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As climate change worsens, and with populations rising worldwide, water shortages are a top threat to human development and security.

people on Earth face shortages of water for drinking, sanitation, agriculture, and economic development. Water scarcity is in regions like the Middle East and North Africa, which 6% of the global population but only 1% of the world¡¯s freshwater resources.

Conventional water sources ¡ª which rely on snowfall, rainfall and rivers ¡ª are not enough to meet growing freshwater demand in water-scarce areas.

Fortunately, Earth has other sources of water: millions of cubic kilometres of water in aquifers, in fog and icebergs, in the ballast holds of thousands of ships, and elsewhere.

Our book, , based on the most up to date information, identifies eight broad categories of unconventional water sources.

Unconventional water sources

Cloud seeding and fog collectors

The atmosphere contains 13,000 km? of water vapour. Annual global freshwater demand today is roughly

Some of the atmosphere¡¯s water vapour can be captured through  ¡ª sowing clouds with small particles of commonly used silver iodide to make them rain or snow ¡ª and the collection of water from fog and mist

Cloud seeding can enhance rainfall by under the right conditions. Direct delivery of seeding material to the clouds using aircraft and rockets gets the highest yield.

Fog harvesting is in parts of the world. Remote communities in Chile, Morocco and South Africa have used vertical mesh nets to harvest fog for over 100 years. Viable sites are open locations with a fairly high elevation, exposed to wind flow.

Advancements in materials and local knowledge have helped develop designs that are efficient in water collection. At times 20 litres can be collected on a dense fog day for every square metre of mesh. Average cost per litre can be less than one US cent.

Desalination

Desalination ¡ª removing salt from seawater ¡ª contributes 100 million cubic metres of water a day, supporting about 5% of the world¡¯s population. Almost half (48%) of the global desalination capacity is located in the Middle East and North Africa region.

in desalination will likely make it the lowest-cost unconventional water supply resource worldwide. Innovative technologies are reducing energy inputs by 20% to 35%.

Desalination produces enormous quantities of brine, a pollutant of concern. But extracting salts from brine to yield commercially viable products the cost of desalinated water production in the next decade.

Reusing water

Advanced treatment systems can convert wastewater into potable water. Treated wastewater 25% of the potable water supply of Windhoek, Namibia¡¯s capital, for example.

Today of municipal wastewater in high-income countries is treated, but only 8% in low-income countries. The annual volume of untreated municipal wastewater in low-income countries globally is estimated at just 171 km?. This is because water use per capita in the municipal sector is low. Sub-Saharan Africa the lowest annual amounts of wastewater per capita (46m?); North America produces almost five times more.

Acceptance of reused wastewater by people and policymakers remains a challenge.

Agricultural drainage water

Irrigation generally results in two types of drainage water: water on the surface, and water that seeps into the earth. Surface runoff can be collected and used again to grow food. Salinity of drainage water is higher, but salt-tolerant crops and new varieties can meet this challenge.

Brackish groundwater offshore

There are vast quantities of water (an 300,000¨C500,000 km?) in aquifers off the shores of continents around the world. These aquifers (bodies of permeable rocks that hold groundwater) were created millions of years ago when sea levels were much lower. They are at shallow depths and less than 100km from shore.

Today new marine electromagnetic exploration methods provide detailed images of offshore freshwater. Horizontal drilling technologies make it possible to pump the water to shore.

To date, no offshore freshwater resources have been developed. The technology is still quite new and exploiting the resource would be expensive. It would also need to be combined with desalination.

Inland brackish groundwater

Deep inland aquifers with brackish or salty water exist in volumes estimated to total . Some countries, like Israel and Spain, already tap into them. It¡¯s expensive, but there are ways to reduce high costs, such as reusing the salt recovered. And farmers can benefit from technologies by switching to high-value crops.

Micro-scale capture of rainwater

In dry environments of rainwater is typically lost to evaporation and surface runoff. Micro-catchment rainwater harvesting is an ancient practice designed to trap and collect water from a relatively small catchment area, usually 10¨C500m?. It employs a wide range of techniques, from rooftop and cistern collection to farm and landscape systems including contour ridges, bunds, small runoff basins and strips.

Move water physically to water-scarce areas

Ships transport around of the goods traded worldwide and discharge some of ballast water (10km?) every year. Ballast water is fresh or saltwater held in the ship to provide stability and manoeuvrability during a voyage.

Under international , all of 400 gross tonnage and above must have onboard treatment options to desalinate ballast water, remove invasive aquatic organisms and unhealthy chemical compounds, and make it usable for other economic activities such as irrigation.

This water could be sold to port cities in arid regions.

Another water source that can be physically moved to water-scarce areas is ice. The more than 100,000 Arctic and Antarctic icebergs that melt into the ocean each year freshwater than the world consumes.

A financial feasibility of towing icebergs to Cape Town, South Africa suggests it is an economically attractive option if the icebergs to be towed are big enough: at least 125 million tons. Wrapping icebergs in a net and then a mega-bag prevent breakup and reduce melting, studies suggest.

Increasing water scarcity is a , social unrest and migration. Water is also being seen as an instrument for international cooperation to achieve sustainable development. It¡¯s vital to tap into every available option.

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