Water towers threatened

The earliest agricultural civilisations in the Middle East were built around managing mountain water supplies, holding back water to irrigate lowland areas. Mountain water is still critical in many regions. But it is increasingly threatened by the impacts of climate change, Anthony King reports

 ‘The high mountains contribute over 50% of freshwater supply for the world’s populations,’ says John Pomeroy, director of the Global Water Futures Programme at the University of Saskatchewan, Canada.

As moist air rises, its temperature drops and it can hold less water vapour, which then falls as precipitation. At high altitude, such water freezes in snow and glaciers, especially over winter. Then, as spring and summer approaches, the glaciers melt and can provide a fairly constant supply of water to downstream areas.

‘Mountain water towers are large topographic features with snow and ice,’ explains Bethan Davies, geologist at Royal Holloway University of London, UK. ‘Mountain processes cause lots of precipitation and then store it, releasing it in the dry season.’

An international team of scientists that included Davies recently ranked 78 water towers around the world (Nature, doi: 10.1038/s41586-019-1822-y). The team estimated that 1.6bn people live in places that receive water from such water towers.

The ranking depended on how much water was in the mountains and also the demand for water from irrigation, industry and domestic uses. The highest ranking was the Indus Basin in Asia, due to its prodigious water resources in the Hindu-Kush, Himalayan and other mountain ranges, combined with densely-populated and intensively-irrigated downstream areas, the report notes. The research predicts demand for water there will rise exponentially in the coming decades.

‘The Indus has the largest irrigation schemes in the world, but the climate there is very dry,’ says study scientist Walter Immerzeel, mountain hydrologist at Utrecht University in the Netherlands and a lead scientist on the study. ‘All that water comes from very large reservoirs there in the mountains.’ Especially important is that water towers like this ‘regulate water downstream and produce a more dependable supply of water and provide a buffer against drought,’ explains Davies.

Almost all the top water towers were based in Asia. Other significant water towers include the Fraser and Columbia river basins in North America; the Patagonian Andes and Cordillera Principal in South America; and the Alps in Europe. Some water towers such as in Iceland and the Tibetan Plateau hold colossal quantities of water but demands from people are low.

However, where mountain water is in big demand, that demand is often increasing. The Indus basin is home to 235m people, but this is projected to rise by 50% by 2050, while its GDP is predicted to jump eight-fold.

Meanwhile, there is the threat of climate change. This is especially true for water towers in South America and Asia, according to the study. ‘The most important water towers are also the most vulnerable ones,’ says Immerzeel.

The most important water towers are also the most vulnerable ones.
Walter Immerzeel mountain hydrologist at Utrecht University, the Netherlands

The average annual temperature in the Indus water tower is predicted to increase by 1.9°C between 2000 and 2050. ‘The key vulnerability is probably climate change, because mean temperatures are rising everywhere and glaciers are retreating as a result,’ says Immerzeel. ‘In all mountain regions, you see that the temperatures are rising faster than the global average.’ The melting of mountain glaciers is well under way. According to the International Global Panel on Climate Change, glaciers outside Greenland and Antarctica lost mass at an average rate of 220Gt/year from 2006 to 2015. ‘Many glaciers have lost over half of their volume since the 1960s,’ says Pomeroy. ‘They’re already becoming insignificant in areas such as the Pyrenees in Spain and Rocky Mountains in the US.’

The good news is that because of the laws of physics, mountains are always going to push air laden with water vapour to high altitudes and thereby be excellent generators of precipitation. Moreover, many climate models predict an acceleration of hydrological cycles and more precipitation in the mountains in future. Rain will continue falling on the mountains. ‘In some mountain ranges, particularly as you go further north in the Northern Hemisphere, there will be increasing precipitation with a warming climate, and that precipitation can still fall as snow,’ comments Pomeroy. ‘It’s not a given that mountains are going to dry out.’

However, there may be a problem with timing of water supplies to downstream areas. Many areas of the world rely on meltwater from snowfall that is built up at higher altitudes that then melts in late spring and summer. ‘This provides a reliable source of water,’ says Pomeroy. In Europe, this is true of water from the Alps, which flows into the Rhine and Rhone and many other rivers. More than 25% of the water flowing from the river Rhone into the Mediterranean Sea in August originated in alpine glaciers, it has been estimated. In Bolivia, the snow melts shortly after falling, but the glaciers hang on and keep melting during the summer, explains Pomeroy. ‘And the summer period is very, very dry, so that glacier water is of extremely high value,’ he adds.

Key to the glaciers and ice is that they can slowly water the land at lower elevations. Also, mountain snow packs can have a critical role in forming a dam and holding back the water for months at a time. Pomeroy and colleagues in Spain recently investigated how climate change will impact mountain snow packs and spring melt in 44 mountain areas worldwide (Environ. Res. Lett., 2020, 15, 114006). Mountains in coastal areas, especially at mid-latitudes, were most impacted by climate warming. ‘With warming, we predict a shift from snowfall to rainfall in many mountains, and that is causing the snow damming effect to be lost,’ says Pomeroy.

Also, melting glaciers can provide a misleading signal for communities downstream by increasing water flow. Once this passes a point called ‘peak flow,’ the glacier is consuming older ice and will no longer maintain the expected flow of water. In Central Asia, says climatologist and glaciologist Maria Shahgedanova at the University of Reading, UK, this is predicted to occur by the 2040s. ‘Glaciers will shrink to a point when peak flow has been passed and then summer flow will decline,’ she explains.

For now, melting glaciers in countries such as Kyrgyzstan and Uzbekistan are not visible downstream. ‘They are losing water fast, and probably we are seeing in some regions an increase in water discharge, and that deflects attention away from the problem,’ says Shahgedanova. It has been estimated that glaciers in the Tien Shan have lost 27% of their mass between 1961 and 2012 (Nature, 2015, 8, 716). These mountains make up 80% of Kyrgyzstan.

The confluence of the Indus and Zanskar rivers in Leh Ladakh, India

In Central Asia, an apparent abundance of meltwater has encouraged wasteful use of water for irrigation. ‘Sometimes you drive along the foothills of the Tien Shan mountains and you see fields simply water logged, because of poor maintenance of irrigation systems and a wasteful approach,’ says Shahgedanova, referring to lands near the city of Almaty in the southeast of Kazakhstan.

Shahgedanova says in Central Asia, snowmelt provides water in May and June, after which glacier melt begins to supply water in July and August. If glaciers melt, then there could be too little water for agriculture during the hottest time of year. The same situation will occur elsewhere too. ‘Glaciers are particularly good drought fighters,’ Pomeroy explains. ‘They melt most quickly in the hottest and driest summers, so the conditions when you need water most. Then in cold and wetter years, they tend to accumulate, so they’re a great regulator of water supplies and we are losing them.’

Glacier meltwater is less important in the Canadian Rockies than in Central Asia, the Indus plain and much of South America. Nonetheless, Pomeroy says the impact is not negligible in North America. When he runs hydrological models for the Canadian Rockies, snowmelt and runoff occur in April instead of June and trees must live without so much water later in summer. ‘In many of our simulations, vegetation just runs out of water in late summer. Then it is dry and is more liable to burn, such as we’ve seen in California [with forest fires],’ says Pomeroy.

There are still uncertainties in hydrological models for high mountain regions of the world. Even regional climate models take grid squares of about 40km2 averaged to obtain an altitude data point for climate simulations. This could lead a model to miss altogether Snowdonia in Wales, for example, since the nearby sea would be included in elevation averages. ‘Even in the Himalayas the high mountain peaks are missed quite badly, so you start off underestimating their elevation by at least half, probably more, and therefore underestimate the snowfall and overestimate the air temperature’ warns Pomeroy.

Models are improving all the time, but need to get down to less than 1km resolution to better model the water cycle of high mountains, says Pomeroy. This requires not just raw computer power, but a better understanding of the processes controlling water supply, such as snow accumulation and redistribution by wind, avalanches, interception by forest canopies and faster melting on south-facing slopes in the northern hemisphere.

Since 2000, over 200,000 people have died in water towers as a result of natural disasters. Climate change and urbanisation is likely to worsen the effects of natural disasters such as rock falls, avalanches, floods and landslides. As part of an EU funded project, Immerzeel is studying Asian water towers using satellite observations and drones to monitor glacier dynamics. The aim is to reveal spatial differences in the water cycle in the Himalayan mountains to better predict future changes in water availability and extreme events.

Others say experts under-appreciate the staying power of rock glaciers in their models – the ice that is mixed with rock and moraine and which is often more resistant to melting than the pure glacier ice we picture atop mountains. Glaciers account for about 95% of all the freshwater stored on the planet. ‘As a glacier retreats, the mountain sides fall apart, and that actually plays a major role in shielding the remaining ice from further warming,’ says Stephan Harrison, professor of climate and environmental change at the University of Exeter, UK. Harrison recently took part in an inventory of rock glaciers in Nepal and recorded over 6,000 of them, with reserves of billions of m3 of water (Global and Planetary Change, 2018, 160, 123).

The presence of rock glaciers makes Harrison ‘slightly less concerned about glacier melt in the Himalayas,’ and he believes glaciers will be more resistant to a complete meltdown due to climate change than models currently predict.

Nonetheless, mountain hydrologists say steps towards reducing greenhouse gases will not save many of the glaciers or threats to water tower ice, with temperature rises now baked into our future climate. Therefore, there will need to be ‘mountain-specific conservation and climate-change adaptation policies,’ according to the Nature study, to ‘safeguard the mountain ecosystems and mountain people and simultaneously ensure water, food and energy security of the millions of people downstream.’

‘We will not solve the problem by reducing greenhouse gas emissions only,’ says Immerzeel. Countries will need to invest in efficient irrigation methods and also consider constructing reservoirs to replace glaciers that melt. ‘By creating a reservoir or building small dams, you can store water in the mountains, so like an artificial glacier basically, and so store the water and decide when to release it.’

In the Swiss Alps, as glaciers melt, it is freeing up areas that the Swiss are considering using for dams and reservoirs and to increase the generation of hydropower, but also to release water to agricultural areas. ‘Glaciers are frozen reservoirs. If they are replaced by lakes, they can still be used as reservoirs,’ comments Shahgedanova.

Another way to conserve water is to create lakes and protect a mountainous area as a national park, which will protect water quality and often provide a recreational resource for people to visit. With climate change, ‘people understand that sea levels might rise,’ says Davies, ‘but don’t think much about the impacts of glaciers melting away.’