THE CITY of Washington is running a budget deficit of at least $90 million for the year ending next September. One way or another, that gap is going to have to be closed over the next seven months.

The $90 million figure is Mayor Barry's. Other reckonings put it even higher. A deficit on that scale is not yet insoluble in a city budget of $1.5 billion. But it is getting worse. Action is imperative to avoid the kind of crisis into which other large American cities have fallen over the past five years.

The immediate necessity is to meet that current shortfall of $90 million or more. Congress can fairly be asked to appropriate a federal payment that comes a little closer to the amount that Congress itself authorized. But it is mainly the city's job to reach a balance through higher taxes or lower spending -- or both. For this immediate requirement, we propose four steps:

1. An absolute freeze on all new hiring, with reduction of city payrolls through attrition, is the obvious first step.Mayor Barry already has it under discussion. But that is only a stopgap.

2. The city's gasoline tax is now 10 cents. It ought to be 20 percent -- which would raise it to 24 cents at once and continue to take it up as the price rises.

3. The school board has a duty to close its unneeded school buildings -- there are 18 on the superintendent's list -- and begin to sell some of those buildings. It's a slow and painful process, but now is the time to get going.

4.If these measures do not suffice, the next recourse is to consider a gross-receipts tax on businesses.

If the city does not get a better grip on its finances now, larger and more dangerous deficits lie ahead. Here we offer three more proposals to provide voters and taxpayers, Congress and the city government alike timely warning of trouble in budgets to come.

5. The District Buildings' budget process needs to be sharpened and opened up to earlier public discussion. The present shortfall was known to some city officials in September, but became public only last month. Quarterly reviews are desirable. Zero-based budgeting is essential. The present forecasting is inadequate, and the federal grants -- the city's largest source of income -- need to be integrated into the estimates.

6. Underfunded pensions have become a fiscal trap for cities all over the country. Congress has left Washington's short of full funding. Mayor Barry might usefully convene an independent commission on city pensions to review the system's integrity.

7. The city government is overstaffed. The mayor needs a firm plan to bring the city's payrolls down to levels more nearly comparable with public services elsewhere.

All of these steps are difficult, and most are unpleasant. Nobody likes to close schools, or trim payrolls, or raise taxes. But keep in mind this city's history. In the bad old days, Congress ruled Washington directly. It was not good government, but it was Congress' province, and Congress met the deficits. Thourh all those years the people of Washington pressed for the right of self-government. Now we have that right -- and the responsibilities that go with it. One of them is the responsibility to balance the budget.