Global cases of groundwater recovery after interventions.

Groundwater depletion poses a challenge to irrigated agriculture and water access. Depleted aquifers can be refilled, but the interventions that have successfully refilled depleted aquifers are rarely reviewed. This work reviews 67 cases of groundwater recovery, where groundwater levels rose after a prolonged period of decline. The interventions that spurred groundwater recovery included policy changes, artificial groundwater recharge, and increased reliance on another water source instead of groundwater. Groundwater recovery can improve water access, restore ecosystems, slow seawater intrusion, and halt land subsidence. However, excessive groundwater recovery can waterlog soils, destabilize buildings, increase liquefaction risks, and intensify flood hazards. This Analytical Review describes how groundwater depletion trends have been reversed through interventions. Stakeholders and managers may consider adapting aspects of these interventions to address groundwater depletion elsewhere. Editor's summary: Groundwater is being depleted, often at rapid and unsustainable rates, in regions across the globe as agricultural, domestic, and industrial demands continue to grow. Is such groundwater loss inevitable? Jasechko reviews an array of examples that demonstrate that it is not and that well-designed human intervention can slow, stop, or even reverse depletion trends. Knowing which methods are effective in countering groundwater depletion could facilitate additional efforts to fight it, thereby mitigating land surface subsidence, water table deepening, reductions in crop yields, and seawater contamination of coastal aquifers. —Jesse Smith BACKGROUND: As the world's largest perennial and distributed freshwater resource, groundwater will play a central role in one of the most critical scientific and political challenges of this century: improving human access to fresh water for the production of food and safe drinking water while sustaining key ecosystems in a warming world. Groundwater is critical to irrigated agriculture and domestic water access. Unfortunately, unsustainable pumping is depleting groundwater reserves at rapid and accelerating rates, especially in cultivated drylands. This groundwater depletion poses a challenge to global urbanization and may represent the world's greatest threat to irrigated agriculture. Depletion trends can be reversed by human interventions, but the interventions that have refilled depleted aquifers have not been compiled and compared. This Analytical Review focuses on a central question that cuts across physical- and social-science studies of groundwater resources: How have groundwater depletion trends been reversed? ADVANCES: Recent work has highlighted that groundwater depletion is not an inevitability and that interventions can slow, stop, and even reverse depletion trends. This work profiles dozens of cases of groundwater recovery, where groundwater levels rose after a prolonged period of decline. These cases span a wide range of climate and land-use conditions and highlight how groundwater recovery can take place in both urban and rural areas and in both wet and dry climates. The interventions that drive groundwater recovery span three general categories. First, most groundwater recovery cases involve access to an alternative water source to offset groundwater demands. These alternative water sources usually involve diversions of rivers, highlighting the critical importance of coordinating surface-water and groundwater management. Because such diversions reduce river discharges, some groundwater recovery episodes have exacerbated water scarcity elsewhere. Second, about half of the cases involve changes to policies or market conditions that reduced groundwater demands, such as the implementation and enforcement of groundwater withdrawal fees or restrictions on pumping. Third, many cases of groundwater recovery involve artificial groundwater recharge. This artificial recharge can be deliberate (e.g., diverting river water into intentionally leaky ponds) or incidental (e.g., leaky irrigation canals). Altogether, many cases of groundwater recovery involve coincident implementation of multiple interventions, demonstrating the merits of multipronged strategies to reverse groundwater declines. OUTLOOK: Refilling depleted aquifers can have cobenefits such as halting land subsidence, slowing seawater intrusion, reducing drought vulnerability, restoring ecosystems, and improving water access. However, new challenges can emerge when groundwater rises to very shallow depths, such as waterlogged soils, destabilized buildings, increased liquefaction risks, and intensified flood hazards. Together, groundwater recovery cases highlight the benefits of intervening to stop groundwater depletion while also cautioning against overcorrections that risk overfilling aquifers. Scientifically, greater pan‑disciplinary study of interventions designed to halt groundwater declines can help identify transferable tactics to address groundwater depletion. Strategically, stakeholders and managers can consider borrowing and implementing components of the strategies reviewed here, which have proven to be successful in one or more cases, thereby enabling successful strategies of the past to empower ongoing efforts to combat groundwater depletion. Interventions that have reversed groundwater depletion trends.: Conceptual model of groundwater depletion (left), interventions (middle), and groundwater recovery (right). Interventions that have reversed groundwater depletion trends include changes to policies (yellow), use of alternative water supplies (blue), and the artificial replenishment of aquifers (red) (see legend for details). ILLUSTRATION: J. SPAHR [ABSTRACT FROM AUTHOR]