Water intake is the amount withdrawn from the source for a particular activity over a specific period of time.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 comsumption 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. Gross water use represents the total amount of water used during a process.