Given current economic policies in the United States and other major industrial countries and the decline in value of the dollar through December, the huge U.S. international deficit should fall by only $30 billion to $40 billion over the next two years and then begin to grow again, according to a Brookings Institution study released yesterday.

Economist Ralph C. Bryant, a senior fellow at Brookings, said the current account deficit -- a measure that includes trade in goods and services, net foreign investment income, tourism and other transfers -- would drop from about $150 billion last year to about $110 billion in 1989, but would go back up about $20 billion by 1991.

He indicated that if the dollar continued to decline or if economic policy adjustments were made, such as a large reduction in the U.S. government budget deficit, future international deficits would likely be smaller.

Bryant's figures were based on the combined results of several models of the international economy in use here and abroad. They were presented in a paper updating work described in a new Brookings book, "External Deficits and the Dollar, the Pit and the Pendulum." Bryant edited the book along with Peter Hooper, assistant director of the Division of International Finance at the Federal Reserve, and Gerald Holtham, who is on the staff of Credit Suisse First Boston in London.

Hooper told reporters that a combination of a tight, anti-inflation monetary policy and a stimulative fiscal policy caused the value of the dollar to rise sharply between 1982 and early 1985. The increase in the dollar, plus somewhat faster economic growth in the United States than in other major industrial countries, produced the unprecedented increase in the current account deficit.

The current account deficit must be financed each year by capital from abroad, either in the form of loans or investments. This financing need and the willingness of private foreign investors to provide it have become major forces in U.S. financial markets. For instance, last year, when private investors became less willing to place money here, U.S. interest rates rose and foreign central banks had to make up the financing shortage, which some analysts put at about $100 billion.

Reagan administration officials generally have been counting on the fall of the dollar to reduce the trade deficit by making U.S. exports less expensive in terms of other nations' currencies and by making foreign imports more expensive here.

Bryant said, though, that an analysis by economist Paul Krugman of the Massachusetts Institute of Technology, included as a chapter of the book, suggests that the dollar has not yet fallen to a level that can be sustained.

"If present {economic} policies more or less stay where they are, you get good news for a couple of years and then it gets worse," Bryant said. "What does that imply about the dollar? All these models indicate more dollar depreciation is likely to be necessary to produce external balance ... but I am not convinced as some academic economists are that further depreciation of the dollar will be enough."