Before the first Americans carved out rectangular borders and called it “the Southwest,” before the Diné established their tribal homeland between the four sacred mountains, the area was water and immense layers of rock. Over millions of years, water cut through rock, forming the Colorado River. Today, the river’s drainage basin, into which precipitation collects, crosses seven states and supplies drinking and municipal water to 40 million people. However, the river also faces severe challenges. For two decades, the Southwest has been in a “megadrought” projected to continue beyond 2100. Driven by the continuous release of greenhouse gases, precipitation in the region has reached historically low levels. Higher temperatures have increased snow and surface water evaporation, further reducing river flow. Yet, even in the face of drought and desertification, US water law continues to allow and encourage unsustainable groundwater pumping. Southwestern groundwater regulatory frameworks must be made sustainable by aligning them with modern hydrological science.
When precipitation sinks through the ground, it fills an aquifer (water-permeable rock layers underlying a drainage basin) and becomes groundwater. The Southwest has resorted to pumping more groundwater as river flows decrease. Currently, the rate at which Southwestern states pump basin groundwater outpaces the rate at which precipitation replenishes it. This imbalance—called overdraft—is so severe that the basin lost 27.8 million acre-feet (over nine trillion gallons) from 2002 to 2024. Continuous overdraft destroys wetland habitats, reduces aquifers’ storage capacities, sinks the land above them, makes groundwater extraction more difficult and expensive, and opens groundwater to subsurface contamination by salt, chemicals, or industrial waste.
Additionally, as surface and groundwater are linked by the water cycle, groundwater overdraft can negatively impact river flows. During dry seasons, groundwater seeps into rivers, replenishing them. In turn, when surface water evaporates from rivers, it later becomes precipitation, which fills aquifers. Less evaporation means less precipitation and therefore less groundwater, forming a vicious cycle of depletion. Additionally, contamination in one form of water can affect the other correspondingly. Despite these inextricable, natural connections, legal frameworks in the United States regulate groundwater and surface water separately.
The fact that rivers cross state boundaries was obvious in the early 20th century, allowing the Colorado River to be governed as a public, interstate good. Since Congress enacted the Colorado River Compact in 1922, interstate surface water has been tightly regulated by the “Law of the River”: a complex interstate system of legal agreements, treaties, Supreme Court cases, and federal laws. The law gives each state a numerical, annual surface water allocation and enforces basin-wide goals (such as limiting depletion to 75 million acre-feet over 10 years in Upper Basin states to ensure water remains available for Lower Basin states). River disputes between states are litigated at the Supreme Court. Arizona, for instance, has sued California numerous times over water allotments. Since the river reaches Mexican states, the law even includes international water allocations and agreements. These regulations, while imperfect, allow for greater coordination between states.
Groundwater was historically less well understood than surface water. In the 1850s, Henry Darcy’s mathematical laws of flow sparked groundwater studies, yet, by 1900, the US Geological Survey only had three groundwater scientists. Now, we know that overpumping groundwater in one state can significantly reduce the surface water bound for another state. Groundwater itself also crosses state lines: 90 percent of US watersheds exchange groundwater with neighboring watersheds that cross state borders. Paradoxically, Southwestern groundwater remains managed at the state level, with highly inconsistent laws across states. When Americans colonized the West, territorial lawmakers linked the “right” to groundwater to property ownership of the overlying land. This principle also gave states free rein to boundlessly pump groundwater within their borders.
Lacking a shared framework, each Southwestern state regulates groundwater according to its own rules, with little consistency across borders. In New Mexico, for instance, lawmakers recognize links between surface water and groundwater, tightly regulating all in-state land and statutorily requiring special permits to pump any groundwater. The Office of the State Engineer controls both sources under the prior appropriations doctrine, under which the first party to claim and use water in an area has the dominant right to it (especially in times of scarcity). Next door, Arizona’s framework treats surface and groundwater separately. While surface water is governed by prior appropriations, groundwater pumping (in approximately 80 percent of Arizona’s area) remains virtually unregulated under the reasonable use doctrine. Since the 2010s, this framework has enabled for-profit water firms and industrial farming operations (some domestic, others Emirati or Saudi) to purchase thousands of acres in Arizona, where they can pump unlimited groundwater. Such pumpers mainly face civil liabilities, which remain largely unenforced against wealthy corporations. In contrast, Arizona’s managed urban areas tightly regulate groundwater pumping.
The unlimited pumping of groundwater by individual states impacts the whole Colorado River system. While some states have taken interstate disputes over “excessive groundwater pumping” to the Supreme Court, there exists no cohesive framework: This patchwork system is unsustainable. Congress must finally recognize that groundwater resources—and its effects—cross state lines and organize an interstate groundwater compact like the Law of the River. Such a groundwater compact, which should cap the annual groundwater quantity taken by states, appears constitutional. As Sporhase v. Nebraska designates groundwater an “article of commerce,” Congress could find such constitutional authority under the Commerce Clause.
Within this structure, state governments should also exert greater regulation over in-state parties’ groundwater use to ensure future clean municipal water. Some regulations of this type include banning for-profit water firms and foreign corporations from purchasing unregulated land, reforming the reasonable use doctrine, enforcing stricter pumping limits, and incentivizing domestic agribusinesses to use more water-efficient irrigation techniques. States could also pursue creative policies to replenish groundwater, such as flooding overlying land with fresh water from oceanic desalination or advanced wastewater treatment plants allowing water to soak into the ground and replenish the aquifers.
A federal compact would also make it easier to mediate water conflicts between tribal and state governments. Throughout history, the United States government has broken its resource-related contracts with Native American tribes. Today, that must change. Many tribal governments are denied both a federal articulation of their water rights and the infrastructure to pump groundwater, and nearly half of all tribal homes lack clean drinking water or sanitation. While still restricting unsustainable pumping, a federal compact should secure water for tribes that have historically been denied it.
In turn, updating groundwater law with an interstate framework could spur sustainable updates to other areas of water law. The law currently unsustainably overallocates Colorado River water. In the 1920s—an unusually wet decade—the river’s annual flow was measured as 16.4 million acre-feet (MAF). Using this quantity, the government allocated 15 MAF per year to the states surrounding it. Today, we know that the river’s real average flow from 1906 to 2024 was 14.6 MAF, a discrepancy (on the order of trillions of gallons) that continues to widen during the megadrought. Future river allocations must recognize this hydrologic reality.
Worldwide, groundwater overdraft is a problem and a paradox. Water is essential for life, so it should be freely accessible to everyone. However, without legal frameworks to limit endless pumping by corporate parties, water will become increasingly inaccessible to everyone. Since nature will not yield to man-made water laws, we must design them with respect for the laws of nature.