The destruction of wetlands not only endangers wildlife, but also hinders flood control. The amount of water in the air may be described by absolute or relative humidity . Absolute humidity refers to the total amount of water in a volume of air and is usually expressed in grams of water per cubic meter of air. Warmer air can hold more water than cooler air. In comparison, relative humidity is the percentage of water in the air compared to the total amount of water that the air could hold if it was saturated with water.
Human activities have a profound impact on the hydrologic cycle.
For example, the irrigation of crops sends large amounts of water vapor into the atmosphere. Pavements on roads, parking lots and in other urban areas prevent infiltration and increase the amount of runoff. In highly populated and dry areas, such as southern California and Arizona, streams have been diverted to provide needed water for Phoenix and Los Angeles. The average amount of time that a water molecule remains in a cloud, ocean, or another reservoir is called its residence time. As shown in the table, a typical water molecule that evaporates and enters the Earth's atmosphere will stay there for 9 days before precipitating and moving into another reservoir, such as a lake, ocean or the subsurface.