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Watersheds

Withdrawal uses


Withdrawal uses, or partially consumptive uses, are water uses that consume some of the water withdrawn, while the remainder is returned to the water source, often in an altered or degraded condition. The volume of water involved in these uses is measured by the quantities of intake, discharge and consumption. The amount withdrawn for its intended purpose is referred to as intake. The water that is returned to the source is usually returned near the intake, and is referred to as the discharge amount. The difference between the intake and discharge amounts represents the consumption amount. This is the portion of the water that is permanently removed from the source. In the cases of water uses where the intake amount is reused repeatedly, which is becoming more common with industrial uses, the efficiency of that use is measured by comparing the gross water amount, which represents the total amount used during that particular process, and the recirculated amount. Since the gross water use could easily be much greater than the intake amount, the difference between the gross water use and the intake amount – which represents the recycled amount - is referred to as the recycling rate.

The main withdrawal uses are:

  1. Thermal power generation, which includes both conventional and nuclear power generation, withdraws more than 60 per cent of Canada’s total intake amount annually. Some of this water is converted to the steam which drives the generator that produces the electricity. The bulk of it however, is used to convey heat from the reactor core to the steam turbines, and to remove and dump surplus heat from the steam circuit. A nuclear or coal plant that is located next to a large water source, such as a lake, river or sea, can accomplish its cooling needs by running water through the plant and discharging it at a slightly higher temperature. Although there would be some evaporation of the water as it cools downstream, the consumption amount would be relatively low. In locations where large bodies of water are not nearby, surplus heat can be discharged through the use of recirculating systems that rely on the physics of evaporation, usually though a cooling tower, which transfers the water’s heat to the air.
  2. Irrigation and agricultural uses comprise the largest consumptive use of water. The amount of water required to grow crops varies from year to year, mostly depending on the amount that is provided by local rainfall and by winter snowfall, which provides spring run-off, and also by weather and soil moisture during the growing season. In Canada, dry regions, particularly the Prairies and the B.C. interior, are prone to long-term drought conditions. These areas, particularly southern Alberta, contain the majority of irrigated cropland in Canada. Irrigators cooperate with private industry, governments and researchers to implement the most efficient means of storing, conveying and applying water in the field, such as switching from gravity-fed systems to more efficient sprinkler systems, installing water meters to measure water use, and converting saline land back to dry land. While irrigation can have positive effects on the environment, including the increase of fish habitat, improved recreational opportunities and increased brush and weeds providing cover and nesting habitat, it can also have negative effects. Those might include flooding of forest and riparian habitat by the construction of reservoirs, sediment build-up behind reservoirs and the risk of contamination to groundwater and streams from increasing pesticide application.
  3. Municipal water uses in Canada are greatest in regions within 200 miles of the U.S. border, where the population is densest. About 30 per cent of municipal water goes to industrial and commercial uses, while about 55 per cent is used in Canadians’ homes. The largest of those uses is showers and baths, followed by toilet flushing, laundry, kitchen and drinking, and cleaning. On average, Canadians use approximately 330 litres per person per day, ranking Canada second to last in water conservation efforts among peer developed countries, behind only the U.S. Water is also used to clean streets, douse fires, fill public swimming pools, and to water ornamental lawns, gardens and golf courses. Municipal water is drawn from rivers, lakes and underground aquifers, then passed through a water purification plant which filters out undesirable chemicals, materials and biological contaminants. Municipalities seek to enforce strict water quality standards for domestic use. Water from our toilets and street drains is collected and passed through a sewage treatment plant to remove contaminants with an aim toward producing a waste stream (effluent) and solid waste (sludge) suitable for discharge back into the natural environment. Unfortunately, the discharged material can be inadvertently contaminated with toxic and inorganic compounds.
  4. Industrial and manufacturing sectors represent about 16 per cent of freshwater use. In industries that produce metals, wood and paper products, chemicals, gasoline and oils, cosmetics, food, cars, toys and even novelty household gadgets, water is used for washing, diluting, cooling, fabricating, processing and transporting. Many thousands of litres of water are required for the manufacture of plastics, fabrics and glass, and yes, even computers.
  5. Mine operations, including coal, metal mining and non-metal mining are also water consumers. Mining operations use water to separate ore from rock, to cool drills, to wash the ore during the production process and to flush away any unwanted or leftover materials. Water might also be used for the generation of electricity required to crush the ore, for on-site processing, smelting and other aspects of improving the properties of the materials being mined. Most mining operations recirculate a great deal of their water. Abandoned mines that release contaminated water into the surrounding environment can pose a significant threat to humans, animal and plant life reliant on downstream freshwater sources in that region.

 
 

Synopsis

This piece uses a pie chart and animated diagram to illustrate Toronto’s municipal water cycles, for both drinking water and wastewater.

















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Permanent removal uses


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