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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 how air pollution affects water quality
    • Recall the large consumers of water in the United States and the world.

    Water merely is two atoms of hydrogen and one atom of oxygen bonded together. Despite its simplicity, water has remarkable properties. It expands when it freezes, has high surface tension (because of the polar nature of the molecules, they tend to stick together), and has a high boiling point for a liquid. Without water, life might not be able to exist on Earth, and it certainly 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 even the 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, many millions of kilometers away, provides the energy that drives the water cycle. Our nearest star directly 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. Precipitation can be rain, sleet, hail, or snow. Sometimes precipitation falls back into the ocean and sometimes it falls onto the 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, stormwater 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 the land, it either evaporates, seeps into the ground or becomes runoff (water running on the surface). When water runs on the land surface it usually converges as it moves towards lower elevation. The converging runoff can concentrate into single channels of conveyance called creeks, stream, or rivers. Usually these start as small rill and rivulets that would join up downhill into larger streams and creeks which can also join up downstream to form even bigger rivers. The 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 catchment or river basin.

    The area adjacent to a river that floods frequently is a called a floodplain. Floodplains are areas that rivers use to temporarily store excess water during storm events and frequently contain very fertile soils. This has historically encouraged humans to move into floodplains and use them for agriculture, resulting in a reduction in the capacity of the floodplain to act as temporally storage for excess water during storm events, causing increased damaging flooding downstream. Properly functioning floodplains reduce the negative impacts of floods (by reducing severity of flood), and they assist in filtering stormwater and protecting the water quality of rivers. They also act as areas of recharge for groundwater.

    Figure \(\PageIndex{5}\): River systems. (A) A river with a small tributary (B) A meandering river with a mature floodplain (C) A satellite image of river system with multiple tributaries (Source USGS)

    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 meets up 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

    Figure \(\PageIndex{6}\): Major watershed of Georgia representing the main rivers in the state. (CC BY-SA 1.0;

    These rivers are very important for supplying water to the cities and populations of the states. The rivers also contain important biological communities and provide opportunities for recreation such as swimming, fishing, and white water rafting. Rivers are so important and largely control settlement patterns all over the world. Major cities, communities, factories, industries, and power stations are located along rivers. It is, therefore, very important to protect the quality and integrity of rivers all over the world.

    Unfortunately, most of the rivers in the world are too polluted to support certain human activities, especially swimming, fishing, and drinking. Close to half of the rivers in the US have been deemed too polluted to support swimming and fishing. A lot of the rivers have also been channelized, dredged, or impounded by dams which have ruined their ability to support a lot of human and biological activities. It is estimated that over 600, 000 river miles have been dammed in the US. Benefits of dams to humans include providing a source of water (reservoirs and farms ponds), recreation waters and reducing local flooding. On the other side, dams can also have negative impacts on people and the environment. They can lead to increased severe flooding downstream of the dam, especially during high rain events.

    The impoundments can trap stream sediments resulting in reduced sediment supply downstream as well as increased deposition behind the dam. This shift in sediments flow can disrupt and damage aquatic habitats and can increase downstream stream erosion due to lack of sediment supply. The impoundments can also prevent certain aquatic organisms from migrating either upstream or downstream, therefore reducing their range and abilities to survive environmental changes as well cutting them off from spawning areas. Construction of dams can also result in displacement of the local people and loss of traditional lands and cultural history. Reservoirs and ponds usually form behind these impoundments.

    Lakes, Reservoirs and ponds

    If water flows to a place that is surrounded by higher land on all sides, a lake will form (Figure \(\PageIndex{7}\)). A lake, pond or reservoir is a body of standing water on the land surface. When people build dams to stop rivers from flowing, the lakes that form are called reservoirs. It is estimated that over 300 million water bodies in the world are lakes, reservoirs, and ponds. Most of the Earth’s lakes (about 60%) are found in Canada. Even though lakes and rivers contain less than 1% of the Earths water, the US gets over two thirds (70%) of its water (for drinking, industry, irrigation, and hydroelectric power generation) from lakes and reservoirs. Lakes are also the cornerstone of the US’s freshwater fishing industry and are the backbone of the nation’s state tourism industries and inland water recreational activities. (

    Figure \(\PageIndex{7}\): Lake Sinclair in Baldwin and Putnam counties (Photo Credits: GCSU Hydrology Research lab)


    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 very 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. Oceans are an important and large part of the hydrologic cycle, with lots biological diversity and many landforms. Did you know that the average depth of the oceans is about 3.6 km with a maximum depth that can exceed 10 kilometers in areas known as ocean trenches? The ocean is also home to many forms of life uniquely adapted to survive in this habitat. Unfortunately, humans have degraded the oceans and their life through pollution, overfishing, carbon dioxide acidification and resource exploitation. Figure \(\PageIndex{9}\) shows a couple of examples of human impacts on the ocean environment.

    Figure \(\PageIndex{9}\): Trash washed up on the beach (A) and seal tangles up and being struggled by plastic trash in the ocean (B).

    Storage and Flow

    Almost 99% of the available fresh water is found below the surface as groundwater. Groundwater is not created by some mysterious processes below ground, but is part of the recycled water in the hydrologic cycle. When precipitation falls, some of the water runs off on the surface while some infiltrates 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.

    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

    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