The Hydrologic Cycle

On the surface, Earth is a water planet.

Over 70% of the Earth's surface is covered in water, with an estimated volume of around 1,360,000,000 cubic kilometers. Over 97% of all water is found in oceans, leaving the other 3% in lakes, rivers, glaciers, groundwater, and atmospheric moisture.

Explore the above choices to learn about how water is moved around this planet.



Water and energy are circulated across planet Earth in what is known as the Hydrologic Cycle. This cycle is responsible for creating storms and weather, moving water through river channels, and providing land with fresh water. The cycle involves four general steps: moving water from the surface to the atmosphere in a gaseous form, moving the water vapor through the atmosphere, returning the water to the surface, and moving the water across the surface.



Evaporation is the main mechanism for moving water back into the atmosphere. When water absorbs enough energy it will change from a liquid state to a gaseous state. This is done by turning water into water vapor. The oceans are the source of about 86% of evaporation. The over 14% occurs on land. This is due to the fact that the sun's energy is captured by the vast expanses of the oceans.



Water vapor enters the atmosphere from the evaporation of water. While most of this comes from large bodies of water, some of it comes from biological organisms. Vegetation releases water vapor as a byproduct of photosynthesis. The release of water from plants is known as transpiration. This water vapor will be added to the water content of the surrounding air.



A small component of the hydrologic cycle, yet still a contributor in the polar regions. Sublimation is the process of changing water from a solid directly into water vapor. Glaciers in polar regions can experience the loss of water due to this process even when there isn't enough enery to lose water mass from evaporation.



As water vapor moves through the atmosphere it loses energy and cools down. Once the vapor has cooled down enough it will be able to return to a liquid state. Particles of dust in the air act as condensation nuclei, places where molecules of water vapor can collect and turn back into liquid. Clouds are pockets of condensed water. When enough water has condensed, precipitation can occur.



The transfer of water from the atmosphere back down to the surface of the Earth. Precipitation occurs when the atmosphere can no longer hold the condensed water. There are various reasons why the atmosphere can no longer hold any more water, from the air becoming saturated to a drop in the air temperature. When precipitation reaches the surface it can be intercepted by vegetation, infiltrate down into the ground, or run off of the surface.



The movement of water across the surface of the Earth. There are two ways water can move across the land: sheet flow and channel flow. Sheet flow, or overland flow, is the movement of water across the entire surface of the land. This occurs during storms when the ground can no longer soak up any more water. Channel flow is the movement of water through a defined channel. Rivers and streams are types of channel flow.



The water that infiltrates down into the ground will still flow back towards the oceans, albeit it will be slower then runoff. Groundwater movement rates are determined by the type of material (soil, rock) that the water is flowing through. Soil also dictates how much water can be held, and how much can be readily used by plants or even animals. Groundwater maintains base stream flow levels when runoff is not available to supply the river with water.



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