Water is one of the most ubiquitous, yet scarce, resources on Earth.

Even though Earth is covered in more than 1.4 billion cubic kilometers (370,000,000 trillion gallons) of water—about 75% of its surface—only about 1/100 of 1% of that water is usable by humans.

Water provides many important ecosystem services that animals and plants require to live. Up to 75% of the human body, for instance, consists of water. But humans need liquid freshwater (which has few dissolved ions such as salt); ocean water is too salty for human consumption and is toxic in large doses.

freshwater

Water that has few dissolved ions such as salt.

Complicating things, nearly 80% of the freshwater on the planet is trapped in ice caps at the poles and glaciers around the world, which contain more than 35 million cubic kilometers (9,000,000 trillion gallons)—enough to fill 140 billion Olympic-sized swimming pools. INFOGRAPHIC 14.1

DISTRIBUTION OF WATER ON EARTH

Most of the water on Earth is found in the oceans, and most of the freshwater is tied up in ice and snow. Only about 0.001% of all of Earth’s water is available for us to use, but with more than 1,300 trillion liters (350 trillion gallons) of water on the planet, that is still a lot of water.

Based on this diagram, what percentage of the total water supply on Earth is found in groundwater? What percentage is found in rivers?

Freshwater makes up 3% of all water and 12% of that is groundwater, therefore 0.03 x 0.12 = 0.0036 or 0.36% of all water is groundwater. Surface water is only 1% of all freshwater (0.03 x 0.01 = 0.0003 or 0.03%). Rivers are 1% of that so 0.0003 x 0.01 = 0.000003 or 0.0003%.

WHERE IS ANAHEIM, CALIFORNIA?

Wherever there is water, it is constantly moving through the environment via the water cycle (hydrologic cycle). Heat from the Sun causes water to evaporate from surface waters (rivers, lakes, oceans) and land surfaces. At the same time, plant roots pull up water from the soil and then release some into the atmosphere in a process called transpiration. Plants with deep roots, like trees, may bring up thousands of gallons of water a year, releasing much of this to the atmosphere. Altogether, the combination of evaporation and transpiration—evapotranspiration—sends more than 66,000 cubic kilometers of water vapor into the atmosphere every year, equivalent to 17,000 trillion gallons. Once aloft, that water condenses into clouds (condensation) and may fall back to Earth as precipitation (rain, snow, sleet, etc.). INFOGRAPHIC 14.2

water cycle

The movement of water through various water compartments such as surface waters, atmosphere, soil, and living organisms.

surface water

Any body of water found above ground, such as oceans, rivers, and lakes.

transpiration

The loss of water vapor from plants.

evaporation

The conversion of water from a liquid state to a gaseous state.

condensation

The conversion of water from a gaseous state (water vapor) to a liquid state.

precipitation

Rain, snow, sleet, or any other form of water falling from the atmosphere.

THE WATER CYCLE

Water cycles between liquid and gaseous forms as it moves through space and time. Ocean water (which we cannot use) is converted to freshwater when it evaporates and falls back to Earth as precipitation, refilling freshwater surface and underground water supplies. Liquid freshwater is a renewable resource as long as we don’t use it faster than it is naturally replenished.

The trees of tropical rain forests are said to contribute as much as half of the rain that falls back on the forest. Explain how this occurs.

The trees pull up significant amounts of water through their roots and this is then lost to the atmosphere via transpiration to condense as clouds and fall back as rain.

KEY CONCEPT 14.1

Only 3% of water on Earth is freshwater, and very little of that is accessible to humans. Fortunately, even this small percentage represents a large amount of water.

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Crystal-clear purified water from the Groundwater Replenishment System is piped to the Orange County Water District’s percolation ponds in Anaheim, California.
Courtesy of Mark Greening/Orange County Water District

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Almost all precipitation ends up falling on the oceans, and a tiny remainder falls on land. This latter portion is the part humans can harvest for their own use. We access freshwater from lakes and rivers (surface water) and from groundwater. Worldwide, the biggest drain on freshwater supplies isn’t the water used to shower, flush the toilet, and wash dishes. Globally, about 70% of all freshwater withdrawals go to agriculture. Industry is the next biggest consumer of water. Domestic use accounts for only 10% of freshwater usage.

groundwater

Water found underground in aquifers.

KEY CONCEPT 14.2

Water cycles through the environment via the water cycle, a process that constantly recycles water on Earth.

People don’t always live near abundant sources of freshwater, making access a vital issue. Around the world, many areas suffer from water scarcity—not having sufficient access to clean water supplies. In some dry regions, there is simply not enough to meet needs; many arid nations like those of the Middle East, parts of Africa, and much of Australia face water shortages as a way of life. The Middle Eastern countries of Bahrain, Qatar, Kuwait, and Saudi Arabia have the lowest per capita water availability in the world, but these oil-rich nations can afford to invest in costly technology to access water (like facilities to remove salt from seawater). In other areas, particularly in Sub-Saharan Africa, there may be enough water, but people do not have the money to purchase it or dig wells to access it. People in these areas may be getting by on just a few gallons of water a day—and that water may not even be safe to use. By far, per capita domestic (household) use of water in developed nations is much higher than that in developing nations; in the United States the average person uses more than 300 liters (about 80 gallons) of water a day.

water scarcity

Not having access to enough clean water.

KEY CONCEPT 14.3

Water scarcity can be physical or economic. Poor sanitation causes health problems and can contribute to scarcity when it contaminates local water sources.

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The United Nations estimates that as many as 3.5 billion people—half of the world’s population—lack access to enough clean water; 2.5 billion lack access to sufficient sanitation facilities (safe disposal of human waste). In developing nations where water and funding for basic sanitation are scarce, people use nearby surface waters to meet their basic cooking, drinking, and washing needs. These waters can be contaminated with raw sewage, which increases the chance for disease transmission. According to the World Health Organization (WHO), more than 1.1 trillion liters (300 billion gallons) of raw sewage enters the Ganges River of India every minute. In Africa, almost 3,000 people die each day from waterborne diseases like cholera and typhoid fever as a result of poor sanitation and contaminated water.

As human populations increase, so will scarcity and sanitation issues; according to the United Nations, two of every three people will face water shortages by 2025. INFOGRAPHIC 14.3

GLOBAL WATER USE AND ACCESS

How might steps to address physical water scarcity be different from steps to address economic water scarcity?

In areas with physical scarcity, conservation could make supplies go further; accessing untapped water sources (i.e. wells) could also provide more water in these areas. For regions with economic scarcity, however, the problem may not be availability of water; the problem is the ability to pay for water. In these areas, programs to decrease poverty could help, as could subsidies to underwrite the cost of water. Cultural obstacles that limit access to water for some groups might also need to be addressed.

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The United Nations estimates that as many as 3.5 billion people—half of the world’s population—lack access to enough clean water.