ROCK GLACIERS

Rock glaciers (Fig. 1, part a) are cryospheric landforms developed from accumulations of ice and debris, generally occurring in high mountainous terrain (Harrison et al., 2008; Berthling, 2011). In shape they resemble small glaciers and form a mix of angular rock debris with a core of ice or ice-cemented fine clasts, usually with a distinct ridge and furrow surface pattern (Potter, 1972). Rock glaciers are often classified by their activity status and subdivided into “active” and “inactive” (containing ice, also known as “intact”) or “relict” (not containing ice, sometimes referred to as “fossil”) (Barsch, 1996; Baroni et al., 2004; Krainer and Ribis, 2012; Falaschi et al., 2014). Active rock glaciers are considered indicators of contemporary permafrost (Barsch, 1996; Burger et al., 1999; Haeberli, 2000). It is estimated that active rock glaciers contain a range of between 40% and 60% ice covered by a surface layer of rock, which insulates the ice from low amplitude and high frequency temperature changes (Brenning, 2005a). They have a steep front (snout) and side slopes and give a visual representation of a swollen body due to ice content (Baroni et al., 2004). It is these surface features of ridges and furrows and steep front and sides that allow for rock glacier identification from high-resolution satellite images for inventory mapping (Paul et al., 2003). However, there is considerable variation in surface morphology occurring between locations in response to both topographic and lithologic variables (Giardino and Vitek, 1988).

FIGURE 1.

(a) Example from July 2011 of a Bolivian rock glacier with the sides and frontal slope indicated with a red dashed line (Pico Austria rock glacier in the Cordillera Real region, 16°11′35″S, 68°15′53″W). . Photo by S. Rangecroft. (b) The location of active rock glaciers in the Bolivian Andes on an elevation map of the study region, with annotations of the three main regions discussed in this paper: Cordillera Real, Sajama, and Western Cordillera.

This study only focuses on active rock glaciers (those containing ice) and therefore those with relevance for water supplies Active rock glaciers have been shown to contain significant volumes of water in the Chilean and Argentinean Andes where they are key stores and sources of water, especially during the dry season (e.g., Croce and Milana, 2002; Brenning, 2005a; Brenning et al., 2007; Azócar and Brenning, 2010). The hydrological role of the active layer in a rock glacier is important because it stores a significant quantity of water or ice due to the open debris texture (Croce and Milana, 2002). Rock glaciers trap and conduct water that enters with the active layer and sub-permafrost material of active rock glaciers producing temporary aquifers and a seasonal release of water from the active layer (Burger et al., 1999; Croce and Milana, 2002).

The hydrological function and importance of rock glaciers is relatively well studied in other mountain regions of the world (Giardino et al.,1992; Cecil et al., 1998; Arenson et al., 2002; Williams et al., 2006; Azócar and Brenning, 2010); however, the hydrological importance of rock glaciers in Bolivia has never been assessed. Therefore, this research addresses this knowledge gap. We have documented the extent and size of the rock glacier resource in Bolivia in a previous study (Rangecroft et al., 2014), which concluded that there are 54 active rock glaciers covering a total surface area of 7 km² across the national extent of the Bolivian Andes (ranging from 15°S to 22°S) (Fig. 1, part b). However, the water storage potential of these permafrost features has not been explored. Therefore, this paper aims to estimate the ice content of Bolivian rock glaciers, calculate the water equivalent for Bolivian rock glaciers and ice glaciers, and therefore allow a comparison of these hydrological stores.

STUDY AREA

Bolivia is located in the center of the South American Andes (Payne, 1998), a region also known as the Dry Andes. The rock glacier inventory (Rangecroft et al., 2014) encompasses the two Cordillera mountain ranges of Bolivia between 15°S and 22°S (Fig. 1, part b), home to 20% of the world's tropical glaciers (Rabatel et al., 2013). Bolivia has a distinctive climate consisting of a dry season (May-August) and a wet season (December-February) (Francou et al., 2003), with aridity increasing toward the south of the country, resulting in no current ice glaciers in the south (Jordan, 1998). The Bolivian Cordillera Oriental is estimated to host 98% of Bolivia's glaciers (Jordan, 1998).

Three climatically and topographically distinct regions were identified along the Bolivian Andes: the Cordillera Real (15°-16°S), Sajama (17°-18°S), and the Western Cordillera (18°-22°S) (Fig. 1, part b). Along the Cordillera Oriental, the first region, “Cordillera Real,” is a glaciated mountain range situated close to La Paz (Fig. 1, part b). With the wettest climate of the Bolivian Andes, this region contains the highest density of glaciers in the Bolivian Andes. The second and third regions, “Sajama” and “Western Cordillera,” are based along the Bolivia-Chile border in the mountain chain of the Cordillera Occidental. The Sajama region is situated around the isolated ice-capped volcanic mountains in the Sajama National Park (Fig. 1, part b). The Western Cordillera covers the area southward of Sajama, along the dry, barren mountain range of the Cordillera Occidental (Fig. 1, part b). Almost no ice glaciers exist in this region (Francou et al., 1999), as rainfall is very low. Annual precipitation here is estimated to be less than 250–300 mm on the summits (Vuille and Amman, 1997). With its dry climate, this area is the best example of arid high mountains in the inner tropics (Francou et al., 1999). Population densities differ in the three regions, with the Cordillera Real providing water for the cities of La Paz and El Alto (Vergara, 2009; Soruco, 2012). There are no cities located in close proximity for the other two regions, only mountain communities and towns. For all three regions, rock glacier volume and water equivalent were estimated and analyzed.

USING THE ROCK GLACIER INVENTORY

Prior to this study, the first Bolivian rock glacier inventory was produced using fine spatial resolution satellite data, available through the platform of Google Earth (version 7.1.1.18888, Google Inc.), supported by a field mapping and validation program undertaken in 2011 and 2012 (Rangecroft et al., 2014). Satellite data available through Google Earth ranged from 5 to 30 m spatial resolution imagery. Rock glaciers were identified and classified according to Baroni et al. (2004) (Rangecroft et al., 2014). The resulting inventory contained spatial data describing the location, shape, elevation, and surface area of the rock glaciers along the Andean mountain ranges of Bolivia, the Cordillera Oriental and the Cordillera Occidental (Fig. 1, part b). A full list of active rock glaciers and their coordinates are available in Table 1.

The calculated mean minimum altitude at rock glacier front (MAF) for active rock glaciers was 4983 m (±30 m) (Rangecroft et al., 2014). The majority (88%) of the active rock glaciers had a main direction of flow as southerly (SE, S, and SW) (Rangecroft et al., 2014). Average active rock glacier length was 550 m (Rangecroft et al., 2014) and average active rock glacier area was 0.13 km², with a minimum of 0.01 km² and a maximum of 0.79 km² (Table 1). Average rock glacier area for the Cordillera Real region was 0.07 km², 0.20 km² for Sajama, and 0.13 km² for the Western Cordillera (Table 1).

ESTIMATING ROCK GLACIER THICKNESS

Calculations of the water content of the rock glaciers were carried out, based on an estimation of the volume of ice contained in the rock glacier (e.g., Barsch, 1996; Brenning, 2005a; Azócar and Brenning, 2010). To generate a value for ice volume, estimates of rock glacier thickness and surface area were multiplied together. However, in this study, rock glacier thickness was an unknown variable because this cannot be measured from satellite- and airborne remote sensing instruments, and to-date there are no direct measurements of the thickness and average ice content of active rock glaciers in the Bolivian Andes. Existing studies elsewhere in the Andes have estimated the mean thickness of rock glacier ice-rich permafrost through an empirical rule established by Brenning (2005b) (Azócar and Brenning, 2010; Perucca and Esper Angillieri, 2011) (Equation 1; Table 2). However, it should be noted that more research is needed to improve the area-thickness relationship established by Brenning (2005b) for other regions.

ESTIMATING ICE CONTENT AND WATER EQUIVALENTS OF ROCK GLACIERS

Once the thickness of a rock glacier was established, ice content was accounted for, as rock glacier permafrost is not composed of 100% ice. Ice content varies considerably within rock glaciers; therefore, the amount of water in rock glaciers is naturally difficult to estimate due to the inherent variability and difficulty in determining exact genesis and subsequent depth and distribution of ice (Seligman, 2009). Currently, the worldwide estimates for ice content within rock glaciers ranges between 40% and 60% by volume (Barsch, 1996; Haeberli et al., 1998; Arenson et al., 2002; Hausmann et al., 2007; Brenning, 2008; Krainer and Ribis, 2012). A lack of studies investigating the ice content of rock glaciers in the Andes does not help to reduce this large range of uncertainty.

TABLE 1

ID and coordinates of active rock glaciers identified in the Bolivian Andes, divided into three corresponding regions. Adapted from Rangecroft et al. (2014).

First attempts to quantify ice content in the Andes were established by Croce and Milana (2002), who found an average ice content of 55.7% for an Argentinean rock glacier. More recently, Arenson et al. (2010) working in Argentina showed that test pits dug in rock glaciers contained over 50% ice content. Similarly, Perucca and Esper Angillieri (2011) used the estimate of 50% ice content by volume for calculating water equivalents of rock glaciers at 28°S in the Argentinean Andes. Monnier and Kinnard (2013) found a much lower range of ice content in a small rock glacier in Chile (15%-30%); however, there are some limitations about the coring technique utilized, and they concluded that the rock glacier was degrading. Therefore, due to the lack of ice content studies in the semi-arid Andes, it can be argued that using the range 40%-60% ice content offers the best estimate currently available and allows a lower and an upper estimate of ice content to be calculated. From this, water content can be determined using the conversion factors for the density of ice and density of water shown in Table 2. An ice density of 0.9 g cm^-3 (≡ 900 kg m^-3) was used to convert ice volume into a water equivalent (Azócar and Brenning, 2010; Perucca and Esper Angillieri, 2011).

ESTIMATING THICKNESS AND WATER EQUIVALENTS OF ICE GLACIERS

One of the aims of this work was to compare the water equivalent storage of rock glaciers to that of ice glaciers. In order to make this comparison, it was first necessary to establish current ice glacier coverage of the Bolivian Andes. Recent data describing the distribution of ice glaciers across the whole of Bolivia were not available, so it was necessary to use data from Ramirez et al. (2012) for the Cordillera Real and Jordan (1998) for the Cordillera Occidental. For the Cordillera Real only, we were able to use a recent inventory by Ramirez et al. (2012) in which they estimated ice glacier coverage of 185.5 km² (Ramirez et al., 2012). Along the Cordillera Occidental, the only ice glaciers existing are those in the Sajama region, which Jordan (1998; Jordan et al., 1980) estimated to cover 10 km². South of this region, ice glaciers are absent (Jordan et al., 1980; Jordan, 1998).

TABLE 2

Values used to calculate estimates of permafrost thickness, ice content, and water volume in rock glaciers and ice glacier thickness and their sources.

Using the empirical relationship established by Chen and Ohmura (1990) used by Azócar and Brenning (2010) and Perucca and Esper Angillieri (2011), ice glacier thickness (and subsequent water equivalent assuming 100% ice content) was estimated using Equation 2:

Ice glacier thickness had to be calculated on an individual basis to be comparable with the rock glacier estimates. However, data on all individual ice glacier sizes were not available; therefore an estimate of average ice glacier size was established using known total surface area and number of ice glaciers. For the Cordillera Real, the total number of ice glaciers within the inventory of Ramirez et al. (2012) was 476, giving an average ice glacier size of 0.39 km². Using this average, the mean ice glacier thickness equation could be applied and multiplied by the number of ice glaciers to find total ice volume and water equivalent. For the Sajama region, the 10 km² coverage was known to be composed of the glaciated cone of Sajama (4 km²), Payachata (4 km²), and Quimsachata (2 km²) (Jordan et al., 1980; Jordan, 1998).

To directly compare the water equivalents of ice glaciers and rock glaciers (as a ratio), the average ice content of 50% was used as a midrange between the lowest (40%) and the highest (60%) bounds of rock glacier ice contents.

FIELD WORK

To investigate the internal composition of rock glaciers, direct and indirect methods can be applied. Direct methods (e.g., coring, drilling, test pits) are labor- and time-intensive, whereas indirect methods (e.g., geophysical surveys) allow relatively rapid and inexpensive acquisition of data. Fieldwork was conducted on the Chiguana active rock glacier (21°06′09S, 67°51′13W) in the Western Cordillera region as this is a representative active rock glacier for the Bolivian Western Cordillera. Electrical resistivity surveys and test pits were conducted on lower parts of the rock glacier, close to the snout of the rock glacier. Electrical resistivity surveying used a Geo-receiver (JP), Geo-transmitter (JP-600), and electrodes to perform the vertical electrical sounding (SEV) method.

Electrical resistivity is one of the most commonly used geophysical methods to investigate permafrost and rock glacier internal content and structure (Croce and Milana, 2002; Bucki et al., 2004) as the equipment is light and easy to use (Croce and Milana, 2002). Refraction seismics can provide detailed information on the structure of the upper layers, and when complemented with geoelectric methods, deeper information can be gained, such as permafrost thickness estimates (Croce and Milana, 2002). Additional geophysical methods should be applied to reduce ambiguities if possible. Geophysical surveys require verification by coring/drilling or other direct methods (Degenhardt and Giardino, 2003) such as test pits. Well drilling combined with high-resolution reflection seismics is usually viewed as the best option for rock glacier investigation (Kääb et al., 1997), but is usually expensive.

ROCK GLACIER PERMAFROST THICKNESS AND ICE VOLUME

Rock glacier permafrost thickness estimates ranged between 20 and 48 m (Table 2). Overall, these calculations resulted in an estimated total ice volume of 0.23 to 37.60 million cubic meters (Table 3).

ROCK GLACIER WATER EQUIVALENT CALCULATIONS

Results in Table 3 show that throughout the Bolivian Andes active rock glaciers are estimated to contain a total water volume equivalent of between 0.01 and 0.13 km³ of water. Regionally, rock glaciers of the Cordillera Real (15°-16°S) have been estimated to store the smallest amount of water, containing between 0.01 and 0.02 km³ of water (Table 3). Sajama (17°-18°S) and Western Cordillera (18°-22°S) have been estimated to hold triple this water equivalent each (Table 3). In Sajama, rock glaciers are estimated to hold 0.04 and 0.06 km³ of water (Table 3). In the Western Cordillera, rock glaciers contained between 0.04 and 0.05 km³ of water (Table 3).

RATIOS OF ROCK GLACIER TO ICE GLACIER WATER EQUIVALENTS

On average, the 1.07 km² of rock glaciers in the Cordillera Real were estimated to contain 0.01 km³ water, yet the 185.5 km² of ice glaciers (Ramirez et al., 2012) were estimated to contain 3.34 km³ of water (Table 4). This resulted in a ratio of rock glacier to ice glacier water equivalence of 1:228, implying that ice glaciers contained a store of water 228 times bigger than that of the rock glaciers in the same region. In Sajama (17°-18°S), the 2.98 km² of rock glaciers were estimated to contained an average of 0.05 km³ of water, with 10 km² of ice glaciers in the same region, which equated to 0.39 km³ of water (Jordan, 1998) resulting in a ratio of rock glacier to ice glacier water equivalence of 1:7 (Table 4). In the Western Cordillera (18°-22°S), there are no ice glaciers due to limited precipitation (Jordan, 1998; Vuille, 2007), yet the 2.88 km² of rock glaciers in this region contained an estimated 0.05 km³ of water, indicating a hydrological store in the absence of ice glaciers. However, this regional water store is for the largest region analyzed (Table 4; Fig. 1, part b).

TABLE 3

Table of ice volume (km³) and corresponding water volume (km³) equivalents for Bolivian rock glaciers, regionally and nationally (total). These calculations cover a range of ice content estimates with a lower (40%), average (50%), and upper (60%) bound. Rock glacier surface areas were taken from Rangecroft et al. (2014).

ELECTRICAL RESISTIVITY

From fieldwork conducted in the Bolivian Andes, the presence of ice was confirmed within the Chiguana active rock glacier (Fig. 2, parts a and b). Test pits confirmed the presence of ice (Fig. 2, part c). However, these surveys did not extend deeper than 5 m, therefore ice content values through the entire depth of the rock glacier were not established. Previous research suggests that resistivity readings greater than 20,000 Ω represent an ice-rich layer (e.g., Francou et al., 1999). Survey site 1 (Fig. 2, part d) (21°06′06.93S, 67°51′17.95W) showed that within the first meter readings were characteristic of an ice-rich layer (19,817 Ω at 0.89 m), followed by a layer characteristic of substrate without ice (2,003 Ω) until a depth of 5.22 m. Below this resistivity, values of 31,640 Ω were measured, implying ice underneath an active layer of ∼5 m. Site 2 (Fig. 2, part e) (21°06′06.46S, 67°51′19.44W) showed a similar pattern; an ice-rich layer at 1 m depth (512,580 Ω between 0.14 and 0.79 m), then a zone of substrate without ice (220 Ω) followed by an ice-rich layer below this at 2.39 m (Fig 2, part e). These results from the SEV surveys at Chiguana rock glacier in Bolivia (Fig. 2, parts d and e) demonstrated a similar pattern found by Croce and Milana (2002) for a rock glacier in the Argentinean Andes. Resistivity values greater than 20,000 Ω were found within the few meters of the survey sites in Argentina, and then again below what could be considered the active layer (the first 1–5 m), implying an upper dry deposit layer was found over a second wet debris layer with poor conductivity (2–5 m thick) followed by a highly resistive layer of frozen debris (permafrost) (Croce and Milana, 2002).

ROCK GLACIER WATER CONTENT

In comparison to the limited number of studies on rock glacier water stores, Bolivian rock glaciers contained far less water than those of the Chilean Andes (Azócar and Brenning, 2010) and the Argentinean Andes (Perucca and Esper Angillieri, 2011). Azócar and Brenning (2010) found 147.5 km² of rock glaciers in Chile (27°-33°S), which they estimated to hold the water equivalent of 2.37 km³. Therefore the hydrological store of rock glaciers is much greater farther south in the Andes (27°-33°S) than Bolivia (15°-22°S). Our results are more comparable to those of Perucca and Esper Angillieri (2011) in Argentina who calculated a water equivalent of 0.12 km³ for 6 km² of rock glaciers in a 1 degree region at 28°S. This result is a similar estimate to that stored in Bolivian rock glaciers but covering a much smaller area. Overall, rock glaciers are relatively more important as water stores in Bolivia with a national ratio of 1:33 rock glacier to ice glacier water equivalent (Table 4) than they are in the European (Swiss) Alps, where their ratio is calculated to be ~1:83 (Brenning, 2005b).

However, it is the regional impact of these water resources that is important to consider when evaluating the absolute values, and their presence in comparison to other water stores (such as ice glaciers). For example, Table 3 shows that there are noticeable differences in rock glacier water content when viewed regionally. Rock glaciers of the Sajama region contained the largest amount of ice covering the smallest regional area (Table 4) and therefore have the greatest potential as water sources. Conversely, the Cordillera Real's rock glaciers contained the least amount. Exploring these regional differences in the context of population pressures and other water stores (e.g., glacial ice) is important if the significance of the rock glacier water storage is to be understood.

HYDROLOGICAL IMPORTANCE OF ROCK GLACIERS

Rock glaciers of the Cordillera Real stored the lowest amount of water out of the three regions (corresponding to the smallest surface area) (Table 3), yet this region of Bolivia is the most densely populated region, with the basins of the Cordillera Real supplying water for around 2.3 million people living in La Paz and El Alto (Vergara, 2009; WMO, 2011). From the comparison of rock glacier to ice glacier water equivalent in Table 4 it is clear that rock glaciers contribute less to regional water supply in the Cordillera Real, where ice glaciers dominate. However, with rapid ice glacier recession evident in this region (Soruco et al., 2009; Ramirez et al., 2012), we hypothesize that the relative importance of rock glaciers to ice glaciers will increase. The ice in rock glaciers is protected from small thermal changes by an insulating debris layer, resulting in a longer lag time than ice glaciers in their response to climate change, which is known to be on decadal time scales (Schrott, 1996; Haeberli et al., 2006). This makes rock glaciers more resilient water stores (Millar and Westfall, 2008), so it is likely that rock glaciers will persist as longer-term stores of water in mountain regions, even when ice glaciers have retreated or disappeared. It is important to note that response to climatic change is a size dependent process, with smaller rock glaciers being more susceptible to thawing. However, we also argue that, given appropriate conditions, retreating ice glaciers may also begin to incorporate significant debris cover and, over time, may undergo a transition to rock glacier forms.

TABLE 4

Regional and total area (km²) and corresponding water equivalents (m³) for Bolivian rock glaciers and ice glaciers, with a final column comparing these water equivalents directly as a ratio. Values are reported to two decimal places. Rock glacier water equivalents use the 50% ice content (average ice content).

In Sajama, rock glaciers have a higher relative importance as water stores in comparison to ice glaciers than in the Cordillera Real region, with a ratio of 1:7 rock glacier to ice glacier water equivalence (Table 4). This ratio is similar to that of the Andes of Santiago, Chile, which is also 1:7 (Brenning, 2005b), but of lower importance than that of the Arid Chilean Andes between 27°S and 29°S, where ratios are 1:2.7 (Azócar and Brenning, 2010), and between 29°S and 32°S, where rock glaciers dominate with a ratio of 3:1 (Azócar and Brenning, 2010). It can be noted that with recent glacier recession the ice coverage of the Sajama region is likely to be currently less than 10 km², especially given glacier recession trends in the Cordillera Real; however, there is a lack of published data for this region. Also, similarly to the Cordillera Real, the estimated rock glacier reserve of Sajama will increase in relative importance with projected glacier recession in Sajama.

In the Western Cordillera (18°–22°S), without the contribution of ice glaciers to the hydrological cycle in this region, and with the very limited rainfall (<300 mm a^-1), here potentially rock glaciers may act as an important water store. However, rock glaciers are still sparse in this region in comparison to those of the Chilean Andes (Azócar and Brenning, 2010), and population levels along the Western Cordillera are low.

FUTURE RESEARCH

Rock glaciers are long-term stores of frozen water, but the total water stores calculated in this research may not represent water that is readily available for release for human consumption. Research and data on rock glacier and ice glacier discharge are crucial for improving scientific understanding of local cryospheric reserves and their importance to the hydrological cycle and for future water resources management. It is also clear from this paper that more recent data on ice glaciers across the whole of the Bolivian Andes are needed and that field studies on ice content of Bolivian rock glaciers is necessary to improve and refine our understanding and the information established here. Furthermore, research on the impacts of future temperature and precipitation changes on the hydrological function of rock glaciers is needed to fully implement the information provided here in a future context. As rock glaciers are known to exist close to the mean annual air temperature 0 °C isotherm (Payne, 1998; Avian and Kellerer-Pirklbauer, 2012), it is important to investigate the potential implications of projected climate change for rock glaciers and their water stores.