Water, air, and food are people’s most important natural resources. Humans can live only a few minutes without oxygen, less than a week without water, and about a month without food. Water also is essential for our oxygen and food supply. Plants break down water and use it to create oxygen during photosynthesis.

Water is the most essential compound for all living things. Human babies are approximately 75% water, and adults are 60% water. Our brain is about 85% water, blood, kidneys are 83% water, muscles are 76% water, and even bones are 22% water. We constantly lose water by perspiration; in temperate climates, we should drink about 2 quarts of water per day, and people in hot desert climates should drink up to 10 quarts of water per day. Loss of 15% of body water usually causes death.

Earth is truly the Water Planet. The abundance of liquid water on Earth’s surface distinguishes us from other bodies in the solar system. About 70% of Earth’s surface is covered by oceans, and approximately half of Earth’s surface is obscured by clouds (also made of water) at any time. There is a very large volume of water on our planet, about 1.4 billion cubic kilometers (km3) (330 million cubic miles) or about 53 billion gallons per person on Earth. All of Earth’s water could cover the United States to a depth of 145 km (90 mi). From a human perspective, the problem is that over 97% of it is seawater, which is too salty to drink or use for irrigation. The most commonly used water sources are rivers and lakes, which contain less than 0.01% of the world’s water!

One of the most important environmental goals is to provide everyone with clean water. Fortunately, water is a renewable resource and is difficult to destroy. Evaporation and precipitation combine to replenish our fresh water supply constantly; however, water availability is complicated by its uneven distribution over the Earth. Arid climates and densely populated areas have combined in many parts of the world to create water shortages, which are projected to worsen in the coming years due to population growth and climate change. Human activities such as water overuse and pollution have significantly compounded the current water crisis. Hundreds of millions lack access to safe drinking water, and billions lack access to improved sanitation as simple as a pit latrine. As a result, nearly two million people die yearly from diarrheal diseases, and 90% of those deaths occur among children under the age of 5. Most of these are easily prevented deaths.

Water Reservoirs and Water Cycle

Water is the only common substance that occurs naturally on Earth in three forms: solid, liquid, and gas. It is distributed in various locations, called water reservoirs. The oceans are by far the largest reservoirs, with about 97% of all water, but that water is too saline for most human uses (Figure 1). Ice caps and glaciers are the largest reservoirs of fresh water, but this water is inconveniently located mostly in Antarctica and Greenland. Shallow groundwater is the largest reservoir of usable fresh water. Although rivers and lakes are the most heavily used water resources, they represent only a tiny amount of the world’s water. If all of the world’s water was shrunk to the size of 1 gallon, then the total amount of freshwater would be about 1/3 cup, and the amount of readily usable fresh water would be two tablespoons.

Bar chart distribution of Earth’s water including total global water, freshwater, surface water, and other fresh water and pie chart of water usable by humans and sources of usable water
Figure 1. Earth’s Water Reservoirs. Bar chart distribution of Earth’s water including total global water, freshwater, surface water, and other fresh water and pie chart of water usable by humans and sources of usable water. Source: United States Geographical Survey Igor Skiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources

The water (or hydrologic) cycle (covered in Chapter 3.2) shows the movement of water through different reservoirs, including oceans, the atmosphere, glaciers, groundwater, lakes, rivers, and the biosphere. Solar energy and gravity drive the motion of water in the water cycle. Simply put, the water cycle involves water moving from oceans, rivers, and lakes to the atmosphere by evaporation, forming clouds. From clouds, it falls as precipitation (rain and snow) on both water and land. The water on land can either return to the ocean by surface runoff, rivers, glaciers, and subsurface groundwater flow or return to the atmosphere by evaporation or transpiration (water loss by plants to the atmosphere).

The Water Cycle
Figure 2.. Arrows depict the movement of water to different reservoirs located above, at, and below the Earth’s surface. Source: United States Geological Survey

An important part of the water cycle is how water varies in salinity, which is the abundance of dissolved ions in water. The saltwater in the oceans is highly saline, with about 35,000 mg of dissolved ions per liter of seawater. Evaporation (where water changes from liquid to gas at ambient temperatures) is a distillation process that produces nearly pure water with almost no dissolved ions. Water vaporizes and leaves the dissolved ions in the original liquid phase. Eventually, condensation  (where water changes from gas to liquid) forms clouds and sometimes precipitation (rain and snow). After rainwater falls onto land, it dissolves minerals in rock and soil, which increases its salinity. Most lakes, rivers, and near-surface groundwater are called freshwater and have a relatively low salinity. The next sections discuss important parts of the water cycle relative to freshwater resources.

Primary Fresh Water Resources: Precipitation

Precipitation levels are unevenly distributed around the globe, affecting freshwater availability (Figure 3). More precipitation falls near the equator, whereas less precipitation tends to fall near 30 degrees north and south latitude, where the world’s largest deserts are located. These rainfall and climate patterns are related to global wind circulation cells. The intense sunlight at the equator heats the air, causing it to rise and cool, which decreases the ability of the air mass to hold water vapor and results in frequent rainstorms. Around 30 degrees north and south latitude, descending air conditions produce warmer air, which increases its ability to hold water vapor and results in dry conditions. The dry air conditions and the warm temperatures of these latitude belts favor evaporation. Global precipitation and climate patterns are also affected by the size of continents, major ocean currents, and mountains.

World Rainfall Map
Figure 3. World Rainfall Map. The false-color map above shows the amount of rain that falls worldwide. High rainfall areas include Central and South America, western Africa, and Southeast Asia. Since these areas receive so much rainfall, they are where most of the world’s rainforests grow. Areas with very little rainfall usually turn into deserts. The desert areas include North Africa, the Middle East, western North America, and Central Asia. Source: United States Geological Survey Earth Forum, Houston Museum Natural Science

Surface Water Resources: Rivers, Lakes, Glaciers

Surface Runoff
Figure 4. Surface runoff is part of overland flow in the water cycle Source: James M. Pease at Wikimedia Commons

Flowing water from rain and melted snow on land enters river channels by surface runoff (Figure 4) and groundwater seepage (Figure 5). River discharge describes the volume of water moving through a river channel over time (Figure 6). The relative contributions of surface runoff vs. groundwater seepage to river discharge depend on precipitation patterns, vegetation, topography, land use, and soil characteristics. Soon after a heavy rainstorm, river discharge increases due to surface runoff. The steady normal flow of river water is mainly from groundwater that discharges into the river. Gravity pulls river water downhill toward the ocean. Along the way, the moving water of a river can erode soil particles and dissolve minerals. Groundwater also contributes a large amount of the dissolved minerals in river water. The geographic area drained by a river and its tributaries is called a drainage basin or watershed. The Mississippi River drainage basin includes approximately 40% of the U.S., a measure that includes the smaller drainage basins, such as the Ohio River and Missouri River, that help to comprise it. Rivers are an important water resource for cropland irrigation and drinking water for many cities worldwide. Rivers that have had international disputes over water supply include Colorado (Mexico, southwest U.S.), Nile (Egypt, Ethiopia, Sudan), Euphrates (Iraq, Syria, Turkey), Ganges (Bangladesh, India), and Jordan (Israel, Jordan, Syria).

Groundwater Seepage
Figure 5. Groundwater Seepage. Groundwater seepage can be seen in Box Canyon in Idaho, where approximately ten cubic meters per second of seepage emanates from its vertical headwall. Source: NASA

In addition to rivers, lakes can also be an excellent source of fresh water for human use. They usually receive water from surface runoff and groundwater discharge. They tend to be short-lived on a geological time scale because they constantly fill in with sediment supplied by rivers. Lakes form in various ways, including glaciation, recent tectonic uplift (e.g., Lake Tanganyika, Africa), and volcanic eruptions (e.g., Crater Lake, Oregon). People also create artificial lakes (reservoirs) by damming rivers. Large changes in climate can result in major changes in a lake’s size. As Earth was coming out of the last Ice Age about 15,000 years ago, the climate in the western U.S. changed from cool and moist to warm and arid, which caused more than 100 large lakes to disappear. The Great Salt Lake in Utah is a remnant of a much larger lake called Lake Bonneville.

River Discharge Colorado River
Figure 6. River Discharge Colorado River, U.S. Rivers are part of overland flow in the water cycle and an important surface water resource. Source: Gonzo fan2007 at Wikimedia Commons.

Although glaciers represent the largest reservoir of fresh water, they generally are not used as a water source because they are located too far from most people (Figure 7). Melting glaciers do provide a natural source of river water and groundwater. During the last Ice Age, there was as much as 50% more water in glaciers than there is today, which caused sea level to be about 100 m lower. Over the past century, the sea level has risen partly due to melting glaciers. If Earth’s climate continues to warm, the melting glaciers will cause an additional rise in sea level.

Mountain Glacier in Argentina
Figure 7. Mountain Glacier in Argentina Glaciers are the largest reservoir of fresh water but are not used much as a water resource directly by society because of their distance from most people. Source: Luca Galuzzi – www.galuzzi.it

Groundwater Resources

Although most people worldwide use surface water, groundwater is a much larger reservoir of usable fresh water, containing more than 30 times more water than rivers and lakes combined. Groundwater is a particularly important resource in arid climates, where surface water may be scarce. In addition, groundwater is the primary water source for rural homeowners, providing 98% of that water demand in the U.S. Groundwater is water located in small spaces, called pore space, between mineral grains and fractures in subsurface earth materials (rock or sediment). Most groundwater originates from rain or snowmelt, infiltrating the ground and moving down until it reaches the saturated zone (where groundwater completely fills pore spaces in earth materials).

Other groundwater sources include seepage from surface water (lakes, rivers, reservoirs, and swamps), surface water deliberately pumped into the ground, irrigation, and underground wastewater treatment systems (septic tanks). Recharge areas are locations where surface water infiltrates the ground rather than running into rivers or evaporating. Wetlands, for example, are excellent recharge areas.  An aquifer is a large area of sub-surface, porous rock that holds water. Aquifers are commonly drilled, and wells are installed to provide water for agriculture and personal use. Examples of aquifers are earth materials with abundant, large, well-connected pore spaces such as sand, gravel, uncemented sandstone, and any highly fractured rock.

Water Use in the U.S. and World

People need water, oftentimes in large quantities, to produce the food, energy, and mineral resources they use.  Consider, for example, these approximate water requirements for some things people in the developed world use every day: one tomato = 3 gallons; one kilowatt-hour of electricity from a thermoelectric power plant = 21 gallons; one loaf of bread = 150 gallons; one pound of beef = 1,600 gallons; and one ton of steel = 63,000 gallons. Human beings require only about 1 gallon per day to survive. Still, a typical person in a U.S. household uses approximately 100 gallons per day, which includes cooking, washing dishes and clothes, flushing the toilet, and bathing.  The water demand of an area is a function of the population and other uses of water.

Trends in total water withdrawals in the U.S. from 1950 to 2005 by water use category, including bars for thermoelectric power, irrigation, public water supply, and rural domestic and livestock. The thin blue line represents total water withdrawals using a vertical scale on the right. Source: United States Geological Survey
Figure 8. Trends in Total Water Withdrawals by Water-use Category, 1950-2005 Trends in total water withdrawals in the U.S. from 1950 to 2005 by water use category, including bars for thermoelectric power, irrigation, public water supply, and rural domestic and livestock. The thin blue line represents total water withdrawals using a vertical scale on the right. Source: United States Geological Survey

Figure 9. Trends in Source of Fresh Water Withdrawals in the U.S. from 1950 to 2005 Trends in the source of freshwater withdrawals in the U.S. from 1950 to 2005, including bars for surface water, groundwater, and total water. The red line gives the U.S. population using the vertical scale on the right. Source: United States Geological Survey

Global total water use is steadily increasing at a rate greater than world population growth (Figure 10). During the 20th century global population tripled, and water demand grew by a factor of six. The increase in global water demand beyond the population growth rate is due to an improved standard of living without an offset by water conservation. Increased production of goods and energy entails a large increase in water demand. The major global water uses are irrigation (68%), public supply (21%), and industry (11%).

Trends in World Water Use from 1900 to 2000 and Projected to 2025 For each water major use category, including agriculture, domestic use, and industry trends. Darker colored bar represents total water extracted for that use category, and the lighter colored bar represents water consumed
Figure 10. Trends in World Water Use from 1900 to 2000 and Projected to 2025 For each water major use category, including agriculture, domestic use, and industry trends. The darker-colored bar represents the total water extracted for that use category, and the lighter-colored bar represents water consumed (i.e., water not quickly returned to surface water or groundwater system) for that use category. Source: Igor A. Shiklomanow, State Hydrological Institute (SHI, St. Petersburg) and United Nations Educational, Scientific and Cultural Organisation (UNESCO, Paris), 1999


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Introduction to Environmental Sciences and Sustainability Copyright © 2023 by Emily P. Harris is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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