Wimpy rainfall is prompting the Board of Water Supply to ask Windward residents to cut back their water use by 50 gallons a day for an average family of four — about 10 percent — starting Sunday until further notice.

More than half of the Windward area's — Makapu'u to Hau'ula — water comes from dike sources, so communities are more affected by the light rainfall, said Su Shin, Board of Water Supply spokeswoman. Dike sources are found closer to the surface, have much smaller compartments in which to hold water, and refill more slowly than aquifers, making them much more susceptible to drought, a BWS news release said.

On the Windward side, most gages show less than 50 percent of normal amounts of rainfall this year over last, according to the National Weather Service.

If the conservation efforts do not work, and there is no additional rain, the Board of Water Supply will consider imposing mandatory conservation measures.

The 50 gallons a day savings applies to all water users, who are being asked to water their lawns on Sundays, Tuesdays and Thursdays after 5 p.m. and before 9 a.m.

By cutting back overall demand the BWS can build up its storage supplies and prevent over pumping, which could damage fresh water aquifers.

The last time O'ahu experienced similar conditions was in 2003 when an island-wide voluntary conservation was put into effect after a prolonged period of limited rainfall, Shin said.

Homeowners also are being asked not to install new lawns, which require constant irrigation, and to limit car washing during this period.

In addition the Marine Corps Base Hawaii at Kane'ohe Bay will restrict landscape watering to two days a week. The city Department of Parks and Recreation also will not water city parks during the heat of the day.