More than 5 billion people could suffer water shortages by 2050 due to climate change, increased demand and polluted supplies, according to a UN report on the state of the world’s water.

The comprehensive annual study warns of conflict and civilisational threats unless actions are taken to reduce the stress on rivers, lakes, aquifers, wetlands and reservoirs.

The World Water Development Report – released in drought-hit Brasília – says positive change is possible, particularly in the key agricultural sector, but only if there is a move towards nature-based solutions that rely more on soil and trees than steel and concrete.

“For too long, the world has turned first to human-built, or ‘grey’, infrastructure to improve water management. In doing so, it has often brushed aside traditional and indigenous knowledge that embraces greener approaches,” says Gilbert Houngbo, the chair of UN Water, in the preface of the 100-page assessment. “In the face of accelerated consumption, increasing environmental degradation and the multi-faceted impacts of climate change, we clearly need new ways of manage competing demands on our freshwater resources.”

Humans use about 4,600 cubic km of water every year, of which 70% goes to agriculture, 20% to industry and 10% to households, says the report, which was launched at the start of the triennial World Water Forum. Global demand has increased sixfold over the past 100 years and continues to grow at the rate of 1% each year.

This is already creating strains that will grow by 2050, when the world population is forecast to reach between 9.4 billion and 10.2 billion (up from 7.7 billion today), with two in every three people living in cities.

Demand for water is projected to rise fastest in developing countries. Meanwhile, climate change will put an added stress on supplies because it will make wet regions wetter and dry regions drier.

Drought and soil degradation are already the biggest risk of natural disaster, say the authors, and this trend is likely to worsen. “Droughts are arguably the greatest single threat from climate change,” it notes. The challenge has been most apparent this year in Cape Town, where residents face severe restrictions as the result of a once-in-384-year drought. In Brasília, the host of the forum, close to 2m people have their taps turned off once in every five days due to a unusually protracted dry period.

By 2050, the report predicts, between 4.8 billion and 5.7 billion people will live in areas that are water-scarce for at least one month each year, up from 3.6 billion today, while the number of people at risk of floods will increase to 1.6 billion, from 1.2 billion.

In drought belts encompassing Mexico, western South America, southern Europe, China, Australia and South Africa, rainfall is likely to decline. The shortage cannot be offset by groundwater supplies, a third of which are already in distress. Nor is the construction of more dams and reservoirs likely to be a solution, because such options are limited by silting, runoff and the fact that most cost-effective and viable sites in developed countries have been identified.

Water quality is also deteriorating. Since the 1990s, pollution has worsened in almost every river in Africa, Asia and Latin America, and it is expected to deteriorate further in the coming two decades, mainly due to agriculture runoffs of fertiliser and other agrochemicals that load freshwater supplies with nutrients that lead to the growth of pathogens and choking algae blooms. Industry and cities are also a significant problem. About 80% of industrial and municipal wastewater is discharged without treatment.

Crucially, the report emphasises a shift away from watershed management towards a wider geographic approach that takes in land use in distant areas, particularly forests. Although farmers have long seen trees as a drain on water supplies, the authors recognise more recent studies that show vegetation helps to recycle and distribute water. This was apparent in the São Paulo drought of 2014-15, which the city’s water authorities and scientists have linked to Amazon deforestation.

The key for change will be agriculture, the biggest source of water consumption and pollution. The report calls for “conservation agriculture”, which would make greater use of rainwater rather than irrigation and regularise crop rotation to maintain soil cover. This would also be crucial to reverse erosion and degradation, which currently affects a third of the planet’s land, a different UN study found last year.

Perhaps the most positive message of the report is that the potential savings of such practices exceed the projected increase in global demand for water, which would ease the dangers of conflict and provide better livelihoods for family farmers and poverty reduction.

Nature-based solutions can be personal – such as dry toilets – or broad landscape-level shifts in agricultural practices. The report contains several positive case studies that show how environments and supplies can improve as a result of policy changes. In Rajasthan, more than 1,000 drought-stricken villages were supported by small-scale water harvesting structures, while a shift back towards traditional soil preservation practices in the Zarqa basin in Jordan are credited with a recovery of water quality in local springs.

The authors stress the goal is not to replace all grey infrastructure, because there are situations where there is no other choice, for example in building reservoirs to supply cities with water. But they urge greater take-up of green solutions, which are often more cost-effective as well as sustainable. They also encourage more use of “green bonds” (a form of financing that aims to reward long-term sustainable investments) and more payments for ecosystem services (cash for communities that conserve forests, rivers and wetlands that have a wider benefit to the the environment and society).

Audrey Azoulay, the director-general of Unesco, which commissioned the report, noted two-thirds of the world’s forests and wetlands have been lost since the turn of the 20th century – a trend that needs to be addressed.

“We all know that water scarcity can lead to civil unrest, mass migration and even to conflict within and between countries,” she said. “Ensuring the sustainable use of the planet’s resources is vital for ensuring long-term peace and prosperity.”

The World Water Forum is the biggest single gathering of policymakers, businesses and NGOs involved in water management. It is being held in the southern hemisphere for the first time, and is expected to draw 40,000 participants.

Among them are indigenous and other grassroots activists who believe the event is too close to government, agriculture and business. They are staging an alternative forum in Brasília that puts greater emphasis on community management of water as a free public resource.