G roundwater, the water in aquifers accessible by wells, is a critical component of the U.S. water supply. It serves as a water source for domestic use and as irrigation water for agriculture, and it is used in mining, oil and gas development, industrial processes, livestock, and thermoelectric power, among other uses. Nearly half of the U.S. population relies on groundwater to meet their everyday needs. Groundwater makes up a portion of the water supplied to about 149 million people (46% of the U.S. population in 2015) for their domestic indoor and outdoor water uses. Groundwater and Irrigation

The greatest volume of groundwater used is for agriculture, nearly entirely for irrigation. Among all states, California uses the most groundwater for irrigation, withdrawing 13.9 bgpd in 2015. The Federal Role in Groundwater Supply

The federal government directly and indirectly influences how groundwater is managed in the United States. This includes the two principal federal water resources agencies: the U.S. Army Corps of Engineers (USACE, which operates nationwide) and the U.S. Bureau of Reclamation (Reclamation, which operates in the 17 coterminous states west of the Mississippi River). Congressional interest has increased in the potential to assist with state, local, and private groundwater management efforts, including efforts to use surface water to recharge and/or store water in aquifers and to recover (i.e., pump to the surface) the stored groundwater when needed. Currently, there are no general federal restrictions on the nonfederal use of water delivered by Reclamation or stored by USACE for aquifer recharge, storage, and recovery purposes; however, some state restrictions and federal environmental protection laws may affect the use of these waters for groundwater recharge. Some legislation would attempt to clarify standards applying to specific nonfederal aquifer storage projects. For decades, USGS has monitored and reported groundwater conditions across the country; developed groundwater models and software tools for characterizing aquifers; and provided long- and short-term forecasts of changing groundwater conditions as part of local and regional groundwater studies. Other agencies, such as the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), make observations and collect data that also are relevant to groundwater monitoring and assessment. Congress has provided other federal agencies with the authority to make water delivered from or water stored at federal water resource projects available for groundwater recharge, storage, and recovery. USDA also collects groundwater data related to irrigation. Reclamation also reports that some state restrictions affect the use of these waters for groundwater activities. Climate Change and Other Long-Term Influences on Groundwater Supply

Long-term changes to the climate affecting the United States, particularly rising temperatures and changes in the patterns, quantities, and type of precipitation (i.e., rain versus snow), could affect the availability of groundwater in the future. Summary and Conclusions

Congress generally has deferred management of U.S. groundwater resources to the states, and that practice appears likely to continue. Several bills introduced in the 115 th Congress, for example, contain language directing that the federal government not "assert any connection between surface and groundwater that is inconsistent with such a connection recognized by State water laws." Those same bills also would prohibit the federal government from requiring the transfer of water rights to the United States, or obtaining a water right in the name of the United States, as a condition for receiving, renewing, amending, or extending "any permit, approval, license, lease, allotment, easement, right-of-way, or other land use or occupancy agreement." In addition, the bills contain language asserting that federal reserved water rights would not be limited or expanded by the legislation.
Groundwater, the water in aquifers accessible by wells, is a critical component of the U.S. water supply. It is important for both domestic and agricultural water needs, among other uses. Nearly half of the nation's population uses groundwater to meet daily needs; in 2015, about 149 million people (46% of the nation's population) relied on groundwater for their domestic indoor and outdoor water supply. The greatest volume of groundwater used every day is for agriculture, specifically for irrigation. In 2015, irrigation accounted for 69% of the total fresh groundwater withdrawals in the United States. For that year, California pumped the most groundwater for irrigation, followed by Arkansas, Nebraska, Idaho, Texas, and Kansas, in that order. Groundwater also is used as a supply for mining, oil and gas development, industrial processes, livestock, and thermoelectric power, among other uses.
Congress generally has deferred management of U.S. groundwater resources to the states, and there is little indication that this practice will change. For example, several bills introduced in the 115th Congress contain provisions that would direct that the federal government recognizes that aspects of groundwater, such as the connection between surface water and groundwater, be consistent with state water laws (surface water includes streams, rivers, lakes, ponds, and is not groundwater or atmospheric water, such as rain or snow). Those same bills also would prohibit the federal government from requiring the transfer of water rights to the United States or obtaining a water right in the name of the United States as a condition for receiving, renewing, amending, or extending "any permit, approval, license, lease, allotment, easement, right-of-way, or other land use or occupancy agreement." In addition, these bills contain language asserting that the legislation would not alter certain reserved rights associated with federal and tribal lands.
Congress, various states, and other stakeholders recently have focused on the potential for using surface water to recharge aquifers and the ability to recover stored groundwater when needed. Some see aquifer recharge, storage, and recovery as a replacement or complement to surface water reservoirs, and there is interest in how federal agencies can support these efforts. In the congressional context, there is interest in the potential for federal efforts to facilitate state, local, and private groundwater management efforts (e.g., management of federal reservoir releases to allow for groundwater recharge by local utilities). The two primary federal water resources agencies are the U.S. Bureau of Reclamation (Reclamation) and the U.S. Army Corps of Engineers (USACE). No significant federal restrictions apply to Reclamation's authorities to deliver water for purposes of aquifer recharge, storage, and recovery. USACE authorities also do not contain restrictions on the nonfederal use for groundwater recharge of water stored or released from USACE reservoirs. Both agencies acknowledge that some state restrictions affect the use of the delivered or stored waters for groundwater activities. Some legislative proposals would provide the agencies with additional directives and mechanisms regarding their authority to support nonfederal groundwater recharge.
Other federal agencies support activities that inform groundwater management. For example, the U.S. Geological Survey monitors and reports groundwater conditions across the country, develops groundwater models and software tools for characterizing aquifers, and provides long- and short-term forecasts of changing groundwater conditions as part of local and regional groundwater studies. The National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration also make observations and collect data that are relevant to groundwater monitoring and assessment. The U.S. Department of Agriculture collects groundwater data related to irrigation.
Long-term changes to the climate affecting the United States, particularly rising temperatures and changes in the patterns, quantities, and type of precipitation (i.e., rain versus snow), could affect the availability of groundwater in the future. Other factors, such as changes to land use, irrigation practices, and patterns of water consumption, also may influence groundwater supplies.