Groundwater is the water held underground in the cracks and spaces within soil, sand, and rock, primarily stored in porous geological formations called aquifers. This hidden resource represents the largest source of usable freshwater on Earth, dwarfing the amount found in all surface lakes, rivers, and streams combined. Because it is naturally filtered and often less susceptible to seasonal changes than surface water, groundwater provides a reliable and high-quality supply. This reliability makes it an indispensable resource that supports human life, industry, and food production worldwide.
The Dominance of Agricultural Irrigation
Globally, the vast majority of groundwater withdrawn—approximately 70%—is used for agriculture, making it the single largest consumer. In the United States, this pattern holds true, with irrigation accounting for roughly 68% of the total groundwater extracted. This massive reliance is particularly pronounced in arid and semi-arid regions, where a lack of rainfall necessitates supplemental watering to grow crops. In some countries with dry climates, the percentage of groundwater dedicated to irrigation can soar to 90%.
Farmers depend on aquifers to sustain crops during dry periods or in areas where surface water is scarce or unreliable, such as the US High Plains. A significant portion of this groundwater is pumped from massive systems like the Ogallala Aquifer, which underlies eight states and provides nearly one-third of the groundwater used for irrigation across the entire country. The ability to irrigate allows for the profitable cultivation of water-intensive crops like corn, cotton, and soybeans. Furthermore, high-value crops like almonds in California rely heavily on groundwater extraction in the state’s Central Valley.
Groundwater for Public and Private Supply
While agriculture dominates groundwater use, other sectors rely on it for human and industrial needs. Groundwater provides nearly half of all drinking water worldwide and supplies 38% of the drinking water in the United States. In the US, the public supply sector, which includes municipal systems that deliver water to homes and businesses, accounts for approximately 18% of the total groundwater withdrawn.
Beyond large municipal systems, many individual households in rural or remote areas use private wells for their water. This self-supplied domestic use represents 4% of the total national groundwater withdrawal. Groundwater also supports industrial activities, including manufacturing, mining, and the cooling processes required for thermoelectric power generation. Although these industrial uses are necessary for the modern economy, their combined total remains substantially lower than the amount used to grow food.
Physical Consequences of Excessive Groundwater Withdrawal
The scale of groundwater extraction, driven primarily by irrigation, has led to several physical consequences. The most immediate is aquifer depletion, which occurs when the rate of water withdrawal consistently exceeds the rate of natural recharge. This over-pumping causes the water table to decline, forcing well owners to drill deeper wells or incur higher energy costs to lift the water to the surface.
In coastal regions, excessive pumping can lead to saltwater intrusion, where the removal of fresh groundwater allows denser seawater to migrate inland and upward, contaminating the freshwater supply. Another consequence is land subsidence, the compacting and sinking of the ground surface. When water is removed from the pore spaces within unconsolidated sediments in an aquifer, the overlying land loses support and collapses, a phenomenon observed in over 150 cities globally.
Pumping also impacts surface water systems because streams, lakes, and wetlands are often hydraulically connected to underlying aquifers. When groundwater levels fall, the aquifer can no longer discharge water into these surface bodies, which reduces streamflow and diminishes the water supply for aquatic ecosystems. This connection means that groundwater extraction in one area can reduce the flow in a distant river.