Andean mountain hydrology

By Rike Becker

Andean glaciers and snow fields are shrinking due to rising temperatures, but what exactly does this mean for hydrology, for future freshwater availability and water security further downstream?

There is no universal answer to this question. This is particularly true when we look at the mega-diversity of Andean mountain river catchments. In some basins, particularly in the dry Andean regions where water scarcity is already high, a small decrease in streamflow due to reduced glacier and snow melt may have a big impact on water security. In other basins where water is abundant, a big change in glacier and snow melt runoff may not make any difference at all. It is therefore important to look at the different regions and natural environments in more detail.

From north to south, Andean mountain catchments range from tropical to desert to high mountain glacier catchments, and finally, to catchments hosting large icefields in Patagonia, such as the large South Patagonian Icefield. Each of these natural environments has their own particular responses to changes in snow and glacier melt, which controls water availability for energy production, irrigation and drinking water for downstream societies.

climatology and glaciology along the Andes. — The climate zones of the Andes and the different controls upon them. Figure courtesy of Deplete and Retreat. Numbers on map refer to catchments studied as part of Deplete and Retreat.

A few general observations can be made for each of these environments.

Andean glacierized tropical catchments

Water availability in high Andean tropical mountain catchments is mainly controlled by precipitation during the wet season and by water released by glaciers, lakes, wetlands or groundwater during the dry season. In these regions, there is no secure and long-lasting freshwater supply from snow melt. The reason is a missing “winter season”, with long-lasting cold temperatures.

The climate in tropical mountain regions has a very strong diurnal cycle and can best be described as “it is summer every day and winter every night”. Hence, solar radiation is high throughout the year and there is not much seasonal variation in temperature. Snow can therefore not accumulate to build a seasonal snowpack which could supply water during dry seasons. Water availability during dry periods, when it’s most needed, is therefore highly dependent on glacier meltwater and other long-term water storages, such as lakes or wetlands (figure below).

Hydrology of the tropical Andes includes wetlands (bofedales), mountains, glaciers lakes. — Wetlands (‘bofedales’) in the valley bottoms at the foot of the Auzangate range, in the Chilca valley near Cusco. Wetlands can act as important water stores and can buffer low flows in dry seasons. Image credit – Anthony Ross, Imperial College London.

In recent decades, increasing meltwater from tropical glaciers in the Andes has led to a temporary increase in dry season runoff (Vuille et al., 2018). However, the predicted continuous shrinking of glaciers will lead to a reduction of this important water supply once peak water has passed. Alternative ways for water storage (e.g. increasing nature-based or human-made water storage capacities) must be found to ensure a secure water supply in these regions.

Andean glacierized desert catchments

In contrast to the tropical catchment, desert catchments receive very little rainfall. Our ‘Deplete and Retreat’ study region of the Copiapó catchment in North-central Chile for example, receives less than 50 mm/year (Favier et al. 2009). Here, most of the precipitation occurs in winter, either as rain or as snow. The summers are dry and often experience months-long periods without any rainfall. During these extended dry periods, snow and glacier melt are the main suppliers of freshwater.

The constant increase in water demand of populations further downstream (e.g. due to large agricultural expansion) has led to declining river flows and an overuse of water resources. This makes catchments in this region highly sensitive to further reductions in glacier and snow melt. Storing and saving water during wet periods to overcome long dry spells is becoming increasingly important.

Study catchments in Deplete and Retreat. Glaciers shown in red.

Andean mountain glacier catchments

Going further south along the Andean mountain chain, precipitation increases, particularly in winter months. Colder winters are leading to higher accumulation of snow and ice, which provide an important freshwater source during long and dry summer periods. During dry summers, glacier and snow melt can contribute substantially to river runoff and secure freshwater resources for a constantly growing population in this area.

For the past decades, glaciers have been shrinking in this area, causing reduced river runoff (Ayala, 2020). Hence, a secure water supply for dry periods is already at risk. This is of particular importance in our ‘Deplete and Retreat” study areas, in the catchments of Mendoza, Maipó and Rapél. These areas are home to large urban settlements in Chile (Fig. 2) and Argentina and host important agricultural areas, which provide food for the local population as well as valuable cash crops for international markets.

Maipó river catchment and the megacity of Santiago de Chile, which demands an immense, constant and secure water supply. Image credit – Rike Becker, Imperial College London.

Glaciar universidad in Rapel catchment. — Glaciar Universidad, site of AWS installation, Rapel catchment. Image credit: Bethan Davies

South Patagonian catchments

In the most southern part of South America, mountain regions receive abundant precipitation and snowfall, mainly during the wet winter season. A combination of rainfall, snow- and glacier melt provides the Patagonian rivers with plentiful water, particularly along the humid western slopes of the Andes. Yet, shrinking glaciers and particularly the fragmentation of glaciers, give reason to carefully study the long-term impact on water supply.

Glacier ‘Grey’ – part of the South Patagonian Icefield which is experiencing increasing glacier detachments. Image credit – Rike Becker, Imperial College London.

The drier eastern slopes of the Andes (for example our case study region of the Santa Cruz catchment) are highly dependent on melt water from the South Patagonian Icefield and changes in glacier melt will have important consequences for fresh water supply for vulnerable rural populations further downstream.

Developing suitable adaptation strategies for mountain water security

Despite their mega-diversity, all of the above-mentioned natural environments are affected by changes in glacier or snow melt. While some areas in the Andes might still experience increasing glacier melt in the upcoming years before fresh water supply from glaciers and snow will reduce, other areas are already experiencing decreased glacier and snow melt. Strategies to adapt to this reduction in water supply and to mitigate its negative impact are therefore becoming increasingly important.

There is not much we can do on a local level to stop glacier shrinkage, yet there are measures we can implement to buffer the lack of glacier and snow meltwater and to reduce the negative impact on water security. To find suitable strategies it is important to not only look at the difference of glacier or snow melt changes from north to south, but also along the course of the river (i.e., upstream or downstream). Depending on the location within a basin, the impact of reduced glacier or snow melt might differ markedly. The magnitude of impacts also depends on the communities being affected and their capabilities of establishing mitigation strategies (Drenkhan et al., 2022).

Within the Deplete and Retreat project, a team at Imperial College London will investigate how water security in Andean catchment is affected by glacier and snow melt and which measures can be taken to increase water security. The team will focus on investigating nature-based solutions and their benefits for local water availability.

References

Ayala et al. (2020)

Drenkhan et al. (2022)

Favier et al. (2009)

Vuille et al. (2018)

About the Author

Rike is a passionate hydrologist and PDRA at Imperial College London. She wants to better understand how climate change alters hydrological processes and how these changes affect future water availability. To improve the understanding of these rapidly changing hydrodynamics, Rike likes to combine the use of hydrological models with in-situ and remote sensing data. Within the DaR project she will contribute to Work Package 4, which goal it is to find out how changes in glacier and snow melt will affect downstream water availability.