Mountains that store water: Pariacaca from a satellite perspective

Following the observance of International Mountain Day on December 11, 2025, the attention of scientists and the public is once again directed toward fragile mountain ecosystems around the world. One of these is the Pariacaca mountain range in Peru, part of the Central Andes, observed from space by the Copernicus Sentinel-2 mission. The latest false-color image, captured on December 7, reveals how the high-Andean landscape is changing under the influence of climate, but also how important it is for the water supply of millions of people.

Pariacaca, also known as the Cordillera de Huarochirí, stretches across the Lima and Junín regions in central Peru. It is a segment of the Cordillera Central, the mountain range that forms the "backbone" of the country. The highest peak, also named Pariacaca, rises to about 5,750 meters above sea level and has been revered for centuries as a sacred "apu," a mountain spirit in Andean cosmology. Today, this same massif is crucial in a completely modern sense: as a natural reservoir of ice and snow that feeds important river systems.

The satellite photograph taken with the Multispectral Imager (MSI) instrument on Sentinel-2 uses the near-infrared channel to display vegetation in shades of red. In this way, high-Andean grasslands, shrubby vegetation, and rare forest remnants are separated from bare rocks, ice, and water surfaces. In the false-color display, red areas indicate living plant cover, darker shades belong to denser forest pockets, and lighter, almost orange shades indicate grasslands and pastures that follow valleys and lake shores.

Despite the impression of intense red color in the image, Pariacaca is actually a predominantly rugged and arid area. The climate is characterized by large daily fluctuations, long dry periods, and relatively modest precipitation. Vegetation is therefore adapted to extreme conditions: high-altitude grasses and hardy shrubs prevail, while wet pastures and smaller forest complexes appear in wetter valleys and along lakes. The dark red islands in the image reveal the presence of keñuala forests and other high-mountain species that rely on moist micro-locations.

The most striking motif in the Sentinel-2 image, however, are the white surfaces of snow and ice covering the highest parts of the ridge. The Pariacaca peak and neighboring peaks form a relatively compact ice system, whose glaciers, such as Nevado Pariacaca, descend down the slopes toward glacial lakes lower in the valleys. These masses of ice may not look huge compared to Himalayan or Alpine glaciers, but for central Peru, they play a decisive role: in the dry season, through melting, they ensure a stable flow of water into river courses that feed Metropolitan Lima and the agricultural valleys of the interior.

The satellite image reveals a series of dark and greenish spots – these are lakes, mostly of glacial origin. The highest ones, located directly below the ice fields, have a characteristic emerald or turquoise color. Such a shade is caused by tiny particles of eroded rock material that glaciers introduce into the water. These particles scatter light and give the lakes a unique glow. Lower lakes appear in darker shades, almost black, reflecting greater depth and a different sediment composition.

Pariacaca and the surrounding lakes are part of a protected area known as the Nor Yauyos-Cochas Landscape Reserve. This vast reserve zone, declared in the early 21st century, protects the upper courses of the Cañete and Cochas-Pachacayo rivers, rich high-altitude flora and fauna, and an impressive mosaic of waterfalls, canyons, and lagoons. For local communities, but also for the inhabitants of distant Lima, it is a "water tower" that collects precipitation water in the form of snow, ice, soil saturated with moisture, and natural reservoirs.

The concept of mountains as "water towers" is particularly pronounced in the Andean context. During the rainy season, glaciers and soil store huge amounts of water, while in the dry months, they gradually release melted snow and ice into rivers. In this way, extreme flow fluctuations are mitigated, and a relatively uniform water supply is ensured for irrigation, household supply, and energy production in hydroelectric power plants. In the case of Pariacaca, this cycle is of vital importance for the Lima-Cañete system and for the agricultural areas of the Junín region.

Unlike classic photographs, Sentinel-2 allows for quantitative monitoring of these processes. The MSI multispectral instrument collects data in 13 spectral channels of different spatial resolutions, from 10 to 60 meters. By combining visible, near-infrared, and shortwave infrared channels, scientists can precisely distinguish snow, ice, water, vegetation, and bare soil and track changes over time. In the case of Pariacaca, comparisons of images over decades clearly show glacier retreat and the expansion of gray, denuded slopes where there used to be ice.

Research based on satellite and archival aerial images confirms that in the Pariacaca massif, from 1970 to the end of the 2010s, more than half of the glacial surface area was lost. The retreat was particularly rapid during the 1980s and 1990s, when rising air temperatures accelerated the melting of tropical glaciers. Although the rate of retreat has partially slowed in recent times, the trend remains negative: the upper limits of the ice have retreated to higher altitudes, while the lower slopes have been left without permanent snow cover.

In the Sentinel-2 image, this process literally leaves a mark. Once icy zones now stand out as wide, gray-brown surfaces without vegetation. This is an area that is only gradually being colonized by pioneer plant species. Compared to the red-colored grasslands and shrubs, these denuded parts of the mountain act like scars in the landscape. At the same time, newly formed glacial lakes and expansions of existing lagoons testify to the connection between ice melting and the increase in water volume in natural reservoirs.

Glacier loss has a dual effect on water resources. In the short term, increased melting can briefly increase water flow downstream, which seems favorable. However, in the long term, as the glacial mass decreases, its capacity for water storage also decreases. at some point, the system transitions into a phase where rivers receive less water during the dry season than in the past. Scientists call this transition "peak water" – the moment when the annual water inflow from glaciers reaches a peak and then begins to decline.

In Peru, this process is already noticeable. National analyses show that the country has lost more than half of its total glacial surface area in the last sixty years or so, and some smaller glaciers have completely disappeared. Pariacaca may not be the most famous mountain range in the country – that role belongs to the Cordillera Blanca and renowned peaks like Huascarán – but data indicates that the same scenario is unfolding here as well: strong ice retreat, change in water regime, and increased risk of natural disasters.

One of the greatest threats are sudden floods from glacial lakes, known as GLOF (glacial lake outburst flood). As glaciers melt, lakes expand, and their natural dams – moraine embankments of loose rock and ice – become increasingly unstable. A minor earthquake, landslide, or ice avalanche is enough to disturb the balance and trigger a torrent that can rush down the valley, destroying bridges, roads, fields, and settlements. Given that communities dependent on agriculture and livestock farming live in the valleys below Pariacaca, the prevention and monitoring of such lakes are of immense importance.

Satellite data such as those from the Sentinel-2 mission are crucial for monitoring potentially dangerous lakes. By using repeated imaging, it is possible to measure the expansion of the water surface, track color changes that indicate silt inflow, and assess the stability of natural dams in combination with terrain slope data. Combined with radar and topographic data from other missions, a two-dimensional and three-dimensional risk image is created which then serves local authorities and agencies for protection planning.

The Pariacaca mountain range also has rich cultural significance. Even in pre-Inca times, the mountain was worshipped as a deity connected to rain, springs, and fertility. The Incas included it in their network of sacred places, and the ancient Qhapaq Ñan processional route passed precisely through these ridges, connecting the coast with the interior. Today, trekking routes run along the old paths, and visitors pass through villages where the Quechua language is still spoken and terraced fields with potatoes, quinoa, and other traditional crops are cultivated.

Traces of human presence can also be recognized in the frame of the Sentinel-2 image: narrow mountain paths, scattered shepherd's corrals, occasional buildings along the shores of lakes and waterfalls. Although the area looks almost uninhabited, it is a space of intense, but relatively low-intensity land use – livestock farming, small-scale agriculture, traditional fishing in mountain lagoons, and growing mountain tourism. Precisely these local forms of space utilization are directly dependent on the stability of the water cycle and the health of the glacial system.

The Nor Yauyos-Cochas Landscape Reserve encompasses a spectrum of ecosystems, from wet valleys to high mountain deserts, and even areas of perpetual snow. Condors, foxes, vicuñas, water birds, and numerous endemic plant species inhabit it. Including Pariacaca in this protected framework means that activities such as mining and uncontrolled infrastructure are strictly regulated. However, even the status of a protected area cannot stop global climate processes that accelerate the melting of snow and ice.

International Mountain Day 2025 is dedicated to the theme "Glaciers matter for water, food, and livelihoods in mountains and beyond". That slogan resonates directly in the story of Pariacaca. Glaciers in the Andean mountains feed not only high-altitude villages but also large lowland cities, agricultural valleys, and hydroelectric systems hundreds of kilometers downstream. In arid tropical zones, such as the coast of Peru, mountain ice and snow are often the only reliable source of fresh water.

Peru is, in this regard, one of the countries feeling the consequences of tropical glacier melting the most. Scientific estimates state that between the early 1960s and today, more than 50% of the national glacial surface area has been lost, while some glaciers have completely disappeared. This fact has far-reaching consequences not only for water supply but also for the risk of landslides, erosion, changes in vegetation zones, and loss of biodiversity. Pariacaca is, with its combination of ice fields, lakes, and steep valleys, a concentrated example of these global trends.

This is precisely why missions like Sentinel-2 become an indispensable tool in managing mountain areas. Unlike traditional field measurements that require long and logistically demanding expeditions, satellites enable frequent, standardized, and spatially continuous observations. This means that not only the state of glaciers can be monitored, but also vegetation changes, the expansion of agricultural areas, pasture degradation, or the consequences of fires. In the Nor Yauyos-Cochas Reserve, such data helps balance nature conservation and the needs of local communities.

On the Sentinel-2 image of Pariacaca, it is also easy to spot the hydrological connection of different landscape elements. White fields of snow overflow into a network of streams that feed dark blue and greenish lakes, from which the Cañete and Cochas-Pachacayo rivers then originate. These rivers then travel hundreds of kilometers, descending towards the Pacific coast or joining the vast Amazon river system. In that sense, every snowflake that falls on the Pariacaca ridge eventually becomes part of a much larger hydrological mosaic connecting mountains, lowlands, and oceans.

Pariacaca is also an increasingly sought-after destination for hikers and nature lovers who come from Lima and other Peruvian cities in search of silence and endless vistas. Trekking routes lead along old Inca trails, over passes above 4,500 meters, and along lakes in which the ice domes of the peaks are mirrored. But a visit to that area also implies facing the reality of climate changes: many local guides testify that the snow line "escapes" higher every year, and certain routes that once led over ice now pass over bare rock.

In the context of International Mountain Day, the view of the Sentinel-2 mission on the Pariacaca mountain range reminds us how mountains are simultaneously magnificent and vulnerable. They simultaneously store water for future months, feed rivers, support agriculture, protect biodiversity, and carry deeply rooted cultural meanings. But these very same ecosystems are among the first to be hit by climate change. Monitoring from space, combined with local knowledge and adaptation measures, is crucial to ensuring that the snow and ice of Pariacaca continue to feed springs, lakes, and rivers for the benefit of the communities that depend on them.