. And since 1974, Rockwell, a career Foreign Service officer and former ambassador to Morocco, has been it.

Among other functions, Rockwell, 60, said yesterday, the deputy served as the chief of protocol's "alter ego." Besides "sharing the work load" and "overseeing the office" on occasion, it also meant standing in when the chief was absent, as often as not on the highly visible Embassy Row social circuit.

Now, in a departure mutually agreed upon by Chief of Protocol Evan S. Dobelle and his deputy, Rockwell will be leaving after the first of the year, yet another indication that on payday in the Carter administration one chief, alter or otherwise, is apt to be quite sufficient, thank you.

"I have great respect for Stuart," said Dobelle, "and I asked him to stay on if he would take on other responsibilities. But I think Stuart thinks those responsibilities are beneath him."

Rockwell, a $47,500-a-year carry-over from the Nixon administration, confirmed that what Dobelle had in mind didn't quite match his own ideas about the job. Dobelle offered Rockwell the chance to stay on as deputy, without a cut in pay, if he would assume the additional responsibilities of either administering the office or acting as liaison with the diplomatic community.

"I didn't think such a senior person as I should perform a job that amounts to managing the office," said Rockwell.

Dobelle said he plans to eliminate the deputy's post as it now stands either by neglecting to fill it after Rockwell leaves or merging it with an assistant chiefdom on his 43-member staff.

"Under zero-base budgeting, I cannot rationalize the post of deputy chief unless it carries with it those other responsibilities," said Dobelle.

Then, too, "If the chief of protocol is doing his job, the job of deputy is a plum. And in my mind that is not what this administration is all about."

What it is about, it seems, if fiscal austerity and in the case of the Office of Protocol, a special goal to save about $100,000, or 11 per cent next fiscal year, over the current $896,000 salary outlay.

Besides Rockwell, protocol soon will lose two (out of a total of five) assistant chiefs of protocol: Ben Whitehead (for administration) who is going overseas, and Hampton Davis for diplomatic liaison) who is retiring. And while Dobelle expects to fill those posts, they will not be filled at their present salary levels.

It all fits in with his reorganization which, he said, has been going on since he took over last winter. By scaling down salaries of Carter appointees from those during the Nixon and Ford administrations. Dobelle claimed he has already saved around $40,000 of his $100,000 goal.

"In my office that's enormous."