HONOLULU -- Board of Water Supply officials on Friday called on Windward Oahu customers to voluntarily reduce their usage by 10 percent because of a drop in rainfall this year.

Windward Oahu water wells have declined as rainfall totals are off by 50 percent so far this year, BWS officials said.

A 10 percent reduction in water use would be about 50 gallons a day for a typical family of four, BWS officials said.

Water officials want Windward customers, including businesses and government agencies, to restrict irrigation and landscape watering to Sundays, Tuesdays and Thursdays, with no watering between 9 a.m. and 5 p.m.

The new guidelines take effect on Sunday and will continue until further advised.

Windward water sources like the Waihee Tunnel in Kahaluu are running about 50 percent lower than normal. So the water board is asking windward water customers to reduce their water use by 10 percent.

"This is a necessary and proactive step on our part this time, to avoid having to go to more stringent measures in the future," BWS manager and chief engineer Cliff Lum said.

More than half of the windward side's water comes from dikes that are closer to the surface and have smaller storage capacity that other Oahu wells, making them more susceptible to drought conditions.

BWS also asked that people hold off on installing new lawns and car washing.

The National Weather Service said rainy conditions, which usually begin in October, will not return until the end of the year.

"Not quite as much as we would normally expect and also not enough to make up that deficit, that rainfall deficit that we've been building since January actually, for Windward Oahu," said Kevin Kodama of the NWS.