President Reagan signed a $3.9 billion drought-relief package Thursday, telling farmers it "isn't as good as rain, but it will tide you over" until normal weather returns.

At a ceremony in the White House Rose Garden, Reagan said, "This bill expresses a distinctive American tradition - that of lending a helping hand when misfortune strikes."He said the measure was a product of bipartisanship, as members of both parties in Congress saw that "the need to help our farmers rose far above any partisan politics."

"This legislation is helpful and generous, but at the same time it is not a budget buster," the president said.

"To ensure the continued success of this important bill, we must stick to the principles I have outlined, targeting assistance to the truly needy and emphasizing sound business decisions," he said.

Addressing the nation's farmers directly, the president said, "This bill isn't as good as rain, but it will tide you over until normal weather and your own skills permit you to return to your accustomed role of being the most productive farmers in the world."

The measure provides aid for farmers suffering crop losses of more than 35 percent of their expected harvest because of the drought or other calamities such as hail, insect damage or excessive rain. The payments would be equal to 65 percent of the lost income.

Farmers whose crop losses exceeded 75 percent of the expected yield would receive payments equal to 90 percent of lost income.

"It's a good buffer, and it's probably enough to get most people over the hump this year," said Keith Salter, who figures he will get $12,000 because the drought baked all 420 acres of wheat, barley and oats on his farm near Menoken, N.D.

Farm organization leaders said the bill - the most expensive disaster relief measure ever enacted for agriculture - won't be enough to keep all financially squeezed farmers in business.

"If a farmer has been struggling these past few years, this will not allow him income to pay down debt and interest," said John Leininger, spokesman for the 33,000-member North Dakota Farmers Union. "We are definitely going to see some farmers leave, I am sure."

According to the Farmers Union, a farmer with an average wheat yield of 30 bushels an acre who harvested nothing this year should receive at least $42 an acre in disaster aid. The payment would go up to about $92 an acre if the farmer had federal crop insurance.