Agricultural Water Use

This section of the chapter first examines the general divisions among types of water use before focusing on the primary agricultural use of water: irrigation.

Water uses can be separated into two broad categories:

• Withdrawal Uses. Withdrawn water is, as the name implies, physically removed from its source. Examples of withdrawal uses include groundwater pumping and surface water diversion for irrigation or municipal use. Use of withdrawn water can be further described as either consumptive or non-consumptive. Non-consumptive uses, such as for municipal drinking water, eventually return most of the withdrawn water to the original source. For example, river water used for municipal drinking water is mostly returned to the river as treated sewage. Irrigation, on the other hand, is a consumptive use--water extracted from a river or an aquifer is largely consumed via evapotranspiration. It is therefore crucial that consumptive uses be performed in the most efficient manner possible so as to minimize water loss.
• Non-Withdrawal Uses. When non-withdrawal uses are considered, electric power plants become the largest users of water in the United States. This is because thermal power plants use vast quantities of water for cooling, and hydroelectric plants use water to drive gravity turbines. However, such uses do not require water to be withdrawn from its source, so they are generally not considered true ``users'' of water (Murray, 1995). Additionally, the use of bodies of water to receive and dilute discharges of treated industrial or municipal waste is considered non-withdrawal.

Figure 2.3: 1990 irrigation withdrawals, as the percent of total freshwater withdrawals (USGS, 1990)