Most of the government's 70,000 supervisors here will be shifted to a pay raise system that will mean half-rations for poor performers, and big increases for those who get the most out of their employes.

Managers and supervisors in Grades 13, 14 and 15 will be phased into the new punish-reward pay plan as set up by the Civil Service Reform Act. It begins in October 1981. But the draft of those who will serve under the system will begin immediately. Those identified as managers or supervisors will get a GM (rather than GS for General Schedule) designation. They will negotiate job goals with bosses and be rated on how well they do.

Members of the new GM class will not get longevity raises or QSI (quality step increases), each worth about 3 percent, once the new system begins. And they will be guaranteed only half of the regular annual percentage pay increase the president sets for their rank-and-file subordinates.

Drawing from a salary pool that includes funds not spent on longevity or QSI raises -- plus regular pay funds -- bosses will award individual pay increases to managers and supervisors. Poor performers will get only half the percentage increase due other civil servants each October. Poor performers also could be demoted or fired. Outstanding managers and supervisors may be able to get what amounts to double the regular percantage increase.

Government-wide there are about 291,000 GS13 through 15 employes. Locally there are 70,637. Those designated by their agencies will go into the new GM pay system. Others will stay outside, getting regular raises. The question the 13 through 15 crowd has is what defines a manager or supervisor? A liberal interpretation could include nearly everybody in that category.

Navy Sea Systems Command, for example, says that 94 percent of its 2,100 Grade 13 through 15 people here will be designated as supervisors or managers.

Many would-be managers and supervisors are uncomfortable with the idea of competing against their peers for pay raises that will be divided from a common salary pot. They also are nervous because the system will give top political appointees much more control over mid-managers, who in turn control the bureaucracy.