LAST SUMMER, WHILE THE Occoquan Reservoir's level was dropping, the Fairfax County Water Authority bought only half as much Potomac water as it could have from Falls Church. That fact, brought out by Rep. Herbert E. Harris II (D-Va.) recently, illustrates how the management of this region's water supplies needs to be changed.

The central problem is one of perspective. As Rep. Harris recent hearing showed, officials in the Occoquan basin worried all summer about their reservoir's dwindling supply. Not until August, though, did they really pursue the possibilities for buying water from Manassas and Falls Church. And apparently no one in those cities volunteered that they had some surpluse to sell. According to Rep. Harris, if fullscale cooperation had started in June instead of September, the reservior might have gained two billion gallons - and the curbs on water use in Fairfax and Prince William counties and Alexandria might not have been required.

Rep. Harris may be pushing his argument too far by suggesting that his constituents "saw their lawns turn brown" unnecessarily last fall. Even with earlier, purchases, some conservation would probably have been desirable before November's rains refilled the reservior.

The basic point, though, is exactly right: The area's water systems should be tied together more effectively. Every jurisdiction should be able to get more help from its neighbors in a drought or emergency. The Corps of Engineers now acknowledges this. In testimony excerpted. For the Record on this page today, a Corps spokesman told Rep. Harris's hearing that if a toxic spill got into Washington's reservoirs, the Corps "would have a problem" and suburban connections would help. Moreover, as engineer Daniel Sheer has outlined, the area's three big water systems, if run as a regional network, might meet everybody's needs for several decades - without tapping the estuary or building more reservoirs.

Perhaps the message is getting through. Lots of studies are going on. Now that they have agreed on how to deal with future Potomac shortages, the region's officials should see how much they can do to minimize shortages anywhere. Rep. Harris is trying to encourage more sharping of technical data - and water - in Northern Virginia. As last summer's experience shows, that's a good place to start.