If the grass is still growing, mow higher to encourage the roots to go deeper. The time between waterings needs to be stretched to the maximum. Make sure to water efficiently, wasting no water to runoff, and water thoroughly. During severe water shortages, let the lawn go brown. Many lawns simply go dormant during times of low rainfall. A healthy lawn of a perennial grass, though completely brown, can survive months of no water and recover quickly once rains return.
Learn more about drought for lawns.