New York Mayor Edward I. Koch declared a drought emergency and announced tough conservation measures, including a ban on watering lawns. The city also banned all ornamental uses of water, such as fountains, waterfalls and reflecting pools, even if they recirculate the water. “New York won’t be as green as usual this year,” Koch said. “Tough times require tough measures, and the drought emergency will be a very tough time.” The reservoirs that supply New York City were at 55% of capacity Wednesday. Normally, they are 91.4% full at this time of year. The city and much of the Hudson Valley region are in the grip of a drought that some officials have predicted could result in the worst water shortage in history. Other restrictions require all businesses to reduce their water use by 20% and ban car-washing and the filling of swimming pools.