Glacier mass balance is a key variable for understanding the response of glaciers to climate change and their contribution to sea level rise [1
]. Traditionally, glacier mass balance has been obtained by the glaciological method, which is based on in-situ measurements of accumulation and ablation using snow pits and stakes [1
]. However, the geodetic method has become widely used in recent years, due to the possibility to monitor large glacierised areas (e.g., [4
]). The geodetic mass balance can be derived, for example, from observations of elevation change derived from Global Navigation Satellite System (GNSS) measurements, airborne Light Detection and Ranging (LiDAR) (e.g., [6
]), digital elevation models derived from structure from motion techniques (e.g., [7
]), and optical images or interferometric synthetic-aperture radar (InSAR) (e.g., [4
In the central Andes of Chile, glaciers have undergone retreat and thinning over the last decades [4
], mainly due to atmospheric warming and a decrease in precipitation [15
]. Unfortunately, long-term and consistent glacier mass balance data are still limited in this part of the Andes, mainly due to logistic and cost constraints. Direct measurements of glacier mass balance on the Echaurren Norte Glacier are the only long-term reference for the Southern Andes (1975 to present) and show an overall negative trend [21
]. These records are consistent with several recent studies that have presented negative glacier mass changes for specific areas and for the entire central Andes of Chile and Argentina [4
]. These studies have provided an updated overview of the regional glacier mass balance and its relationship with climatic fluctuations.
Since 2010, a persistent sequence of dry years has affected this region [18
], leading to an unprecedented rainfall deficit in the region laying from 30 to 38°S [19
] and hence, resulting in increased glacier thinning rates [10
]. This precipitation deficit has turned glacier meltwater into an essential hydrological source, further enhancing the role of glaciers as water reservoirs during dry periods [20
In this study, we provide the longest geodetic glacier mass balance record for glaciers in the Maipo River Basin, central Andes of Chile (Figure 1
). Our objective is to analyse the spatial and temporal distribution of elevation and mass changes between 1955 and 2013. For this, we use digital elevation models (DEMs) derived from topographic maps (1955), the SRTM C-band (2000), and TanDEM-X (2013). We complement this dataset with LiDAR measurements for fifteen glaciers, carried out in four campaigns between 2009 and 2015.
Our results revealed that negative values of glacier mass balance dominated between 1955 and 2013, but with some spatial and temporal differences in the responses of sub-basins and glaciers. While glacier mass balance showed negative and positive values in the different sub-basins and glaciers for the first study period (1955–2000), there was a generalised negative trend in the second one (2000–2013), with the exception of the Colorado sub-basin. After 2010, we observed that mass losses accelerate everywhere, with nearly all analysed glaciers showing high negative mass balance. The differences between these periods can be explained by a combination of precipitation variability and a sustained increase of air temperature associated with global warming. Glacier mass balance in the central Andes is very sensitive to precipitation, which is mainly related to El Niño Southern Oscillation (ENSO) events, where positive mass balances have been associated with El Niño events and vice-versa with La Niña events [21
]. However, at longer time scales, the Pacific Decadal Oscillation (PDO) and the Interdecadal Pacific Oscillation (IPO) play an important role in the climatic conditions over the Central Andes [59
]. It is noteworthy that at present, this region is being affected by a severe drought that started in 2010, with an annual rainfall deficit that ranges between 25% and 45% [19
]. Besides the precipitation deficit, recent studies have indicated an increase in the autumn and spring temperature of inland Maipo [61
]. Both factors may affect glaciers at a local and regional spatial scale, but also depend on the response time of the individual glaciers to those forcings, latitude, and the glacier characteristics, such as elevation and aspect, and the variability of ice melt [20
Our results represent the first estimation of glacier mass changes for the second half of the 20th century in this region. Between 1955 and 2000, the highest thinning rates in the Maipo River Basin were observed in the Olivares sub-basin. One possible reason for this is related to the location of this sub-basin, which is adjacent to two open-pit mines. These mines generate dust that can be transported and deposited over the surface of glaciers in Olivares. This process potentially leads to a lowering of the surface albedo, which in turn increases the absorbed incoming shortwave radiation and energy available for melt. It is beyond the scope of this work to assess and quantify the impact of the deposition of dust originated from mining activity; hence, additional studies are required. Another possible explanation for these differences between sub-basins is that the Maipo River Basin presents a shift between two hydroclimatic regimes, where an increment in the precipitation amounts towards south of the 34°S have been observed [60
]. This could also explain the observed north to south glacier mass balance gradient observed during this period (Table 2
and Figure 5
), where positive and nearly neutral mass balance was observed for Volcan and Upper Maipo for the period 1955–2000, respectively (Figure 5
). The difference between both sub-basins is related to elevation of the glaciers, where the Volcan sub-basin contains big glaciers that are located at higher elevations on the western and southern flanks of active volcanoes (Marmolejo and San Jose Volcano).
Throughout the second period (2000–2013), our results agree well with recent findings in the central Andes of Chile and Argentina [4
], including those focusing on individual glaciers (e.g., [20
]). Again, in the Olivares sub-basin, we obtained the highest negative glacier mass balances during this period. However, in this period, the latitudinal gradient was not observed, instead, we obtained similar negative glacier mass balance, with the exception of glaciers located in the Colorado sub-basin, where it seems that these glaciers are in balance with the present climate, likely due to their high elevation. This was also confirmed by the LiDAR measurements.
Comparing our results with those for the Southern Andes macro-region (RGI 17) [5
] can be misleading, since this macro-region encompasses the Patagonian Icefields which provide nearly all of the regional mass loss [4
]. In Figure 8
, we illustrate the glacier elevation changes of these previous studies that include glaciers of the Maipo River Basin from 2000 onwards. On a local scale, our estimate of glacier mass balance for the Maipo River Basin was more positive than that derived by the glaciological method for the Echaurren Norte Glacier. This glacier shows largely negative mass balance rates [21
], but the representativeness of the Echaurren Norte Glacier is still a matter of further analysis, since there are differences between the glaciological and geodetic methods. The geodetic mass balance of the Echaurren Norte Glacier [23
] was positive between 2000 and 2009, and this trend has been confirmed by other authors [10
]. The acceleration thinning rates based on LiDAR data indicates the role of the extensive drought that has been affecting the region since 2010 [19
]. It is likely that after 2013, the drought exacerbated thinning rates, which may also explain the differences between the two recent region-wide glacier mass balance records for the central Andes [4
]. In fact, observing other studies, it seems that the effects of the drought were not prominent before 2013 for glaciers located in the Yeso sub-basin [20
]. For instance, GNSS measurements on Bello and Yeso glaciers showed −0.24 and −0.17 m a−1
between April 2012 and March 2014 [63
], and measurements during the ablation period of 2012 and 2013 showed 0.67 and 0.44 m, respectively [64
]. The acceleration of thinning rates has also been observed by other authors [10
]. For instance, Hernandez et al. [65
] estimated thinning rates of −1.64 ± 0.08 m a−1
and −0.94 ± 0.10 m a−1
for the Olivares Alfa and Olivares Beta glaciers between 2013 and 2016 respectively, using airborne LiDAR. Our results also showed similar rates: Olivares Alfa Glacier: −1.61 ± 0.08 m a−1
and Olivares Beta Glacier: −1.39 ± 0.08 m a−1
. The slight difference may be related to the time span and the dataset coverage (Table 1
and Supplementary Figure S6
). Conversely, glaciers located in the Colorado sub-basin seem to be in equilibrium after an important reduction of their area size (Figure 3
), as we observed in our elevation change results (2000–2015) (Figure 7
and Table 2
). This may be related to the new mean altitude of these glaciers, which in the case of Yeso 1 and 2, are nearly located at 5000 m a.s.l., whereas in a prior period (1955–2000) both glaciers were part of a larger, lower-lying glacier (Figure 3
We also calculated thinning rates for several debris-covered glaciers during the second study period and we estimated that some of them are thinning at similar rates (Supplementary Figure S2
). Ayala et al. [26
] showed that the glacier runoff contribution of the low-lying, debris-covered Piramide Glacier is similar to that of the high-elevation, debris-free Bello and Yeso of the same sub-basin, suggesting that elevation differences and debris cover can explain the similar mass balance of these two types of glaciers [20
]. In fact, when comparing the Echaurren Norte Glacier and Piramide Glacier (debris-covered), which present a similar mean elevation (~3700 m a.s.l.), thinning rates at the Echaurren Norte Glacier are twice of those at the Piramide Glacier (Figure 7
). Hence, although the Piramide Glacier presents negative rates, the supraglacial debris clearly reduces the thinning compared with debris-free surfaces at the same elevation. Nevertheless, it is likely that ice cliffs and supraglacial lakes are playing a key role by enhancing the total ablation [66
]. Ice cliffs on debris-covered glaciers have been identified as “hot-spots” and hence as major contributors of mass loss, which outplay the insulating role of debris cover. This phenomenon was confirmed by the LiDAR measurements, which show strong ablation at several ice cliffs and supraglacial lakes with depths of up to 40 m on the Piramide Glacier (Supplementary Figures S3, S4, and S6
). Other debris-covered glaciers in the region were also displaying ice cliffs and supraglacial lakes in the SRTM and TanDEM-X elevation changes (Supplementary Figure S4
), but further monitoring studies with high-resolution data (e.g., LiDAR) are required to understand the role of ice cliffs in the mass balance of debris-covered glaciers at the basin or region scale in the central Andes.
We observed signals of surge in some glaciers, although not with the same magnitude as in other regions, e.g., Himalaya [68
]. Overall, only a few surging glaciers have been the focus of historically detailed studies in the central Andes, such as the Horcones Inferior [69
] and Grande del Nevado [71
] in Argentina, and Cachapoal Glacier in Chile [73
]. Other surge events were reported in the past by Lliboutry [35
], such as those of Juncal Sur (1947), Nieves Negras (1927), and Glaciar del Río (1935). However, we did not observe clear surge events in those glaciers between 1955 and 2000, with the exception of Nieves Negras Glacier. Instead, there were small signals of surge at Picos del Barroso, Loma Larga, Sierra Bella, Azufre (Tupungatito volcanic complex), and Oeste del Cerro Alto Glaciers (Supplementary Figure S5
). For the second period, our results also confirmed small signals of surge-type behaviour for a few glacier systems, as noted by Falaschi et al. [74
], in glaciers such as Loma Larga, Oeste del Cerro Alto, and Sierra Bella.
In this study, we have estimated the long-term region-wide glacier elevation and mass changes for the Maipo River Basin in the central Chilean Andes, which was previously sparsely investigated. We estimated a glacier mass balance of −0.12 ± 0.06 m w.e.a−1 with a total mass loss of 2.43 ± 0.26 Gt for the glaciers throughout the Maipo River Basin between 1955 and 2013. Heterogeneous spatial patterns in the elevation changes were observed between sub-basins in the first period (1955–2000), and an increase in the thinning rates in the following analysed period (2000–2013). A strong acceleration in the thinning rates was found until 2015, where fifteen glaciers were observed using in-situ techniques. The acceleration of glacier thinning after 2010 is coincident with the current severe dry period (2010 onwards) in the central Andes of Chile and Argentina.
Over the entire study period, the most negative glacier mass balance was found in glaciers located in the Olivares sub-basin. This large mass loss may be attributed to a decreasing surface albedo and less precipitation amounts in comparison with the southern sub-basins. However, additional studies are required to elucidate the relationship between dust on the glacier surface and anthropogenic activities (e.g., mining and transport of city pollution to mountain areas).
Our results provide the first long-term spatio-temporal glacier elevation and mass change analysis, and again prove the feasibility of using remote sensing techniques to monitor glaciers. However, it is critical to extend the field glacier monitoring over the basin, due the relevance of glaciers in the Maipo River Basin. Here, we also provide a key dataset to further validate hydrological projections as well as for the implementation of water management plans, as the demand for water resources has been considerably increasing during the last decades.