The water supply necessary to sustain a particular land use activity varies. Agricultural land used to grow seasonal crops, for example, may require less water than land used for agro-forestry. Land used for energy development may utilize but not diminish the available water supply. Knowledge associating land use types with water quantity needs is critical to decision makers active in watershed management.
Water use is usually defined and measured in terms of withdrawal or consumption. Withdrawal refers to water extracted from surface water or groundwater sources, while consumption is that part of a withdrawal that is ultimately used and removed from the immediate water environment whether by evaporation, transpiration, incorporation into crops or a product, or other use.
Water consumption varies by water use category, with irrigation consuming the highest percent (81%) and commercial the lowest (1%) in the U.S. Return flow is the portion of a withdrawal that is not consumed, but is instead returned to a surface or groundwater source and becomes available for further use. Some categories of water use, such as irrigation and livestock watering, consume a high percentage of water that is withdrawn from surface water and groundwater sources. Thus, less water is available for return flows from these high-consumptive activities. Other categories of use such as thermoelectric power consume only a small fraction of the water they withdraw.
Water use can be divided into offstream and instream uses. Offstream water
use involves the withdrawal or diversion of water from a surface water
or groundwater source. These uses include:
Municipal/public
supply
Domestic,
residential and commercial uses
Industrial
and mining uses
Agricultural
uses
Energy
development uses
Estimated offstream use of water in the U.S. in 1980 averaged 450 billion gallons per day (bgd), an increase of 8 % from the 1975 estimate and 22% more than the amount used in 1970. (Dzurik 1990)
Instream water uses are those which do not require a diversion or withdrawal from surface water or groundwater sources and include:
Water quality and habitat
improvement
Recreation
Navigation
Fish propagation
Hydroelectric power production
Data from 1993 illustrates that the largest user of water from both fresh and saline sources is thermoelectric generation. Irrigation represents the largest demand for freshwater withdrawal alone.
