Whenever District of Columbia officials testify on Capitol Hill about such budgetary matters as the need for a larger federal payment to the city or the desire to levy a commuter tax, they cite a fearsome figure:

By 1983, they say, there will be a gap of $333 million between the city's anticipated revenue from existing taxes and other sources, and the projected level of municipal spending.

That's the bad news. The good news is that the District government has received international acclaim for the technique it uses to forecast such future financial trends.

Comer S. Coppie, D.C. director of budget and management systems, recently went to Houston to receive the annual Louisville Award, a plaque presented annually by the Municipal Finance Officers Association for outstanding contributions to public financial management.

The award is named for Louisville, Ky., which endowed the prize that has been given every year since 1939. American and Canadian governmental units are eligible. There were 25 nominees this year.

The award received by Coppie represents an area in which the District government, often criticized for slipshod financial practices, has become a model for other cities.

Officially, the process is called a "multi-year expenditure forcasting system." Where city officials once peered into a crystal ball and then jotted down an educated guess about the future, the new system involves a sophisticated, computerized projection of future trends.

In effect, according to Coppie and his aides, every one of the city's financial accounts, for each agency and program, is broken down into separate "building blocks."

Each block is subjected to separate analysis. For example, according to a summary of the program from the budget office, District salary and employe benefit trends differ greatly from costs for supplies, utility services and other payments, and are analyzed separately.

In arriving at a forecast, the building blocks are reassembled and matched against projections of what the city's tax programs will yield in revenue.

That is what produced the forecast of a budget gap of $333 million by 1983.

In the budget gap, the forecast showed that expected higher salaries for city workers is by far the biggest item. Between 1979 and 1983, without new municipal spending programs beyond those in the proposed city budget for next year, the city operating budget is expected to increase by nearly $500 million, to a total of $1.8 billion.

Pay increases would be responsible for $265 million of the $500 million increase.

Other major increases are caused by inflation, the cost of operating new public facilities (for example, the expanded Blue Plains treatment plant), the upward cost trend of special programs (such as public welfare and medical assistance) and these expense of paying off the city's debt to the U.S. Treasury (from borrowing for construction programs, including Metro).

Although the forecasting system was launched in 1969, it has existed in its present form since 1975. For the serious student of municipal finance, its most notable product is a gray-bound paperback document called the "Multi-Year Financial Plan - Fiscal Years 1979-1983," which catalogues the city's problems in text and graphs.

Two persons are chiefly responsible for the forecasts, Thomas J. O'Brien and Jean Sperling. They work under Thomas P. Hoey, director of the resource management improvement division of the budget office.

Coppie, in summarizing how the forecasting program is used, said local governments have passed the point where they can commit their taxpayers to costly, long-lasting programs.

To this end, he said, "Systematic expenditure forecasting has become an indispensable part of our effort to improve program and fiscal management."