Water withdrawal is a foundational concept in the management of global freshwater resources, representing the initial step in nearly all human water use activities. This process involves actively taking water from its natural source for agricultural, industrial, or domestic purposes. Understanding water withdrawal is essential for assessing the pressure humans place on the natural water cycle and for developing sustainable resource policies. Withdrawal figures are critical metrics for monitoring water security and availability in any given region.
Defining Water Withdrawal and Measurement
Water withdrawal is defined as the total volume of water removed or diverted from a surface water or groundwater source for human benefit. This measurement captures the gross amount of water taken, regardless of whether that water is eventually returned to the environment after use. Withdrawal represents the demand placed on the source ecosystem, helping evaluate the level of competition for water resources.
Quantifying water withdrawal involves measuring volume over a specific unit of time. Common units include millions of gallons per day (Mgal/d) or billions of cubic meters per year (km³/year). For example, the United States Geological Survey (USGS) periodically estimates total national withdrawal in billions of gallons per day. This gross volume measurement indicates the strain placed on local water bodies and aquifers.
Primary Sources of Withdrawn Water
The water humans withdraw comes from two main categories of natural reservoirs: surface water and groundwater. Surface water withdrawal involves taking water from visible sources like rivers, lakes, and artificial reservoirs.
Groundwater withdrawal involves pumping water from underground layers of permeable rock and soil known as aquifers. This water is accessed using wells, which can be shallow or deep. Groundwater often offers a more consistent supply and can be of higher quality than surface water, though its extraction is more expensive and can lead to depletion if not managed sustainably. Globally, about half of the freshwater withdrawn for domestic use and 25% for irrigation comes from groundwater sources.
Major Sectoral Uses
Water withdrawal is categorized into different sectors based on its ultimate application, with agriculture typically dominating global figures. Worldwide, agriculture accounts for approximately 70% of all freshwater withdrawals, primarily for irrigation of crops. This sector places the greatest overall demand on renewable water resources.
Thermoelectric power generation represents another major sector, where water is withdrawn primarily for cooling purposes in power plants. This industrial use often involves large volumes of water, making it a significant portion of the total withdrawal in many developed nations.
Public supply encompasses water withdrawn for domestic and municipal use, including household consumption, commercial enterprises, and public services. This water is typically treated to meet drinking water standards before distribution.
Industrial use covers a wide range of applications, such as manufacturing, mining, and aquaculture. Industry globally accounts for around 20% of freshwater withdrawals, though this percentage varies significantly by region, often being much higher in industrialized countries.
Differentiating Withdrawal from Consumption
A crucial distinction in water management is the difference between water withdrawal and water consumption. Consumption is the portion of the withdrawn water that is permanently lost from the local water cycle and is no longer immediately available for downstream users. Water is considered consumed if it evaporates, is transpired by plants, is incorporated into a manufactured product, or is ingested by humans or livestock.
The thermoelectric power sector is an example of high withdrawal but low consumption, as most of the water used for cooling is returned to the source, albeit often at a warmer temperature. In contrast, the irrigation sector typically exhibits high withdrawal and high consumption because a large percentage of the water applied to fields is lost through evaporation and transpiration. This distinction is significant because consumption directly impacts water availability and local water scarcity.