Irrigation is the practice of applying controlled amounts of water to land to help grow crops. This practice relies heavily on water sources, often drawing supply from underground reservoirs known as aquifers. An aquifer is an underground layer of permeable rock, gravel, sand, or silt saturated with water. Agriculture’s withdrawal of this water creates consequences affecting the aquifer’s volume, quality, and physical structure.
Reduction of Aquifer Volume
The most immediate effect of irrigation is the reduction of the aquifer’s stored water volume. Agriculture is a major user of fresh water globally, accounting for nearly half of all freshwater withdrawals in the United States alone. In many regions, the rate of pumping for irrigation significantly exceeds the natural rate of groundwater recharge. This imbalance is often termed “groundwater mining,” signifying unsustainable extraction.
This excessive pumping causes the regional water table to drop persistently. Around each heavily pumping well, the water table declines in a conical shape known as the cone of depression. When multiple cones of depression overlap, they accelerate the regional lowering of the water table. This decline forces farmers to drill deeper, more expensive wells and requires larger, more energy-intensive pumps. For example, the Ogallala Aquifer has seen its saturated volume reduced by an estimated nine percent since 1950 due to agricultural pumping.
Changes to Groundwater Chemistry
Beyond the loss of volume, the remaining water often deteriorates in quality. Pumping alters natural flow dynamics, potentially drawing in poorer quality water from adjacent areas. In coastal regions, reduced freshwater pressure allows denser saltwater from the ocean to migrate inland and upward, a process known as saltwater intrusion. This contamination can render the freshwater unusable for drinking and irrigation.
Agricultural return flow, the water that percolates back into the ground after irrigation, commonly carries concentrated contaminants picked up from the surface. Excessive application of nitrogen fertilizers results in high concentrations of nitrate leaching into the groundwater, posing a health concern if used for drinking. Pesticides and herbicides applied to crops can also dissolve into the percolating water, accumulating in the aquifer. This process changes the groundwater’s chemical signature, often increasing overall salinity and the load of dissolved solids.
Geologic and Structural Impacts
The physical removal of water from the aquifer system results in changes to the subsurface geology and the land surface. Water within the aquifer pore spaces provides hydrostatic pressure that supports the overlying rock and sediment layers. When this pressure declines due to large-scale withdrawal, the aquifer material, particularly fine-grained sediments like clay and silt, loses structural support and begins to compress.
This compression of the aquifer matrix causes land subsidence, which is the gradual sinking of the ground surface over a large area. Subsidence caused by groundwater pumping has been measured at total values of up to 14.5 meters in some areas. This compaction is permanent; even if water pressure is restored, the physical space for water storage within the aquifer is lost forever. The resulting land deformation can cause millions of dollars in damage to infrastructure, including roads, canals, and pipelines.
The Dynamics of Return Flow
Not all water applied for irrigation is consumed by crops or lost to evaporation; a significant fraction cycles back into the groundwater system as return flow. This movement of water from the surface back into the saturated zone acts as a source of aquifer recharge. In arid and semi-arid agricultural regions, this return flow can be a substantial contributor to the overall volume of groundwater.
However, the quality of this input is often mixed, creating a dual effect on the aquifer. As the water moves through the soil, it dissolves and concentrates salts and minerals, along with surface-applied chemicals. While return flow may help maintain the water table elevation, it simultaneously introduces a load of dissolved contaminants, like nitrates and salts, into the receiving aquifer.