Water Use, Withdrawal and Consumption

In concepts & definitions, footprints

 

The Term “Water Use” Gets Misused

“Water use” often fails to adequately describe what happens to water because “water use” can take two forms – consumption or withdrawal. It’s important to understand the difference between the two.

In Handbook of Water Use and Conservation, by Amy Vickers, water withdrawal is defined as “water diverted or withdrawn from a surface water or groundwater source.”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.”

The United States  withdraws a lot of water every day for use in homes, businesses, industries and food production. Every five years, the United States Geological Survey makes an estimate of how much the country withdraws on a daily basis, and , for the first time since 1995, consumptive water use for thermoelectric power and irrigation were reported.

In 2015 (*the last year for which data is available), 322 Billion gallons per day (Bgal/d) of water – almost 488,000 Olympic-sized swimming pools – were withdrawn. Although domestic, at-home water use was a significant portion of that – 39 Bgal/d accounting for almost 12 percent of the total water withdrawn, thermoelectric power plants and agricultural irrigation consistently withdrew the largest portions, but they used that water in different ways.

Withdrawal and Consumption for Thermoelectric Power Generation

In 2015, water withdrawals for thermoelectric power were estimated to be 133 Bgal/d [pdf] or 41 percent of all withdrawals. Most of that was surface water used for cooling at power plants. That’s because 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. The heated water is then released back into the environment. Less than three percent of water withdrawn for thermoelectric power was consumed through evaporation.

This type of cooling has a number of problems associated with it, including killing massive numbers of aquatic organisms, causing shutdowns with decreased output during periods of drought and causing increased water temperatures in receiving water bodies as cooling water is returned to the environment.

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 first circulated 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 withdraw between 95 and 98 percent less water than once-through cooling systems but they consume more water – up to 70 percent of water withdrawn is lost to evaporation.

On the other hand, renewable energy systems like wind and solar PV are great options from a water resources standpoint because they withdraw and consume very little water.

Withdrawal and Consumption for Irrigation

In the US in 1950, there were about 25 million irrigated acres that required withdrawals of 8.8 Bgal/day of water. By 1995, that acreage increased to almost 60 million and withdrawals rose to 134 Bgal/d. By 2015, the number of irrigated acres increased to more than 63.5 million and withdrawals increased to about 118 Bgal/d, representing 37 percent of total withdrawals for 2015.

The good news? New, more efficient irrigation methods require less water to irrigate more acreage. The bad news? Sixty-two percent of water withdrawn for irrigation in 2015 was consumed, but that’s lower than the 1995 consumption estimate of 80 percent.  Consumptive rates are so high because water that is applied to a field is considered lost to the immediate water system. It either gets evaporated, used by the plants 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, often over great distances.

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 because of all the irrigated grain fed to livestock. In addition, it takes a lot of water to grow cotton because cotton is typically grown in arid regions requiring irrigation. On top of that, as food and other goods are imported and exported around the planet, massive amounts of water are moved (and consumed) in the process, even though this water is hidden from sight.

Being water wise means understanding how water is used – sorry, make that withdrawn or consumed – and accepting that lifestyles and behavioral choices have impacts. The Water Footprint Calculator helps users figure out how big their water footprint is and helps them make informed choices by understanding that water is involved in all parts of their lives. By choosing renewable energy, being careful with purchases and moderating diets, everyone can lessen their burden on water resources at home, in town and around the planet.