2.5.4: The Hydrologic Cycle
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The hydrologic cycle refers to the movement of water through its various stores within the Earth system. The amount of water that cycles between the surface and the atmosphere is phenomenal. At any minute, nearly a billion tons of water is delivered to the atmosphere by evaporation and the same amount precipitated from it. The hydrologic cycle not only traces the movement of water through the Earth system, it is a path way for the movement of energy. Water is evaporated from tropical oceans where energy is abundant and is transported on the wind to high latitudes where energy is in short supply. There it condenses and gives off heat to the atmosphere. The exchange of energy from low latitudes to high latitudes helps maintain the energy balance of the Earth system.
The cycle starts with evaporation from the surface, most of which comes from the tropical oceans. The water vapor later condenses into clouds in which precipitation forms. Water falling as precipitation may be intercepted by vegetation or fall directly onto the surface. Water intercepted by plants may ultimately fall to the ground and seep into it. Likewise, water falling directly on the surface may seep into the subsurface or runoff to nearby streams. Water seeping into the ground may become soil water or groundwater. Water in the soil may be taken up by plants then transpired to the air. Groundwater may seep into streams or return to the ocean along along a coast. Water found in streams may also empty into the ocean. A detailed discussion of the hydrologic cycle is found in Chapter 10: "The Hydrosphere".