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Chemistry LibreTexts

8.1: Water Cycle

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  • Learning Objectives

    • Recall the percentages of fresh and salt water sources on Earth.
    • Know the processes and phases of the water (hydrologic) cycle.
    • Understand hydrologic terms:  stream, pond, river, reservoir, wetland, aquifer, and ocean.
    • Recall the large consumers of water in the United States and in the world.

    Water is composed one oxygen atom bonded with two hydrogen atoms (H2O). Despite its simplicity, water has remarkable properties. It expands when it freezes, has high surface tension (because of the polar nature of the molecules that stick together), and has a high boiling point for a liquid. Without water, life might not be able to exist on Earth, and this planet would not have the tremendous complexity and diversity that we see.

    Figure \(\PageIndex{1}\): This Florentine Bell Tower is one of Furman University most beloved landmarks. The original Furman Bell Tower was built in 1854 on what used to be the men's campus in Downtown Greenville. During that time,the bells rang to signal the start of classes and to celebrate victories for the athletic teams. Image courtesy of Elizabeth R. Gordon.

    Distribution of Earth's Water

    Earth’s oceans contain 97% of the planet’s water, so just 3% is fresh water, water with low concentrations of salts (Figure \(\PageIndex{2}\)). The majority of the Earth's water can be classified as being saline (or salt containing). Most freshwater is trapped as ice in the vast glaciers and ice sheets of Greenland. A storage location for water such as an ocean, glacier, pond, or  atmosphere is known as a reservoir. A water molecule may pass through a reservoir very quickly or may remain for much longer. The amount of time a molecule stays in a reservoir is known as its residence time.

    Figure \(\PageIndex{2}\): The distribution of Earth’s water. In the first bar, notice how only 2.5% of Earth's water is freshwater - the amount needed for life to survive. The middle bar shows the breakdown of freshwater. Almost all of it is locked up in ice and in the ground. Only a little more than 1.2% of all freshwater is surface water, which serves most of life's needs.The right bar shows the breakdown of surface freshwater. Source: Igor Shiklomanov's chapter "World Freshwater resources" in Peter H. Gleick (editor), 1993 Water in Crises: A guide to the World's Freshwater Resources. The numbers are rounded.

    The Hydrologic Cycle

    Because Earth’s water is present in all three states, it can get into a variety of environments around the planet. The movement of water around Earth’s surface is the hydrologic (water) cycle (Figure \(\PageIndex{3}\)).

    Figure \(\PageIndex{3}\): Because it is a cycle, the water cycle has no beginning and no end. Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.

    The Sun, while many millions of kilometers away, provides the energy that drives the water cycle. It impacts the water cycle by supplying the energy needed for evaporation. Most of Earth’s water is stored in the oceans where it can remain for hundreds or thousands of years. Water changes from a liquid to a gas by evaporation to become water vapor. The Sun’s energy can evaporate water from the ocean surface or from lakes, streams, or puddles on land. Only the water molecules evaporate; the salts remain in the ocean or a freshwater reservoir.

    The water vapor remains in the atmosphere until it undergoes condensation to become tiny droplets of liquid. The droplets gather in clouds, which are blown around the globe by the wind. As the water droplets in the clouds collide and grow, they fall from the sky as precipitation(rain, sleet, hail, or snow). Precipitation can fall back into the ocean and/or land surface.

    Andy in the snow.jpg
    Figure \(\PageIndex{4}\): Snow is one form of precipitation and a key transport mechanism in the water cycle. Image courtesy of Elizabeth R. Gordon

    When water falls from the sky as rain it may enter streams and rivers that flow downward to oceans and lakes. Water that falls as snow may sit on a mountain for several months. Snow may become part of the ice in a glacier, where it may remain for hundreds or thousands of years. Snow and ice may go directly back into the air by sublimation, the process in which solid changes directly into a gas without first becoming a liquid. Although you probably have not seen water vapor sublime from a glacier, you may have seen dry ice sublime in the air.

    Snow and ice slowly melt over time to become liquid water, which provides a steady flow of fresh water to streams, rivers, and lakes below. A water droplet falling as rain could also become part of a stream or a lake. At the surface, the water may eventually evaporate and reenter the atmosphere. A significant amount of water infiltrates into the ground. Soil moisture is an important reservoir for water (Figure \(\PageIndex{5}\)). Water may seep through dirt and rock below the soil through pores infiltrating the ground to go into Earth’s groundwater system. Groundwater enters aquifers that may store fresh water for centuries. Alternatively, the water may come to the surface through springs or find its way back to the oceans.

    Water trapped in soil is important for plants to grow. Plants and animals depend on water to live and they also play a role in the water cycle. Plants take up water from the soil and release large amounts of water vapor into the air through their leaves, a process is known as transpiration.

    It is important to note that water molecules cycle around. If climate cools and glaciers and ice caps grow, there is less water for the oceans and sea level will fall. The reverse can also happen.

    Components of the Hydrologic Cycle

    Most precipitation falls in the form of rain, but there are other forms such as snow, hail, and sleet. Once it runs sufficiently, surface water runoff is generated when the ground is saturated or impervious. Surface water is a major component of the hydrological cycle and one that we interact with very regularly. It includes lakes, wetlands, storm water runoff (overland flow), ponds, potholes, rivers and streams.

    Streams and Rivers

    A river forms from water moving from a higher altitude to lower altitude, under the force of gravity. When rain falls on land, it can seep into the ground or become runoff (water running on the surface). If the water runs on the land surface, it can converge as it moves towards lower elevation. The converging runoff can concentrate into single channels of conveyance called creeks, stream, or rivers. Streams and rivers that join up to form a larger river are called tributaries, Figure \(\PageIndex{5}\). The land area drained by a river and all its tributaries is called a watershed or river basin.

    The area adjacent to a river that floods frequently is a called a flood plain. These regions temporarily store excess water during storm events and frequently contain fertile soils.  Properly functioning flood plains reduce the negative impacts of floods (by reducing severity of flood) and they assist in filtering storm water which results in protecting the water quality of rivers. Also, flood plains act as areas of recharge for groundwater.    


    In South Carolina, the Santee River system flows throughout the state (Figure \(\PageIndex{6}\)). From North Carolina, the Saluda River runs through the South Carolina upstate region. The Saluda River converges with the Broad River south of Columbia, South Carolina. The combination of these two waterways form the Congaree River. Just north of Rock Hill, South Carolina, the Catawba River enters into the Wateree Reservoir.  Once this water flows to Columbia, the Catawba River becomes the Wateree River.  All of these rivers and water systems provide water for the upstate of South Carolina.

    Figure \(\PageIndex{6}\): Large rivers of South Carolina (CC BY-SA 1.0;

    Rivers are important for supplying water to the cities and populations of any state. In addition, rivers contain important biological communities and provide recreational opportunities like swimming, fishing, and white water rafting. Large cities, communities, factories, industries, and power stations are located along many rivers. It is, therefore, very important to protect the quality and integrity of rivers all over the world.

    Lakes,reservoirs, and ponds

    If water flows to a place that is surrounded by higher land on all sides, a lake will form.  A smaller version of this type of aquatic system is called a pond.  When dams are constructed to stop rivers from flowing, lakes are transformed into reservoirs.  Once constructed, reservoirs can provide water for consumption, cooling capacity for power plants, irrigation for agriculture, and recreation activities.

    In the South Carolina upstate, Hartwell Reservoir provides cooling water for Oconee Nuclear Reactor Station.  In addition, many South Carolina residents enjoy fishing, boating, and residing on the reservoir. Figure \(\PageIndex{7}\): Hartwell Reservoir in upstate South Carolina. Image taken from:,_Georgia_and_South_Carolina._LOC_77696311.jpg



    A wetland is an area which is home to standing water for notable parts of the year, has saturated soils for a large part of the year and has plants that require large amounts of water to survive. Wetlands include swamps, marshes, and bogs. Wetlands are identified using three characteristics: soils (water-saturated soils are present), hydrology (shallow water table) and vegetation (wetland plants that are adapted to areas that are saturated with water for long periods of time). Wetlands are important areas of biological diversity and productivity. These are also important areas where geochemical and biological cycles/ processes are consistently taking place. For instance, wetlands are considered as areas of significant carbon sequestration (storage), which impacts global climate change. They also act as filters for storm-water runoff before it enters rivers and lakes.


    As you have probably already guessed, oceans are an important component of the hydrologic cycle because they store majority of all water on Earth (about 95%). Most of the major rivers drain into them. The five oceans covering the surface of the Earth are the Atlantic, Indian, Pacific, Arctic and the Southern Ocean (Figure \(\PageIndex{8}\)).

    Figure \(\PageIndex{8}\): The five oceans found on planet Earth. The Pacific Ocean is the largest. Source:

    Approximately 90 % of the water that is evaporated into the hydrologic cycle comes from the ocean.  The ocean contains many forms of life uniquely adapted to survive in this habitat. Unfortunately, humans have degraded the oceans and their life through pollution, excess fishing, carbon dioxide acidification, and resource exploitation.


    Storage and Flow

    Almost 99% of the available fresh water is found below the surface as groundwater. This resource results from recycling of water in the hydrologic cycle. When precipitation falls, water travels on the surface and some can infiltrate into the ground. Groundwater is replenished when water moves from the surface, through unsaturated rocks or sediment (unsaturated), all the way down the saturated parts (saturated zone) in a process called infiltration and becomes groundwater (Figure \(\PageIndex{10}\)). The top of the saturated portion is called the water table, which is the boundary between saturated and unsaturated zone.

    Groundwater is found in aquifers, which are bodies of rock or sediment that store (and yield) large amounts of usable water in their pores. Aquifer productivity is controlled by porosity and permeability. Porosity is the percentage of open space in a rock or sediment body. Permeability is the ability of subsurface material to transmit fluids. Groundwater is found in the saturated zone of a rock body where all pores are filled with water. An important concept is that surface water always moves from higher elevation to lower elevation while groundwater always moves from higher energy (hydraulic head) to lower energy.


    Screenshot (123).png
    Figure \(\PageIndex{10}\): Model of groundwater system showing the different components of an unconfined groundwater system:

    Groundwater will continue to flow until it emerges as a spring, or discharges into surface water bodies on the land or in the ocean. To utilize groundwater, we drill holes (wells) into the ground and pump the water out.

    Water Consumption

    One of the most important natural resources is water.  In some areas of the world, water sources are extremely limited or contaminated. If a geographical area lacks this resource, then agriculture and industry will have difficulty producing goods and foods.  If water is contaminated, individuals who reside in these areas will be more likely to develop waterborne illnesses.  Lack of water limits a consumer's ability to drink, eat, and bathe.  

    The table below illustrates who uses the majority of water sources.  Be sure to note the differences between the United States and Global percentage applications.

    Table \(\PageIndex{2}\): Water Use in the United States and Globally
    Use United States Global
    Agriculture 34% 70%
    Domestic (drinking, bathing) 12% 10%
    Industry 5% 20%
    Power plant cooling 49% small