Running dry: How to store more groundwater for dry seasons

Many soils in Germany are far too dry. Rainfall was already lacking in spring, especially in March.

Even the partly above-average precipitation in some regions during April and May was unable to replenish the missing moisture in deeper soil layers. This is shown by the current drought monitor of the Helmholtz Centre for Environmental Research (UFZ).

The city of Munich in Germany's South has banned filling private swimming pools and watering lawns.

Following an extreme heat wave at the end of June, the next one is already torching Europe, putting even more strain on dwindling groundwater resources.

Anyone with a garden and a water tank knows the advantage of collecting rainwater and using it when rain is scarce. But groundwater also functions like a gigantic water reservoir.

Containing nearly half of the world's drinking water, groundwater forms when water seeps into the ground and accumulates above impermeable layers of rock or clay in the subsurface. It's often the only source of water in arid regions.

Groundwater levels are falling worldwide

Precious groundwater is currently being extracted at three times the rate it was 50 years ago.

The problem of more intensive water use is being exacerbated by more frequent drought linked to climate change. Even during heavy rains, the dried-out soil cannot absorb enough water.

More and more land is also being paved over, causing rainwater to run off the surface instead of seeping into the ground — meaning it can't be collected for use during dry periods.

Even in countries with relatively high rainfall, such as Germany, groundwater levels are falling significantly, with wells drilled deeper and deeper in some regions to reach water.

Groundwater reserves can only replenish if enough rain seeps into the soil. But groundwater overexploitation means that more water is extracted than can be restored.

Now more than 2 billion people and 40% of global agricultural production depend on overexploited groundwater reserves.

Targeted groundwater storage to combat drought and flooding

A research team led by hydrogeologist Thomas Baumann from the Technical University of Munich has developed a "Smart Storm Water Storage" device to tackle groundwater depletion.

"Our idea was to combine flood protection with drought prevention," said Lea Augustin, another hydrogeologist working with Baumann.

The facility is located in a hilly hop-growing region about 60 kilometers (about 40 miles) north of Munich, where heavy rainfall frequently causes flooding, and where groundwater is significantly depleted for drinking water and irrigation.

The water storage project works by first collecting water in a retention basin during heavy rain. It is then purified, and pollutants are removed. The clean water then flows through pipes to so-called infiltration wells that channel it about 30 meters (100 feet) deep into the ground. As the groundwater level rises, the water can be pumped back up via wells.

Groundwater does not evaporate

The Namibian capital, Windhoek, is also working to replenish groundwater. The city is located in the country's arid highlands where rainfall is scarce. By the late 1960s, the groundwater was already considered overexploited.

At the time, Windhoek responded by becoming one of the world's first cities to convert wastewater into drinking water.

And since 2002, infiltration wells have been used to channel this treated water into the ground. Water sourced from sometimes distant reservoirs is also used to replenish groundwater.

The advantage is that far less water evaporates underground compared to open reservoirs, allowing better retention of scarce water resources.

California relies on groundwater storage

More and more groundwater remediation techniques, sometimes known as managed aquifer recharge, are being developed worldwide.

These include the redirection of excess rainwater or floodwater into ditches or large basins before it slowly seeps into the ground. The method is less expensive than technical systems but requires a lot of space. It also takes longer for the water to reach the aquifer.

Such infiltration basins exist in numerous worldwide locations, including along the San Gabriel River in the Los Angeles metropolitan area.

The drought-stricken US state of California views aquifer recharge as a key strategy for coping with climate-related weather extremes, according to the state water agency. In 2023 alone, more than 4.1 million acre-feet of water — equivalent to more than 5 billion cubic meters — was injected into aquifers, the agency told DW.

Flooding fields to increase groundwater

Another option is the targeted flooding of agricultural land when extreme rain threatens, allowing floodwaters to be captured and directed into the groundwater. The advantage: The land can continue to be used for agriculture after the water has seeped away.

Since farmland and pastures account for about 40% of the world's land area, this method has great potential. However, it only works under certain conditions.

The plants must not be damaged. Furthermore, no pollutants or pathogens from the floodwaters must enter the groundwater, making implementation difficult.

River restoration replenishes groundwater levels

Nature itself can also help. When rivers are restored, they expand. They can then spread out during floods, creating shallow side channels and pools.

These capture floodwater, some of which also seeps into the ground. Floodplain microorganisms also filter the water, meaning the process both replenishes and purifies the groundwater.

At the same time, the risk of flood damage decreases.

Harvesting groundwater directly from rivers or lakes

In Germany, drinking water is often obtained indirectly from rivers through a process known as bank filtration.

Here, groundwater is pumped out of wells located near rivers or lakes, creating a vacuum. Water from the river then flows in to replace it. As it passes through the ground, it is naturally filtered. Bank filtration provides more than half of Berlin's drinking water.

Infiltration weirs also channel river water into the groundwater. These are structures in the riverbed that reduce the flow velocity, or temporarily dam up the river. When the water flows more slowly, more of it can seep into the ground.

In arid regions such as Kenya, small so-called sand dams are constructed at several points along the river's course. They are designed to retain the sand that rivers carry during the rainy season. The resulting layer of sand at the dams serves as an additional aquifer, further filtering the water as it seeps through the gaps in the sand into the groundwater.

Through wells downstream, the groundwater is then drawn up even during dry seasons. Here, too, groundwater storage results in significantly less loss through evaporation than with aboveground storage. This benefits people, animals and vegetation, which recovers when the groundwater level rises.

This article was originally written in German.