Withdrawal use
Withdrawal use is directly measurable as quantities of intake, discharge, and consumption. Water intake is the amount withdrawn from the source for a particular activity over a specific period of time. This measure is important because it represents the demand imposed by that particular use on the water source at a given location. Usually, however, most of the water taken out is returned at or near the source. This is called water discharge.
Water consumption is the difference between water intake and water discharge. Consumption removes water from a river system and makes it unavailable for further use downstream. The irrigation of crops is by far the largest consumptive use, followed by evaporation in large open water reservoirs and cooling ponds. However, because evaporation is difficult to measure, it is seldom recognized as water consumption.
In the global hydrologic cycle, water is never actually lost. For example, the water evaporated from industrial cooling towers or an irrigated field simply returns to the atmosphere, later to fall again as precipitation somewhere else on earth.
We determine how efficiently we use water in a particular process or economic sector with the help of two additional measurements: gross water use and the amount of water that is recirculated. Gross water use represents the total amount of water used during a process. This would normally be equal to the water intake, except that more and more users (especially industries) reuse the same water one or more times. In such cases, the gross water use could be equal to several times the water intake. The difference between gross water use and water intake is the amount recirculated, which can be expressed as a recycling rate. This is the number of times that the water is recirculated and indicates how efficient a particular water use is.
In 1996, five main withdrawal uses are estimated to have accounted for a gross water use in Canada of 64 421 million cubic metres (MCM), made up of intake (44 611 MCM) and recirculation (19 810 MCM in industrial uses). About 10% of the intake was consumed (mostly industrial uses and agriculture), while the rest was discharged back to receiving waters. 
The main withdrawal uses are:
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