For many years Fairfax County has had a prosperous economy seemingly immune to national economic downturns. Well, times have changed. Fairfax County is still prosperous, and its economic base is still strong, but national economic conditions are starting to catch up with it. Now is the time to plan for the future with some fiscal caution.

During the past few years we have seen four trends that are having an impact on the level of growth of Fairfax County tax revenues and spending.

The first is the downturn in federal financial aid to local jurisdictions. The Reagan Revolution essentially shifted spending responsibility in many areas -- especially human services -- to the local level. Now with renewed interest in tackling the federal deficit, there is no indication that a counter-revolution is imminent. In fact, we're projecting no increase in federal revenues in the years ahead, although costs are rising, and we could well see cutbacks in federal financing.

The second trend is the shift in state spending priorities. An increasing share of the state funds that previously went to affluent counties such as Fairfax are now being directed to less affluent areas of the state. And at the same time, tax revenues at the state level have declined. Gov. Wilder has already initiated spending cuts in state programs to compensate for this decline, and it is probable that the Fairfax share of state spending will be cut.

The third trend is the cooling off of the real estate market. Sales and prices of homes are down almost everywhere in the nation, and we see the effect here in Fairfax County as well. This downturn is already being reflected in property tax assessments -- assessment increases for homes will average 2 to 4 percent next year rather than the double-digit increases of recent years. All this is good news for homeowners but bad news for a county that has traditionally financed new programs with the revenues from new home sales.

The last trend is the downturn in business growth in the county caused in part by cutbacks in federal defense spending and in part by the regulatory fallout from the federal savings and loan crisis. As office vacancy rates increase, and as banks and S&Ls become more cautious in financing new commercial development, the tax revenues we can expect from business will continue to fall.

It is, of course, impossible to predict the exact impact these four trends will have on county revenues in the years ahead. But if these trends are ignored, and the county continues to increase its spending at the same rate as previous years, we could be facing a revenue shortfall of at least $65 million in fiscal year 1992.

I've suggested that the proper response to these trends is to exercise a little fiscal caution now at the county level, just as Gov. Wilder has done at the state level. The projected decline in revenues gives us an excellent opportunity to streamline and prioritize county spending without raising taxes.

A prudent approach would be to build a reserve of funds by deferring nonessential spending. By trimming county and school spending this year, we can create a special reserve that can be used when needed to cushion any revenue shortfall. The remaining gap between revenue and expenditures can be closed by holding increases in school and county spending for the 1992 fiscal year to 4 percent.

If I am right in my calculations, and we are faced with a serious deficit in 1992, then a budget reserve and tight spending guidelines such as I have proposed would go far toward reducing any revenue shortfall without raising taxes or cutting into essential services. If the economic future is not as cloudy as I predict, the reserve could then be used to restore spending to important programs such as schools, roads and human services, or to reduce taxes.

Either way, the citizens of Fairfax County will benefit tomorrow from a little fiscal caution today.

-- Audrey Moore is chairman of the Fairfax County Board of Supervisors.