Humans are depleting groundwater worldwide, but there are ways to replenish it

GROUNDWATER provides about half of the world’s population with drinking water and nearly half of all water used to irrigate crops.

It sustains rivers, lakes and wetlands during droughts.

Groundwater is a renewable resource, but it can take decades or even centuries for some aquifers to recover after they are depleted.

Current understanding of this challenge is based mainly on where and how frequently people record measurements of water levels in wells.

In a newly published study, scientists, water specialists and policy experts compiled the first global-scale dataset of these levels.

We analysed millions of groundwater level measurements in 170 000 wells located in over 40 countries and mapped how groundwater levels have changed over time.

Our study has two main findings. First, rapid groundwater depletion is widespread around the world and that rates of decline have accelerated in recent decades, with levels falling by 20 inches (50.8cm) or more yearly in other countries.

The research also reveals many cases where deliberate actions halted groundwater depletion.

These results show that societies are not inevitably doomed to drain their groundwater supplies, and that with timely interventions, this important resource can recover.

Many factors determine groundwater levels, including geology, climate and land use.

But groundwater levels that are dropping deeper and deeper in a particular location often signal that people are pumping it out faster than nature can replenish it.

Research has shown that groundwater levels have declined since the year 2000 in far more places than they rose.

In many locations, especially arid zones that are heavily farmed and irrigated, groundwater levels are falling by more than 20 inches per year.

The second and more concerning finding is that in most areas, the rate of groundwater decline is accelerating.

Accelerated groundwater decline is common in dry climates where large swaths of land are used for agriculture. This suggests a potential link between groundwater-fed irrigation and intensifying groundwater depletion.

Groundwater is an essential but under-appreciated resource worldwide.

Drinking-water supplies from wells and springs can run dry when groundwater levels decline.

People and communities who rely on those wells can lose access to what may be their sole source of accessible fresh water for drinking.

This problem will continue and worsen unless action is taken to stabilise groundwater reserves.

Wells that run dry can also threaten crop production. Groundwater depletion has long been viewed as one of the greatest threats to global irrigated agriculture, because wells supply nearly half of the water used for irrigation globally.

In areas where groundwater typically drains to rivers, falling groundwater levels can reverse this flow and cause rivers to leak into the subsurface.

This affects the river’s ecology and reduces water supplies downstream.

Leaky streams are more common where groundwater withdrawal rates are high, highlighting how groundwater pumping can directly reduce the amount of water that flows underground into nearby rivers.

Groundwater declines can also cause land surfaces to sink. Land subsidence has increased flood risks in dozens of coastal cities worldwide including in dozens of coastal cities worldwide, including Jakarta, Tokyo, Istanbul, Mumbai, Auckland and the Tampa Bay area of Florida.

Farther from the coast, land subsidence can damage infrastructure. It poses a critical challenge in areas where groundwater levels have declined, including Tehran and Mexico City. In many cases, the main culprit is excessive groundwater pumping.

Falling groundwater can cause seawater to move inland underground and contaminate coastal groundwater systems – a process known as seawater intrusion.

When seawater intrudes, coastal aquifers can become too saline to use for drinking water without energy-intensive desalina- tion.

How to replenish groundwater supplies There are places where groundwater levels are recovering. The strategies that communities used to replenish their groundwater sources include developing new alternative water supplies, such as local rivers; adopting policies to reduce demand for groundwater; and intentionally replenishing aquifers with surface water.

In Bangkok, so many private wells were drilled for domestic, industrial or commercial purposes between 1980 and 2000 that groundwater pumping doubled and groundwater levels fell.

Officials responded by quadrupling groundwater extraction fees between 2000 and 2006. Total groundwater pumping declined, and levels began to recover as users found other water sources.

In wet years, water managers in California, pump water out of rivers and canals into recharge basins. There, the water can sink slowly into the ground and recharge groundwater supplies.

To help replenish the depleted groundwater, leaky ponds can be constructed.

It is important to monitor groundwater levels in many locations.

With groundwater levels declining in many places, communities and businesses that depend on it need accurate information about their water supplies so they can act in time to protect them.