How We Use Fresh Water
There are trillions of gallons of liquid fresh water on Earth. Humans use about half of it, mostly for agriculture. This excessive use of water interferes with the health of freshwater ecosystems. And because water isn't distributed evenly around the world, billions of people don't have enough for basic health and sanitation. So sensible water use is important, especially in the places with the least water.
Where Water Is Scarce
As important as water is, people often live where there isn't enough for basic needs such as drinking and hygiene. On this map, green shows areas with more water available than people demand. Magenta areas are where human water demand is greater than supply. Places with great scarcity and surplus are often surprisingly close to each other.
Cairo, Egypt, and Karachi, Pakistan, are major cities built in deserts.
Beijing, China, a city of over 15 million people in a relatively dry area, must pump extra water from distant parts of southern China.
The Colorado River, tapped by growing cities and for extensive irrigation, dries up completely 50 kilometers (30 miles) before reaching the sea.
In 2007, the Australian government created a water management plan for Murray-Darling River basin, attempting to balance the needs of wildlife, farming, and ranching.
How do we use the world's fresh water?
8 percent: Domestic and municipal uses (global average)
22 percent: Industrial uses (global average)
70 percent: Agricultural uses (global average)
Why so much more for agriculture? We eat more water than we drink. Every crop needs months of water before harvest, and much irrigation water never reaches the crops. In most wealthier countries, industry uses the biggest share--46 percent of U.S. water use is for industry.
How much water do people use each day?
573 liters (151 gallons) per person per day
U.S., average domestic and municipal use
118 liters (31 gallons) per person per day United Kingdom, average domestic and municipal use
10 liters (3 gallons) per person per day Ethiopia, average domestic and municipal use
People in the U.S. and Canada use much more water than residents of most other countries. In the U.K. and most other European countries, people live more water-efficient lifestyles. Most Ethiopians, like many others in the developing world, can't get enough water to ensure basic health and sanitation.
How much water do people have available?
44 million liters (12 million gallons) per person Brazil water resources/capita
10 million liters (3 million gallons) per person U.S. water resources/capita
3 million liters (700,000 gallons) per person United Kingdom water resources/capita
How much water you use depends on how much you have available. Brazil has the world's biggest water supply. The U.S. has the fourth-largest supply but a much larger population. The U.K. is known for its damp climate, yet a large population means there's not much water per person.
Small Dams, Big Impact
We typically think of dams as huge, imposing structures. But a large dam isn't the only effective way to manage a river. For centuries, residents of western China's Chengdu Plain suffered from devastating, unpredictable floods that took lives and ruined crops. Then in 256 BC, governor and engineer Li Bing designed a system of dams, dikes and channels to control the Min River while also delivering a reliable water supply to rice farmers. The basic design of the so-called Dujiangyan project is still in use today.
Tube-shaped bamboo baskets filled with rocks--called "rock sausages"--were key components of the Dujiangyan project. They were stacked to create a dike and also used to anchor diversion dams supported by wooden tripods.
Water to Rice to Dynasty
The Dujiangyan water system supported thousands of small farms in the Chengdu Plain, one of China's most fertile regions. Such agricultural wealth aided the formation of the Qin Dynasty in 221 BC, the first unification of China.
The yuzui or "fish snout," is a manmade island that splits the river into two channels. In the ancient system, workers would temporarily dam each channel, dredging out sediment to maintain proper water flow.
A diversion dam was built from a long row of tripod supports, anchored by "rock sausages" and faced with finely woven bamboo mats.
Workers in the 1950s weave bamboo baskets to fill with river rocks, forming the components of the "low dam," or feishayan. This dam diverts water toward the Chengdu Plain during the low-water season. When water levels rise, the excess rushes over the dam and back to the main river channel instead of flooding the farmlands. Today the dam is made of concrete.
The baopingkou, or "neck of the vase," is a narrow channel cut through a ridge of bedrock. It carries water to farms on the Chengdu Plain. Its narrow opening keeps out most of the flood waters during the wet season.
Farther downstream, this channel branches out like a river delta, feeding water to 5,300 square kilometers (2,000 square miles) of farmland.