Water is Life

When NASA’s deep space probes look for life, they look for water. When humans look for high quality of life, they assess water availability and quality as critical signs of environmental and ecosystem health.

Three separate but important water events—an oil spill, a burning river, and a trip to the moon—spurred the modern environmental movement and its attentiveness to protecting water. In January and February 1969, a massive spill from offshore oil production in the Santa Barbara Channel released over 80,000 barrels (12.7 million liters) of crude oil that lined the nearby beaches of Southern California and killed thousands of birds. Later that year, the Cuyahoga River in Cleveland, Ohio, caught on fire due to rampant pollution.1“America’s Sewage System and the Price of Optimism,” Time Magazine, August 1, 1969, accessed August 26, 2016, http://content.time.com/time/magazine/article/0,9171,901182,00.html. Almost exactly one month later, the first mission to the moon successfully landed. Photographs taken from space revealed Earth to be more visibly blue than previously expected. The view of the oceans from space served as a reminder that despite our planet’s name, our globe’s surface is primarily covered in water, not earth.

The mass public outcry over the Cuyahoga fire is credited by many to be the spark that eventually led to the formation of the Environmental Protection Agency (EPA), the Clean Water Act (CWA), and a myriad of other reforms in the past 50 years. 

Image Credits: Apollo 17/NASA.

Water Footprint

Visit waterfootprint.org to get a snapshot of the impact of your daily lifestyle. Compare how much water is used to make a variety of products.

Water use for agriculture and industry gets embedded into different products and services. In some cases, the water embedded in those goods is significant enough to be tracked. This embedded water, or virtual water, is often known as the “water footprint.”

The Water Footprint Network provides the definitions to differentiate blue, green, black and gray water from a water-footprint perspective.1A. Y. Hoekstra, et al., Water Footprint Manual: State of the Art 2009, technical report, Water Footprint Network, 2009; and “Water Footprint Network,” accessed October 16, 2012, http://www.waterfootprint.org Blue water includes consumption from fresh surface and groundwater (lakes, rivers, aquifers). Green water falls as precipitation on land that does not run off or recharge the aquifer because it stays in the soil, but might eventually be evaporated or evapotranspired during growth of a crop. Gray water becomes polluted during production or dilutes pollutants. Black water contains sewage (fecal matter and urine) and organic matter from dishwater drains. It requires a septic tank or sewer system that transports the blackwater to a treatment facility to remove pathogens.

Fresh surface and groundwater. In other words, the water in freshwater lakes, rivers and aquifers.The precipitation on land that does not run off or recharge the groundwater but is stored in the soil or temporarily stays on top of the soil or vegetation.Wastewater or runoff from precipitation. It may contain soap or other organic residue.Wastewater from toilets and other sewage water containing fecal matter.

In the built environment, washing produces gray water, which looks cloudy or gray from the soaps and other organic residues from sinks, laundry washers, showers, and tubs. Gray water also includes storm runoff from roofs that is harvested in rain barrels.

Some advanced eco-sensitive homes separate the gray water and black water, using the former on-site for irrigation and sending the latter to a facility for treatment. This approach removes some of the energy burden for transportation and treatment but introduces problems if flows of wastewater reduce to levels at which transporting the sewage through the sewer fails. Purple water is treated wastewater effluent that does not meet potability standards, but is suitable for applications such as irrigation and washing cars. Many municipalities use purple pipes to provide a visual clue that the effluent is not potable.

small purple water pipes in a shallow trench
These purple pies carry water that is treated enough for irrigation use, but does not meet the standards for drinking water.

Image Credits: Alistair Scott/Shutterstock.com; Evelyn Sugar/Shutterstock.com; Robert Schlie/Shutterstock.com.


Several demographic factors around the world affect water use: population, urbanization, standards of living, prevailing mix of economic sectors, and number of households.1Jill Boberg, Liquid Assets: How Demographic Changes and Water Management Policies Affect Freshwater Resources (Santa Monica: RAND Corporation, 2005). As these factors change—population grows, number of households increase, and societies switch from agrarian to industrial modes—water demands shift.

In particular, the number of households and household size stand out as key drivers of consumption, which is summarized by the conclusion, “Per capita, smaller households consume more water and produce more waste.” While the number of households increases globally, the number of people living in each household decreases. As affluence increases, multigenerational households shift from three generations under one roof, sharing laundry, cooking, and outdoor watering to individual or single-generation households, driving up water use.

Because the United States is relatively affluent, the per capita withdrawals are high for drinking, washing, and irrigating lawns. In addition to water use for residential life, the United States is a major agricultural producer. Those agricultural needs drive up water withdrawals even further. The United States also has a very extensive industrial sector, with significant power generation, chemical production, refining, mining, and other water-intensive activities. By comparison, other more economically developed countries (MEDC) with robust industrial sectors, such as Japan or the United Kingdom, have relatively less agricultural activity. Consequently, North American per capita water withdrawals (including Canada) are much higher than in other regions of the world.

Freshwater withdrawals vary globally based on both economic and demographic factors. More affluent countries withdraw more water per capita. For example, the United States withdraws more water per person than Mexico.

Hydrologists frame water withdrawals in terms of per capita water availability, which varies globally. In this context, availability means the amount of water that is accessible to people over the course of a year within a reasonable distance. While Asia has the most total water available, it also has the highest population, and subsequently the lowest per capita water availability. This measure of water’s abundance and availability to users is important when evaluating how trends in water withdrawals, coupled with the use of non-renewable water sources, will trigger significant water strains.

Every day the United States withdraws about 1,340 billion liters (355 billion gallons) and consumes 379 billion liters (100 billion gallons).2M.A. Maupin et al., “Estimated use of water in the United States in 2010,” U.S. Geological Survey Circular 1405, November 5, 2014, accessed August 26, 2016, doi: 10.3133/cir1405. That works out to more than 3,780 liters (1,000 gallons) withdrawn per person per day, of which more than 1,140 liters (300 gallons) is consumed per person per day. Withdrawals refer to the water that is taken out of a water body, some of which is consumed and some of which is returned. Consumption refers to the water that is evaporated or otherwise lost; instead of being returned to the water source, it soaks into the soil or comes down as precipitation somewhere else.

That water is withdrawn from a mixture of sources—groundwater and surface water—and from a variety of source qualities, including saline and fresh, for a mix of end uses. 

green golf courses with desert mountains in the distance
Not all irrigation is used for farming. Golfers enjoy thick, lush grass on this desert golf course thanks to extensive irrigation.

Similar to the situation with energy, residents of the United States withdraw about twice as much water per person as Europeans and four times as much as Southeast Asians. Also, while the power sector is responsible for the greatest volume of water withdrawals in the United States, the agricultural sector has the greatest water consumption. That consumption is from evapotranspiration during photosynthesis, when water moves up and out of plants; run-off; and water that trickles back down to aquifers. Most of the water withdrawn for power plant cooling is returned to the source, with very little of it evaporated.3M.A. Maupin et al., “Estimated use of water in the United States in 2010,” U.S. Geological Survey Circular 1405, November 5, 2014, accessed August 26, 2016, doi: 10.3133/cir1405.

Image Credits: GGRIGOROV/Shutterstock.com; Gordon Swanson/Shutterstock.com.

Water and Health

Despite the importance of clean water to sanitation, the close connections between public health and water supplies were not revealed scientifically until the mid-1800s. After an outbreak of cholera in London in 1848, Dr. John Snow determined that public wells drawing water from the Thames River spread cholera.1The story of the discovery of the connection of cholera with contaminated water and the Great Stink in London have been extensively covered by the published literature. I recommend Bill Bryson, At Home: A Short History of Private Life (New York: Anchor, 2011) as one that is accessible and interesting to a general audience. At the time, untreated human waste emptied directly into the river.

The John Snow memorial and pub, located on Broadwick Street, London.

The city of London had disposed of untreated sewage in the Thames for hundreds of years, and the currents had done a great service to the metropolis by washing the waste downstream to sea. However, the combination of heat and drought during the summer of 1858 left less water to dispose of the wastes. Stagnant water and air produced a noxious smell, triggering a temporary suspension of Parliament, whose building sits on the banks of the Thames. The events of the summer marked a turning point in the development of modern water systems by inspiring the creation of an ambitious public works project installing 1,200 miles of sewers into a crowded city of 3 million people. The project addressed the problem of waste disposal and created the river embankments that stand today as a key piece of London’s urban landscape.

In the past, London solved its water problems by simply flushing waste away from the city. Today’s high density, industrialized societies invest energy to dispose of their waste through modern wastewater treatment plants. They also use energy to treat their drinking water supplies. Investing energy to clean water is one of the great public policy achievements of the last 150 years.2David Sedlak, Water 4.0: The Past, Present and Future of the World’s Most Vital Resource (New Haven: Yale University Press, 2014). Energy also lets us heat our water, which is critical for sterilizing medical equipment, washing our hands, ridding our society of many disease-carrying pests, and cleaning wounds.

Despite the multiple benefits, more than 1.1 billion people globally lack access to clean water sources for drinking, cooking, and washing.3For information on global access to water and wastewater, the United Nations is a reliable source. For information on global water stress, Vörösmarty’s work is the standard-bearer: C. J. Vörösmarty, et al., “Global threats to human water security and river biodiversity,” Nature 467 (2010), 555-561, accessed August 26, 2016, doi: 10.1038/nature09440. In addition, the Pacific Institute produces a series of biennial reports on freshwater resources with convenient summaries of water data and in-depth analyses on water topics including availability, access, policies, and technologies: Peter Gleick, et al., The World’s Water: Volume 8 (Washington: Island Press, 2014), (and prior volumes in prior years). Models project the number increasing to 1.8 billion people by 2025. In China alone, 100 million people lack improved water sources, and 2.6 billion people globally remain vulnerable to waterborne diseases because they lack access to sanitation and wastewater treatment. Nearly 4.8 billion people, or 80% of the world’s population in 2000, reside in areas with significant water security or biodiversity threats. Because of the critical relationship between water and life, improving water quality is clearly a significant way to improve public health worldwide.

A woman washes her hair in the Irrawaddy River, Myanmar (Burma).

Image Credits: arindambanerjee/Shutterstock.com; Justinc/CC BY-SA 2.0; LiteChoices/Shutterstock.com.

The Hydrologic Cycle

It can change its intensity over time, but the hydrologic cycle does not stop, and each part of the cycle is interconnected. As described by the U.S. Geological Survey,1United States Geological Survey, “The Water Cycle,” USGS Water Science School, May 2, 2016, accessed December 31, 2014, link.

“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 hydrologic cycle includes major fluxes and volumes of water. The largest movements of water are evaporation and precipitation over the ocean. Other exchanges include significant transport of water vapor in the atmosphere, rainfall and snowfall over land, and other fluxes in the form of runoff, stream flow, and evapotranspiration, which is the evaporation of water through photosynthetic activity from the growth of plants. Earth has an abundance of water, but the purity, salinity, and availability differ greatly over time and place. The image of the hydrologic cycle is a depiction of water abundance, but converting, transporting, and purifying water to meet human needs requires energy.

diagram showing the hydrologic cycle

Solar energy is the key force driving the hydrologic cycle.2 Weston A. Hermann, “Quantifying global exergy resources,” Energy 31 (2006): 1685-1702, accessed August 26, 2016, doi: 10.1016/j.energy.2005.09.006. Evaporating water consumes about half of Earth’s surface’s incident solar radiation.3Vaclav Smil, Energy: A Beginner’s Guide (London: Oneworld Publications, 2006). Essentially, the sun acts as a massive water pump, elevating water into the atmosphere. Gravity pulls that water back to Earth as snow and rain. As water rolls back down to the oceans, we harness it for power, agriculture, drinking, and many other purposes.

In addition to the movement of water from one location and form to another, there are also massive reservoirs of water in oceans; in aquifers, ice, and permanent snow cover; in permafrost (frozen water in the soil); in rivers, lakes, and swamps; in the atmosphere; and within living organisms. The world has plenty of water, but it’s not always available where we want it and in the form we need it.

Water SourceWater Volume (mi3)Water Volume (km3)% of Fresh Water% of Total Water
Oceans, Seas & Bays321,000,0001,338,000,00096.5
Ice Caps, Glaciers & Permanent Snow5,773,00024,064,00068.61.74

Soil Moisture3,95916,5000.050.001
Ground Ice & Permafrost71,970300,0000.860.022

Swamp Water2,75211,4700.030.0008
Biological Water2691,1200.0030.0001
Total Water in the Hydrosphere1,386,000,000100
Total Fresh Water35,030100

The global distribution of water among types (saline and fresh) and locations shows that only a small fraction is easily accessible, fresh surface water.4Igor Shiklomanov and John C. Rodda, eds., “World Water Resources at the Beginning of the 21st Century,” International Hydrology Series (Cambridge: Cambridge University Press, 2003), referencing data from Igor Shiklomanov, “World fresh water resources,” in Water in Crisis: A Guide to the World’s Fresh Water Resources, ed. Peter H. Gleick (Oxford: Oxford University Press, 1993); and United States Geological Survey, “The Water Cycle.”

Image Credits: Alexey Kamenskiy/Shutterstock.com; Jeffrey M. Phillips/Webber Energy Group.