Groundwater levels are declining at rapid and accelerating rates in numerous aquifers around the world, but the decline can be reversed in some cases, finds a new study involving researchers from University of California at Santa Barbara (UCSB), UCL and ETH Zürich.
The research, published in Nature, analysed measurements taken over the last two decades from 170,000 wells in 1,693 aquifer systems across more than 40 countries. The team found that groundwater levels are declining by more than 10 cm per year in 36% of the monitored aquifer systems and are rapidly declining by more than 50 cm per year in 12% of them, with the most severe declines under cultivated lands in dry climates.
Groundwater supplies roughly half of the water used for drinking and irrigation worldwide and sustains rivers and streams in the absence of rainfall. Depleted aquifers cause land subsidence which can damage infrastructure. In coastal environments, this depletion can induce seawater to intrude and contaminate freshwater wells.
Co-author Dr Mohammad Shamsudduha (UCL Institute for Risk & Disaster Reduction) said: "This is the most comprehensive analysis of global groundwater levels published to date. Unlike satellite observations that make coarse approximations of groundwater storage changes over areas the size of the United Kingdom, groundwater levels from monitoring wells provide a direct measure of groundwater resources at the scale at which groundwater is used."
In addition, the researchers found that groundwater-level declines have accelerated over the past four decades in 30% of the world's regional aquifers.
Co-author Professor Richard Taylor (UCL Geography) said: "This widespread acceleration in groundwater-level declines not only threatens drinking water supplies and global food production but also our use of groundwater to adapt to the amplification of floods and droughts caused by climate change."
The team also examined examples where depleted groundwater resources recovered following human interventions. In a series of accompanying case studies ranging from Albuquerque, New Mexico to Bangkok, Thailand, the researchers analyse the practical and policy actions taken to replenish groundwater levels. They highlight how interventions including the successful implementation of water conservation policies, water transfers between basins, and the use of surface water and floodwaters, can replenish depleted aquifers.
Lead author, Dr Scott Jasechko of UCSB, said: "This study shows that humans can turn things around with deliberate, concentrated efforts."
The study focuses on areas where long-term groundwater-level records exist. The 1,693 monitored aquifer systems examined in this study account for about 75% of all groundwater withdrawals worldwide. However, in some regions including tropical Africa and Latin America long-term records are more limited and further work is required to better understand groundwater-level trends in these environments.
This research was supported by the National Science Foundation, the Canadian Institute for Advanced Research, and the Zegar Family Foundation.
