We can see water raining down from the sky, moving from the atmosphere to Earth’s surface, and we can see water in a river flowing toward the ocean. However, even though water is constantly moving from one place to another, the total amount of water for our blue planet is nearly constant. That means all the water we will ever have on the planet is here now. Water on Earth is a closed system.

We can think of water on Earth as a cycling system in which all parts are related: what happens in one part affects the rest of the system. Water cycles through a variety of reservoirs which hold water in various states. The three states of water are solid, liquid, and gas and it can be held at the Earth’s surface, in the atmosphere, or in the ground.

Figure 1 shows the major reservoirs of water and the processes acting to move, or transport water from one reservoir to another. The six major reservoirs of water in the Earth System are the oceans, ice, groundwater, lakes, running water (rivers and streams), and the atmosphere. A much smaller amount of water is also held in the biosphere (plants and animals). Water continually moves from one state to another and from one reservoir to another through various processes such as evaporation, precipitation, infiltration, melting, runoff, and transpiration.

For water to move through the different reservoirs of the hydrological cycle and to change states it must move up into the atmosphere through evaporation and transpiration (by plants) and down the river to the ocean. The energy for all these changes of state and movement is supplied by the sun and gravity. The sun supplies the heat to melt snow and evaporate water. Heat from the sun also creates temperature differences on our planet that drive the winds, a fundamental component of oceanic and atmospheric circulation for our weather and climate. Gravity pulls water out of the atmosphere as rain (precipitation), downhill toward the ocean (runoff), and into the ground (infiltration).

Major Hydrologic Reservoirs and Processes