Charles County issued its third appeal of the summer on Friday for residents to conserve water, citing drought-induced strains on underground supplies.

The call came one day after Maryland Gov. Parris N. Glendening (D) declared a statewide drought emergency and called for voluntary water conservation while he weighs mandatory restrictions.

Washington-area rainfall has been more than one-third below normal over the past 12 months, according to the National Weather Service.

The shortfall poses the most obvious threat to localities that rely upon depleted reservoirs, but Charles County officials said it also is affecting the underground aquifers that supply the county's water.

In large part because of heavy use by people watering lawns, the aquifers have dropped about three feet--about two feet more than the normal summer drop, said Sam McDougal, a supervisor of water operations.

A wet season could reverse such a drop, as water inflows increase pressure in the aquifers and squeeze water toward the surface, said Earl Greene, a groundwater specialist with the U.S. Geological Survey.

In the meantime, Greene said, extensive pumping from stressed aquifers eventually might cause some wells to go dry, especially smaller ones near high-volume wells. Well owners often can remedy such problems, but repairs can be expensive. Such remedies could involve dropping the pump to a lower level in the well or deepening the well.

The county uses roughly 75 wells to tap aquifers for water used by about 51,000 people. The remainder of the county's 120,000 residents rely upon their own wells or upon community water systems.

The county asked residents to refrain from watering lawns or washing cars, to run only full loads when washing clothes or using a dishwasher, to fix water leaks, and not to run water while brushing teeth or shaving.