Mayor Edward Koch declared a drought emergency Wednesday as the city's reservoirs reached dangerously low levels, threatening to dry up the region's water lifeline unless strict conservation measures are taken.

The guidelines could lead to even dirtier sidewalks in New York and brown lawns in the city's five boroughs, which have long been untouched by water shortages.City officials said they hoped the mandatory restrictions - coupled with voluntary water conservation measures - would cut the city's daily water consumption of 1.5 billion gallons by 20 percent.

The Department of Environmental Protection urged residents to take shorter showers, install low-flow water devices and not run water while shaving or brushing teeth.

"New Yorkers must use as little water as possible in their daily lives," the mayor told a news conference at City Hall. "If the lawn at Gracie Mansion has to turn brown, then so be it," the mayor added, referring to his official residence.

Officials predicted the mandatory restrictions, which took effect Wednesday, would last until at least the end of the summer.

The regulations ban the watering of lawns and washing of cars with a hose.

Filling swimming pools with city water is banned, along with hosing down sidewalks, buildings and driveways. Commercial car washes must use well water or close their business until the guidelines are lifted.

Those who violate the ban on watering lawns would be fined $100.

The fines range all the way up to $500 for opening a fire hydrant and could double for repeat offenders.

The Department of Environmental Protection plans to spend $18 million over the next nine months, the bulk of which will be used to hire some 250 "drought-busters," who will patrol streets and buildings looking for violators.

City environmental officials said businesses have also been ordered to cut their water usage by 20 percent.

The city, along with much of the Hudson Valley region, is in the grip of a drought emergency that some health and conservation officials have predicted could result in the worst water shortage in history.