Drought is simply a long period of dry weather where less than normal or no precipitation falls.
During dry summer months, trees drop their leaves. Every living thing needs water to survive. Many trees and shrubs respond to drought conditions through an early dormancy. To do this trees and shrubs drop their leaves earlier in the season than normal. Leaf drop is a trees survival tactic to reduce the amount of water that otherwise might have been lost through the leaf surface by a process known as transpiration.