Water shortages, already affecting billions of people around the world, are expected to worsen in the coming decades - linked to drought, pollution, rising sea levels and poor management - but an "invisible" solution may be hiding underground.

With water usage seen rising by 1% each year over the next three decades, a U.N. report predicted on Monday that so-called groundwater will grow in importance as climate change and human exploitation shrink surface supplies like lakes and reservoirs.

Today, groundwater - which accounts for 99% of the planet's freshwater supplies - is poorly understood and consequently undervalued, mismanaged and even abused, according to the U.N. World Water Development Report 2022.

Globally, 3.6 billion people had inadequate access to water for at least one month of the year in 2018, and this figure is expected to top 5 billion by 2050, researchers say.

"What if the solution to the world's water problems is sitting there right under our feet?" said Richard Connor, editor-in-chief of the new report published by UNESCO.

"There is an enormous opportunity if we can manage and exploit all this groundwater sustainably," he told the Thomson Reuters Foundation.

As the global population grows, hiking pressure on water supplies, here's why we should pay more attention to the huge potential of groundwater and take steps to manage it properly:

Why is groundwater important and what are its benefits?
Only about 1% of water on Earth is freshwater - mostly found in ice caps - with the rest being saline, in the oceans.

Of the planet's liquid freshwater, 99% is found underground, where the quality is generally good. It can therefore be used safely, affordably and without requiring advanced treatment.

Water stored above ground, such as in reservoirs and dams, is a finite resource, often costly and vulnerable to pollution and climate change impacts like severe drought - and the ways it is exploited can have ecological and social consequences.

By comparison, 10-20% of groundwater renews naturally and is found at shallow depths, making it easily accessible.

The rest is "fossil water" that has been in the ground for thousands or even millions of years and, while not renewable, it is abundant.

Groundwater systems are important for supporting nature-rich landscapes such as forests, and provide about a quarter of all water used for farming, according to the U.N. report.

Underground supplies also account for about half of the water used domestically by the world's population and are the cheapest source of drinking water for rural villagers, most of whom are not connected to public or private supply systems.

How are groundwater supplies abused, and what are the consequences?
Over-extraction can have dire consequences, including land subsidence and conflicts linked to scarce supplies.

In 2018, when India suffered what was seen as the worst water crisis in its history, a report by a government think-tank predicted that at least 40% of its 1.3 billion population would have no reliable access to drinking water by 2030.

Droughts are becoming more frequent as the climate heats up, creating problems for India's rain-dependent farmers, while disputes between states are on the rise.

In Indonesia's capital, Jakarta, meanwhile, rapid urbanization and disappearing water catchment areas mean most residents rely on wells that drain underground aquifers, causing the mega-city to sink by about 5-10 cm (2-4 inches) each year.

The planet's groundwater can be contaminated by improper sanitation and pit latrines, as well as industrial pollution from tanning, mining and agricultural chemicals.

U.N. report editor Connor noted that groundwater is less susceptible to pollution than surface supplies.

But once it happens, the contamination is hard to reverse, he said, calling for more action to protect groundwater by strengthening environment agencies, regulation and enforcement.

What are the challenges of tapping more groundwater, and how can they be overcome?
A region like sub-Saharan Africa has poorly developed water infrastructure and little irrigation for farming, leaving it dependent on increasingly erratic rainfall and vulnerable to drought - which can fuel famine, poverty and mass migration.

The region, along with the Middle East, holds significant groundwater reserves that are largely untapped and, if extracted in a controlled manner, could help maintain water security.

Governments must invest in water infrastructure and institutions, and train professionals, in order to access those reserves sustainably, the U.N. report said.

The development of groundwater sources could catalyze economic growth by expanding irrigated farmland and improving agricultural yields and crop diversity, it added.

Outside Australia, Europe and the United States, little data exists on groundwater, including how much is available at different depths, its quality and level of salinity.

But companies involved in oil, gas and mineral exploration often gather huge amounts of information on the underground - including the water it holds.

Corporate responsibility pledges by such firms could include sharing groundwater information with agencies responsible for managing it, to support sustainable use, said Connor.

"You have to have knowledge and data to know how much water (there) is, what its quality is ... but also where is it and how fast is it recharging?" he added.