Sometimes the term "water use" gets, well, misused.
"Water use" often fails to adequately describe what happens to water, so when you see or hear the term you have to ask yourself what's actually being said. Water use can take two forms - consumption or withdrawal - and it's important to understand the difference between the two.
In The Handbook of Water Use and Conservation, by Amy Vickers, water withdrawal is defined as "water diverted or withdrawn from a surface water or groundwater source." Consumptive water use, on the other hand, is defined as "water use that permanently withdraws water from its source; water that is no longer available because it has evaporated, been transpired by plants, incorporated into products or crops, consumed by people or livestock, or otherwise removed from the immediate water environment."
In the United States we withdraw a lot of water on a daily basis for our homes, businesses and food. Every five years, the United States Geological Survey makes an estimate of how much we withdraw each day. Until 1995, they also estimated how much water was consumed each day. In 2005, 410 Billion gallons per day (Bgal/d) of water were withdrawn in the United States -- almost 621,000 Olympic-sized swimming pools. That's a lot of water! Although domestic, at-home water use is significant, thermoelectric power and irrigation consistently are the biggest users, but they use water in different ways.
Withdrawal and Consumption for Thermoelectric Power Generation
In 2005, water withdrawals for thermoelectric power were estimated to be 201 Bgal/d [pdf], representing 49 percent of all withdrawals. Almost all of that water was surface water used for cooling at power plants. Most thermoelectric power plants in this country use what is known as once-through cooling, a process that pulls in cold river, lake or coastal water to cool the steam that turns a power plant's turbines and then releases the heated water back into the environment. In 1995 it was estimated that less than three percent of water withdrawn for thermoelectric power was consumed (lost to evaporation).
This type of cooling has a number of problems associated with it, including killing massive numbers of aquatic organisms, shutdowns with decreased output during periods of drought and increased water temperatures as cooling water is returned to the source. Power plants in water-stressed areas can place heavy demands on local water resources. A solution to this problem is a cooling system that uses air or recirculated water. This latter system, called closed-cycle wet cooling, is a system where cooling water is circulated first through the plant to absorb heat, then through the cooling towers to evaporate heat to the atmosphere and condense steam back to liquid to be recirculated through the plant. Closed-cycle cooling systems consume more water, up to 70% of water withdrawn due to evaporation, but they withdraw between 95 and 98 percent less water than once-through cooling systems. Renewable energy systems like wind and solar PV are great options from a water resources standpoint because they don't rely on thermoelectric power generation and they consume very little water.
Withdrawal and Consumption for Irrigation
In 1950, there were 25 million irrigated acres in the U.S. By 2005, that number had increased to more than 60 million acres. Withdrawals for irrigation in 2005 were 128 Bgal/d [pdf], which represents 31 percent of all withdrawals. By comparison, in 1995, irrigation withdrawals were 134 Bgal/d. The good news? Withdrawals for irrigation are decreasing as new, more efficient irrigation methods are put in place. The bad news? In 1995, it was estimated that 80 percent of water withdrawn for irrigation was consumed. This is because water that is applied to a field is considered lost to the immediate water system. It either gets used by the plants, evaporated or leaves the field as run-off. In addition, leaks or evaporation cause loss while water is being transported from the source to the field.
Because irrigation-related water consumption is so high, it is important to make water-wise food and textile purchases. Meat consumes the largest amount of water, primarily from the water that goes toward the grain used as feed. Eating less meat is a great way to reduce your water footprint. Also, it takes a lot of water to grow cotton and unfortunately, it's typically grown in arid regions where irrigation is required. Chances are, a lot of water was withdrawn and consumed to create the fabric in those jeans you might be considering buying. On top of that, as we import and export food and textiles around the planet, we move (and consume) a lot of water in the process, even though it's hidden from sight.
Being water wise means understanding how water is used - sorry, make that withdrawn or consumed - and accepting that all of our choices have impacts. There are several online calculators that can help you figure out how big your water footprint is and while they don't differentiate between consumption and withdrawal, they do help you understand that water is involved in many parts of your life that you may not realize, like electricity and diet, and that can help you make informed choices. Choosing renewable energy, being careful with your purchases and moderating your diet can lessen the burden on water resources at home, in town and around the planet.