Home > Encyclopedia of Science > Hydrologic Cycle

• Earth's water reservoirs and the water cycle
• An inconsistent cycle

Hydrologic cycle is the phrase used to describe the continuous circulation of water as it falls from the atmosphere to Earth's surface in the form of precipitation, circulates over and through Earth's surface, then evaporates back to the atmosphere in the form of water vapor to begin the cycle again. The scientific field concerned with the hydrologic cycle, the physical and chemical properties of bodies of water, and the interaction between the waters and other parts of the environment is known as hydrology.

The total amount of water contained in the planet's oceans, lakes, rivers, ice caps, groundwater, and atmosphere is a fixed, global quantity. This amount is about 500 quintillion gallons (1,900 quintillion liters). Scientists believe this total...