The Great Lakes states today signed an unusual compact with their Canadian counterparts aimed at barring the booming but parched Southwest from diverting the lakes' vast fresh water reserves to slake southern thirst.

"This is a clear signal to the Sun Belt that we stand united to protect the greatest fresh water resources in the world," said Michigan Gov. James Blanchard (D).

Called the Great Lakes Charter, the agreement binds each of the eight states and two provinces that have Great Lakes shorelines to notifying and consulting each other on any proposed major project for diversion or consumption of Great Lakes water.

It culminates two years of intensive work spearheaded by Blanchard and Wisconsin Gov. Anthony S. Earl (D).

They and other leaders at the signing ceremony here spoke sharply of what they see as an approaching confrontation with the Southwest over access to the lakes' water. They lauded the antipollution efforts of their states in recent years and declared that planning will avert trouble.

The governors included Richard F. Celeste of Ohio (D), Mario M. Cuomo of New York (D); James R. Thompson of Illinois (R), Rudy Perpich of Minnesota (D), and Earl and Blanchard. Indiana Gov. Robert D. Orr (R) arrived late, delayed by the heavy snows that have blanketed the Midwest in recent days. Pennsylvania Gov. Richard L. Thornburgh (R) sent an aide in his place.

Thompson said there is a common belief in the country that "the Sun Belt has it all over us and they plan to snatch the final prize" by succeeding at schemes to divert the lakes' water south through pipelines or canals and rivers.

The governors pledged to cooperate with Quebec and Ontario to guard the lakes.

The charter does not give veto power to any signer and has been criticized in some environmental circles for this apparent lack of power.

But Minnesota's Perpich, describing himself as "the only governor here who is a former dentist," said, "It takes time to grow teeth -- 12 years for wisdom teeth."

After the signing, in a conference room atop the 40-story First Wisconsin Bank, Milwaukee's tallest building, Blanchard said he intends to ask the Michigan legislature to enact "a flat ban" on allowing major diversions of the lakes' water from the region. "Almost everyone agrees that a total ban is desirable," he said.

Illinois, Minnesota, Indiana and Ohio have laws regulating and restricting the diversion and consumption of the lakes' water.

The governors rapped the Reagan administration for offering little to the northern states to ease their recent economic woes or to help them deal with the diversion question.

The rapid buildup of the Southwest in the past 25 years has left the region searching for new and reliable water sources so the boom can continue. At the same time, increased per capita consumption of water throughout the United States is straining potable water supplies everywhere.

Meanwhile, the Great Lakes states have become aware of the value of lakes Superior, Michigan, Huron, Erie and Ontario. Thirty-five million Americans and Canadians live within the lakes' watershed. And 26 million of them get their drinking water from them, a study commissioned by the Council of Great Lakes governors found.

The region's awareness of the issue sharpened in 1981 when a proposal was made to transport coal from Wyoming to the Midwest by pipeline.