THERE WILL BE A LOT of parched lawns and dirty cars in Northern Virginia unless the rains come soon. Despite some voluntary water-saving, the Occoquan Reservior's level is still going down. Mandatory curbs on water use could be imposed within the next several days - and lawn-sprinkling and carwashing are likely to be among the first activities banned or sharply limited. There could be worse to come: If the autumn rainfall is sparse, Fairfax and Prince William counties and Alexandria will have to crack down on water-using businesses in order to husband the dwindling Occoquan supply.

Those prospects should be sobering to anyone who is not yet convinced that the problem is serious. It is - and it is likely to persist. Even with ample rain, the current Occoquan shortage may not be overcome for weeks. Next summer the problem could easily recur. Buying water from other jurisdictions helps somewhat, but permanent water-supply improvements are still years away.

There are, however, several things that could be done right to ease the strain. First, Alexandria and Prince William should adopt procedures like those in Fairfax County to ensure that water restrictions can be imposed quickly when required.

Second, the three jurisdictions should commit themselves to acting in concert when curbs are required, so that water-conservation rules and burdens will be consistent throughout the area.

Third, it should be impressed on the public that mandatory restrictions on some activities do not end the need for voluntary conservation overall.To the contrary, the more people can cut down non-essential water consumption in their homes and working places, the less likely harsh curbs on anyone will be.

Conservation just starts with elementary steps such as fixing leaky faucets and installing water-saving devices in showerheads and toilets tanks. Out in drought-stricken California, people have gotten quite accustomed to putting used dishwater on gardens and otherwise wringing maximum benefit from every drop. While calling for conservation, suburban Virginia officials so far have done less in the way of specific public education than their counterparts in Maryland. It's about time for a much greater effort. No matter what water-supply strategies are adopted for the long run, right now the real question in th Occoquan basin is whether to save every possible drop, or gamble even more heavily on the rain.