Rock glaciers likely play an important hydrological role in the semiarid Andes (SA; 27°–35°S). They supplement streamflow when water is needed most, especially during dry years in the late summer months. Despite their assumed importance, there are no publications that quantify their hydrological contribution to streamflow in the SA of Chile, based on measurements of rock glacier ice loss or discharge. In this study, we assess the available information on the hydrological importance of rock glaciers in the SA and provide suggestions on how future research can address knowledge gaps. We conclude that there is insufficient data available to quantify the hydrological contribution of rock glaciers in the SA. Measurements of glacier discharge are limited to unpublished data sets from which only very limited conclusions can be drawn. There are no ice volume change measurements or proxies available for individual rock glaciers. Approximations of rock glacier ice volume, calculated from areal extent, thickness, and percentage of ice content are available, and these data provide an initial baseline for calculating ice volume change in the future. While these baseline data are very valuable, they represent rough estimates due to a scarcity of studies, especially on glacier thickness and percentage of ice content. With increased temperatures and a decrease in precipitation expected in the future, rock glaciers could become an increasingly critical water resource in this region, especially in the Elqui and Juncal catchments. Improved estimates of rock glacier discharge, water content, processes, and hydrology are required to model their future evolution and evaluate their contribution to water resources.
Water resources, policy, governance, and adaptation in a warming climate
While Chile is fortunate to have a relative abundance of water resources with a mean annual per capita water supply of 51,218 m3/person/year (World Bank 2010; DGA 2016), water resources in Chile are not evenly distributed. In semiarid and arid regions from the Metropolitan Region of Santiago to the north (~ 18°–34°S), the total water supply is ~ 500 m3/person/year on average (DGA 2016). Between Santiago and the Austral Macrozone (~ 34°–44°S), the water supply is ~ 53,900 m3/person/year on average. The average population is 91 and 36 habitants/km2 in these two areas, respectively. There is a sharp increase in water availability in the Austral Macrozone (~ 44°–56°S) with ~ 2,444,600 m3/person/year while the population plummets to 1 habitant/km2 (DGA 2016). The zones of limited water resources from Santiago to the north also correspond to areas with the highest population density (Santiago), significant agricultural development, and extensive mining activity. Water supply for these sectors is largely reliant on melt from seasonal snow and ice bodies from the Andean Cordillera (Favier et al. 2009; Azócar and Brenning 2010). Due to high variability in interannual precipitation, forecasting water supply is often a difficult task. For a large portion of the semiarid region of Chile, the water supply has already been exhausted (DGA 2016). Water availability is in a precarious position as the demand for water resources is expected to rise in the future (Meza et al. 2015). Determining the contribution of different sources to groundwater and streamflow and modeling future change is fundamental to the sustainable management of water resources.
This review is focused on the semiarid Andes (SA) of Chile (27°–35°S) which includes part of the Atacama Region (III), Coquimbo (IV), Valparaiso (V), the Metropolitan Region of Santiago (RM), and the Libertador General Bernardo O’Higgins Region (VI; Fig. S1). East–west-oriented valleys run from the continental divide to the Pacific Ocean. Most human and agricultural development is concentrated in the valleys, while the mountainous terrain is largely unpopulated or unused, with the exception of mining operations. The SA is a transition zone between the extremely arid region north of 25°S and the humid climate south of 40°S (Escobar and Aceituno 1998). Precipitation at high altitudes (~ 4000 m a.s.l.) predominantly falls during the austral winter (May to August) and ranges from ~ 200 mm at the northern edge to 700–800 mm a−1 at the southern end of the SA (35°S; Azócar and Brenning 2010). The southern extent (~ 32°–35°S) is a transition zone between semiarid and temperate conditions (Bown et al. 2008). Precipitation is modulated by the ENSO, with higher than normal precipitation often occurring during El Niño years and lower precipitation during the La Niña years (Escobar and Aceituno 1998). Over the twentieth century, total precipitation has declined (Santibañez 1997; Vuille and Milana 2007) and desertification has been recognized internationally as a critical problem in the SA (UNCCD 1994; Le Quesne et al. 2006).
Future strain on water resources is compounded by predictions that these resources will diminish in a warming climate. The world is expected to warm by 1.1–2.6 °C, relative to the 1986–2005 average, for a moderate warming scenario (RCP 4.5) by the end of the twenty-first century (Collins et al. 2013). High mountain areas are expected to see the largest temperature increases, particularly, in Ecuador, Peru, Bolivia, and northern Chile (Bradley et al. 2006; Souvignet et al. 2010; MRI Working Group 2015). In the upper Limarí basin (~ 30°S, > 1000 m a.s.l.; region IV), temperatures are expected to rise 3–4 °C above the 1960–1990 average temperature by 2100, according to the IPCC scenarios A2 and B2 (similar to RCP 8.5 and 4.5, respectively; Vicuña et al. 2011). Snowmelt is projected to occur earlier in spring in a warmer climate, shifting peak river runoff earlier and subsequently resulting in reduced water availability when the demand is highest in the summer and autumn (Barnett et al. 2005; Souvignet et al. 2010; Vicuña et al. 2011). In addition, precipitation is projected to decrease in the SA (Souvignet et al. 2010), by 10–30% compared to the 1960–1990s in the upper Limarí basin, for example (Vicuña et al. 2011). Change in total precipitation and timing will likely increase the reliance on water stored in glaciers, accumulated snow, and permafrost in the future. However, the cryospheric reservoir has been diminishing in recent decades. Glaciers are retreating (e.g., Rivera et al. 2002; Pellicciotti et al. 2014), precipitation has decreased (Vuille and Milana 2007), and the snowline (minimum elevation where snow exists) has been moving to higher elevations (e.g., Pellicciotti et al. 2007). The projected temperature rise and precipitation decrease in the coming decades will result in a continuation of the observed cryospheric trends (Souvignet et al. 2010; Gascoin et al. 2011; Huss et al. 2017). The combination of increased reliance on a diminishing cryosphere combined with a growing population and water demand (Beniston 2003) requires careful management of these precious water resources.
Water resource management in Chile depends on the interaction among 43 private and public institutions (World Bank 2013). These entities are responsible for 102 functions, which include everything from information collection and delivery to policy formation to administering water rights associated with the water market. Water property rights are included within the Chilean Constitution (1980, updated 2017), and associated rights and responsibilities are outlined in the Código de Aguas (1981, updated 2014; literal translation: Water Code). The Código de Aguas essentially defines water as a national good for public use and grants private users permission for water use. The right to use water includes the possibility to trade water rights, which has led to the development of an active water market in Chile. The Código de Aguas does not adequately protect water resources since there are no environmental regulations inhibiting the use of these water rights, and governance is centralized in Santiago which has insufficient resources available to govern water at a local level. Water management at a national level is primarily managed by the Dirección General de Aguas (DGA), a department of the Ministry of Public Works in charge of promoting the management and administration of water resources with respect to sustainability, public interest, and efficient distribution. At a catchment scale, water right allocations are administered by user groups known as Juntas de Vigilancia which are comprised of individuals who have legal access to water within that watershed. Water rights and management strategies correspond to groundwater and surface water extraction, but solid water reservoirs (e.g., glaciers) are currently excluded from water legislation, even though they are of vital importance. In 2008, the DGA developed a National Glacier Strategy to setup a framework for glacier monitoring in the future (CECS 2009). This framework has led to the construction of a national glacier inventory and the development of a glacier monitoring network led by a centralized, specialized unit in Santiago. Glaciers have also been specifically included within the Environmental Impact Assessment System (EIAS) framework from the Ministry for the Environment since 2013 due to the increased importance the Central Government and wider society have placed on glaciers in the last few years, largely precipitated by conflicts with the mining sector (Fields 2006). Additionally, since 2005, several projects led by the Chilean Congress and Government Ministries have attempted to develop specific glacier protection and preservation legislation, with the most recent attempt shelved in 2018. While these efforts are a good start, the DGA itself recognizes that these regulations and initiatives are not sufficient to adequately protect water resources in Chile in a future warmer and drier climate (DGA 2016).
In order to adapt to a warming climate, the DGA recommends the creation of a National Policy on water and sustainable water management plan that best meets the needs of all users, including municipalities and industry (DGA 2016). An important aspect includes a better system to track the use of water resources. Policy and management would be coordinated by a new Ministry of Water, and new policy and management strategies would focus on management at the watershed scale. Certain regions have developed strategic tools. For example, in the Coquimbo Region, three water resource-specific strategic plans have been implemented (CAZALAC 2013; GORECoquimbo 2013, 2015). However, a legal framework that recognizes the heterogeneity and water shortage as real phenomenon is also needed (DGA 2016). Improved water conservation and investment in additional water sources, such as desalination plants, are also deemed necessary to confront water shortages. In order to implement these changes, the DGA, as well as central government, recognize that additional knowledge of the hydrological system and scientific investigation are required (CNID 2016; DGA 2016).
Implementation of the proposed national policy requires that the total volume of water resources available now and in the future be known. Without this knowledge, it is possible that water could be over allocated to industry, such that there would not be enough available for human consumption or to ensure the preservation of ecosystems. The DGA has an extensive publicly available network of hydrological monitoring stations from which the total amount of water available in the nation’s rivers can be determined. However, to properly model the water availability in the future, the contribution from each component of the hydrological system feeding river flow must be known. This includes precipitation and contributions from the cryosphere (glaciers, snow, and permafrost), which are of critical importance during the late summer months when there is essentially no precipitation, particularly during dry years (e.g., Favier et al. 2009; Azócar and Brenning 2010; Gascoin et al. 2011). In this paper, we will summarize what is known with regard to the contribution of one component of the cryosphere to water resources in the SA. We focus on rock glaciers since they are the part of the cryosphere that has been least studied, and the component which is thought to contain the most significant store of fresh water in the SA, because they are the predominant glacier type (Azócar and Brenning 2010). The available knowledge on rock glaciers will be reviewed and evaluated to understand if it is sufficient to determine the hydrological contribution of rock glaciers and model their response to a warming climate and therefore future water availability.
Landform definitions and geographical distribution
In the SA, the cryosphere includes rock glaciers, debris-free glaciers, and debris-covered glaciers. In contrast to debris-free glaciers composed almost exclusively of snow and ice, rock glaciers are a mixture of rock, sediment, and ice (Cogley et al. 2011). They are the visible expressions of cumulative deformation of ice-rich creeping mountain permafrost (Fig. S2; Barsch 1989; Bodin et al. 2010; Berthling 2011). Debris-covered glaciers are debris-free glaciers with a layer of rock and/or sediment covering part or all of the surface (~> 25%; Janke et al. 2015). In this paper, glaciogenic rock glaciers are defined as those formed from debris-covered glaciers, whereas cryogenic rock glaciers are formed from the geological processes associated with permafrost. Polygenic glaciers have a combined glaciogenic and cryogenic origin. Rock glaciers are differentiated from debris-covered glaciers by having a thick enough debris cover to insulate the ice beneath (~> 3 m), with the percentage of ice content typically < 45% (Janke et al. 2015). Debris-covered glaciers are characterized by exposed ice due to the discontinuity of debris cover or thermokarst collapse, among other features, that create a rough surface. In contrast, no ice is visible on the surface of rock glaciers and they are comparably smooth and convex (Janke et al. 2015; Monnier and Kinnard 2017). Rock glaciers are also defined in terms of their motion (termed activity status) and percentage of ice content. Active rock glaciers are relatively fast moving with a relatively high percentage of ice content, passive rock glaciers contain a lower percentage ice content and by consequence are stagnant or move very slowly, and relict rock glaciers are unlikely to contain ice and are characterized by an irregular surface with signs of degradation, such as subsidence (Putman and Putman 2009; Janke et al. 2015). While these straightforward definitions are provided here for simplicity, the origin and internal composition of rock glaciers have been an ongoing topic of debate (e.g., Potter et al. 1998; Haeberli et al. 2006; Monnier and Kinnard 2017).
Glacier size is limited in the northern part of the SA (Pellicciotti et al. 2014) due to low precipitation and relatively high rates of ablation, of which a large part is attributed to sublimation (MacDonell et al. 2013). In the far north (Copiapó and Huasco basins), debris-free glaciers are dominant, but between (~ 29–35°S) rock glaciers are more abundant. Glacier distribution in the north is highly correlated with aspect, with approximately 80% of glaciers located on slopes facing the southeast or southwest (Nicholson et al. 2009). The majority of debris-free glaciers are found between 5000 and 5200 m a.s.l., while the majority of rock glaciers are found between 4000 and 4400 m a.s.l. Moving southward, active rock glaciers are more common (Pellicciotti et al. 2014). Rock glaciers in the southern end of the SA are generally found above 3000 m a.s.l., and active rock glaciers are consistently found above ~ 3500 m a.s.l. (Brenning 2005a,b).
Hydrological importance of glaciers and rock glaciers in the Chilean semiarid Andes
Snow cover only lasts 1 to 2 months after the last winter snowfall (Favier et al. 2009); so, glaciers are the main water source when river levels are at a minimum, especially during dry years (Gascoin et al. 2011). Glaciers act as natural reservoirs, storing water and releasing it slowly (Dewayne et al. 1998; Jansson et al. 2003; Williams et al. 2006). The mean annual glacier contribution to streamflow within the SA varies from ~ 3 to 44% for most years (Table 1; Gascoin et al. 2011; Ragettli and Pellicciotti 2012), with the highest contributions in years with less snow (e.g., 2008/2009) and at higher elevations. During severe drought in the summer, the contribution can be even greater. For example, during the summer of 1968, glacier contribution to streamflow in the Maipo River basin, Central Chilean Andes, was up to 67% of the monthly discharge according to a snowmelt-runoff model applied in the region (Peña and Nazarala 1987). Likewise, glacier contribution reached 67% during 2015 for the Yeso River Basin, one of the driest years on record (Ayala et al. 2017).
Within the Coquimbo Region (Elqui, Limarí, and Choapa basins), Maipo and Yeso basins, rock glaciers are more abundant, cover a larger surface area, and may represent more stored water than debris-covered and debris-free glaciers combined (Table 2). The amount of stored water in rock glaciers has been roughly estimated, and this has been used to derive approximations of rock glacier contributions to streamflow (Brenning 2005b; Nicholson et al. 2009; Azócar and Brenning 2010; Bodin et al. 2010; Janke et al. 2017). There are no publications that have specifically calculated the contribution of rock glaciers to streamflow in the SA of Chile, but two publications have calculated the contribution of all glaciers in the “La Laguna Basin” (Colorado Catchment), which contains rock glaciers (Favier et al. 2009; Pourrier et al. 2014). Favier et al. (2009) estimated the contribution from the watershed upstream of the La Laguna dam to be 4–9% of the mean annual discharge. Discharge from debris-free glaciers was estimated from mass balance measurements of glaciers in other nearby catchments, and the discharge from rock glaciers was assumed to be 2–3 times lower, following Krainer and Mostler (2002). Comparatively, Pourrier et al. (2014) calculate the contribution of the Tapado catchment (basin A in Fig. 1) to be 13 ± 20% based on discharge measurements at D2 and D4. Neither of these publications use discharge measurements specifically from rock glaciers in SA or elsewhere in the world to estimate their contributions to streamflow.
We are aware of three published studies that calculate the rock glacier contribution to streamflow outside of the SA of Chile. In the Agua Negra catchment, Argentina (adjacent to the Chilean border at ~ 30°S), rock glaciers contribute ~ 13% to streamflow during the summer months (Schrott 1996). In the Utah mountains, rock glaciers contribute 15–30% of the total basin runoff (Geiger et al. 2014). In the Canadian Rockies, a passive rock glacier contributes 50% to streamflow at the end of summer and 100% during the winter, despite draining less than 20% of the watershed area (Harrington et al. 2018). Other publications on rock glacier hydrology have been focused primarily in the Austrian Alps (e.g., Krainer and Mostler 2002; Krainer et al. 2007; Winkler et al. 2016) and the United States (e.g., Utah: Geiger et al. 2014), with one study we are aware of in the Bolivian Andes (Rangecroft et al. 2015).
Using a combination of the minimal glacier discharge data available (all from unpublished data sets) and published discharge values measured at rock glaciers outside of the SA, we analyzed spatial patterns of discharge and estimated the rock glacier contributions to streamflow for the La Laguna Basin (Fig. 1; Table 3). Discharge measurements at the end of summer at three key sites (D2, D3, and D4; Fig. 1) showed increasing discharge in the downstream direction as expected, most likely in response to increasing rock glacier coverage (Fig. 1). The streamflow contribution from rock glaciers in the La Laguna Basin was estimated using the discharge data from the end of summer (January and February) when there is no snow and very minimal precipitation. During this time, we assume that the vast majority of streamflow contribution is from the cryosphere. We assumed that all of the runoff from glaciers in the region contributed to streamflow and that debris-free glaciers contributed six times more runoff than rock glaciers. We determined this ratio by comparing discharge measurements taken during the first week of February 2012 at the toe of Tapado Glacier and the toe of the Tapado rock glacier which were 0.048 and 0.008 m3 s−1, respectively (CEAZA 2012). As there are no measurements of an isolated debris-covered glacier in the area, we assumed the discharge from this glacier type to be three times that of a rock glacier.
We calculated the contribution to streamflow over a likely range and for an extreme maximum scenario. Discharge from rock glaciers varies between ~ 0.005 and ~0.25 m3 s−1 (Bajewsky and Gardner 1989; Barsch 1996; Krainer and Mostler 2002; Harrington 2017). At Las Tolas Rock Glacier (0.29 km2; Fig. 1), the only location where there are measurements of discharge from an isolated rock glacier in the Laguna Basin, the lowest discharge measurements taken were 0.004 and 0.006 m3 s−1 during November of 2011 and 2013, respectively. The average of these measurements equals the lower limit proposed by Barsch (1996) of 0.005 m3 s−1; so, we conclude that this is a reasonable minimum value for rock glacier discharge in the La Laguna Basin.
The highest discharge measurement from Las Tolas Glacier obtained on January of 2016 (0.011 m3s-1), was used for the likely maximum.
There are only two publications that include discharge measurements (rather than approximations): the Hilda rock glacier in the Canadian Rocky Mountains (1.5 km2; Bajewsky and Gardner 1989) and the Reichenkar rock glacier in the Austrian Alps (0.27 km2; Krainer and Mostler 2002). The discharge measurements taken at the end of summer (July and August) for Hilda and Reichenkar rock glaciers were 0.18 and 0.05 m3 s−1 or 0.12 m and 0.19 m3 s−1 per km2 of rock glacier, respectively. Given that both of these discharge values are much higher than the likely maximum (0.038 m3 s−1 per km2), we decided to use the Hilda Rock Glacier maximum of 0.12 m3 s−1 per km2 as the extreme maximum. This value is well within the range of rock glacier discharge values of 0.005 to 0.25 m3 s−1 provided by Barsch (1996).
We multiplied the minimum, likely maximum, and extreme maximum discharge values per km2 of rock glacier by the area of rock glaciers in sub-basins A, B, and C (Table 3) to provide discharge values. The aforementioned ratios were applied to calculate discharge values for debris-covered glaciers and debris-free glaciers (1:3:6). Rock glaciers contributed 0.14, 0.30, and 0.93 m3 s−1 to the La Laguna Basin (basins A + B + C) for the minimum, likely maximum, and extreme maximum scenarios, respectively. All glaciers contributed 0.53, 1.18, and 3.65 m3 s−1 to the La Laguna Basin for each scenario. The contribution to streamflow was calculated by dividing each discharge value by the measured value at D4 (Fig. 1). Rock glaciers likely contributed 9-20% of the streamflow, while the total glacier contribution was likely 37-81%. At higher elevations, the rock glacier percent contribution remains the same, but total glacier contribution becomes more important. For basin A (Fig. 1) all glaciers contributed 46-100% (0.22-0.49 m3 s-1) of the streamflow at D2 (Table 3) for the likely minimum to maximum scenarios. The extreme maximum gave >100% contribution for all basins and was therefore considered an oversitmation. It should be noted that these results only account for water at the surface and that the uncertainties in discharge values can be high and vary with the method used (Pourrier et al. 2014).
This analysis provides a first approximation of the contribution to streamflow using measurements of rock glacier discharge in the SA of Chile. However, our results are a rough approximation because discharge measurements adjacent to a rock glacier terminus only provide the surface water contribution both generated from ice within the rock glacier, as well as water that has flowed through the rock glacier. Piezometric measurements are also needed to quantify runoff lost to the subsurface. These data could also be combined with a hydrological model to provide a complete analysis of contributions to streamflow (La Frenierre and Mark 2014).
Measurements of ice volume change over time can also be used to calculate the contribution to streamflow, assuming that all ice lost contributes. There is one publication we are aware of that calculates rock glacier elevation change in the SA. Monnier et al. (2014) report a surface surfacelowering of 5-10 m in the central part of the Tapado rock glacier and surface rising of 5-15 m near the margins between 1956-2010. These measurements are very valuable, but isolating the rock glacier ice volume change would be complicated since the glacier is part of, and interacts with, a larger glacier complex (Fig. 3). Elsewhere, rough estimates of ice volume for one point in time have been made, and these provide a valuable baseline from which changes in rock glacier ice volume could be calculated in the future. Knowing the ice volume also provides a proxy for the hydrological importance of rock glaciers and is necessary information to determine how long the water reserve will be available. The rate of deterioration of a rock glacier may also provide an indication of the contribution to streamflow. A rock glacier with a stable geomorphology likely contributes less to streamflow than the same glacier showing evidence of downwasting. At some point the glacier will lose so much ice that it will again contribute less to streamflow. The deterioration could be identified based on changes in geomorphology or quantified with a measured change in ice volume. Obvious decreases in rock glacier velocity would also indicate deterioration and possible transition from an active to passive to relict rock glacier.
The remainder of this section is a review of available knowledge of ice volume in the SA of Chile as the minimal information available on rock glacier discharge and ice volume change has been summarized previously. A detailed inventory of rock glaciers has been completed for Chile using Landsat imagery from 2000/03 and 2015, and other inventories have been completed for smaller regions with higher resolution imagery (Table 4). Rock glacier area coverage ranges from 37 to 78% of the total glacier area. There are two publication on rock glacier change over time in the SA, and both conclude that the areal extent of rock glaciers has remained approximately constant between 1975 and 2007 in the Laguna Negra Catchment (~ 33.5°S; Bodin et al. 2010) and between 2000/03–2015 throughout the Chilean SA (Barcaza et al. 2017). Debris-free glaciers have been retreating throughout the region over the past ~ 50 years, with smaller glaciers retreating faster than the larger ones (Nicholson et al. 2009; Barcaza et al. 2017). Rock glaciers may also be experiencing ice loss without a change in area, but there are no published studies that document changes in geomorphology or ice volume for isolated rock glaciers in the SA to verify this.
Thicknesses measured with geophysical techniques on four rock glaciers in the Chilean SA have been documented in published literature: one in the Choapa Province and three in the Elqui Province (CEAZA 2012). The thickness for the rock glacier in Choapa is 10–30 m (Monnier and Kinnard 2013). In the Elqui Province, average thicknesses of ~ 20 and ~ 25 m have been measured at Tapado and nearby Llano de las Liebres rock glaciers, respectively (CEAZA 2012; Monnier and Kinnard 2015a). In the SA of Argentina, thickness has been measured for San Juan at El Paso Rock Glacier (18.5 m maximum thickness; Croce and Milana 2002). These thickness measurements are very valuable, but additional measurements at other rock glaciers are needed to develop a regional model to estimate thicknesses.
Once the rock glacier area and thickness are known, the rock glacier volume can be estimated. However, to determine the amount of ice contained within the glacier, the percentage of ice content must also be known. In the SA, an ice content of 50% has been assumed in most cases (Brenning 2005a, b; Nicholson et al. 2009; Azócar and Brenning 2010; Bodin et al. 2010) but 60% has also been used (Schrott 2002). These ice content values are based primarily on borehole measurements of ice content at three rock glaciers in the Swiss Alps (Arenson et al. 2002) and ice content inferred from seismic data at a rock glacier in the arid Andes of Argentina (Croce and Milana 2002). There is one published direct measurement of the percentage of ice content in Chile from a rock glacier in the Choapa Region (~ 31°S) within the Los Pelambres mine. However, at the time of extraction, this glacier was in an advanced state of deterioration with visible permafrost degradation; so, the measured ice content from two cores of 15–30% may not be representative (Monnier and Kinnard 2013). Indirect measurements of the percentage of ice content using the ground penetrating radar (GPR) have been completed on an active rock glacier in the Elqui Province, Llano de las Liebres. The estimated ice percentage here was between 22 and 83% with an average value of 66% (Monnier and Kinnard 2015a). Geophysical data were also used to estimate the percentage of ice content of a rock glacier in the Elqui Province adjacent to Cerro Tapado. This cryogenic origin rock glacier had an ice content of 20–47% (Milana and Güell 2008). As evidenced by these measurements, rock glacier percentage of ice content can vary considerably. It varies with the rock glacier origin (glaciogenic, cryogenic, or polygenic), the internal structure of the glacier (e.g., debris or boulder matrix), and external factors, such as topography and climate conditions (Schrott 1996; Milana and Güell 2008). Percentage of ice content also varies with the speed at which the rock glacier is moving. Active rock glaciers move downhill under the force of gravity, likely due to the deformation of ice within, and are, therefore, assumed to contain more ice than passive rock glaciers. In contrast, passive rock glaciers are stagnant, and it is assumed that they contain little to no ice (~< 25%; Janke et al. 2015; Nicholson et al. 2009). Given the variability of ice content percent and the importance of this parameter for accurately determining the amount of water stored in rock glaciers, more measurements are needed in the SA to estimate water content at a regional scale.
In order to better quantify the percentage of ice content throughout the SA, a catalog of rock glacier origin and activity status might prove useful. To date, two rock glaciers near Cerro Tapado have been identified as likely glaciogenic (Monnier and Kinnard 2015a; Monnier et al. 2014) and a third as cryogenic in origin (Milana and Güell 2008). It is also possible that these rock glaciers are polygenic. One rock glacier in the Choapa Province has been classified as most likely cryogenic in origin (Monnier and Kinnard 2013). In the Central Andes of Chile, a newly formed rock glacier appearing in the lower part of a debris-covered glacier was identified in the Juncal catchment (Monnier and Kinnard 2015b). Rock glaciers in the SA have been classified as passive or active for the upper Huasco Valley (Nicholson et al. 2009), a sub-catchment of the Yeso River in the Andes of Santiago (Brenning 2003), and for Tapado and Llano de las Liebres glaciers in the Elqui Region (DGA 2010; CEAZA 2012).
Variations in discharge through time
It is important to know the timing of water release at daily, seasonal, and decadal time scales for an improved understanding of the hydrological processes within rock glaciers and to evaluate water resources. The following discussion relies mostly on studies outside of Chile as there is only one study in Chile that examines rock glacier hydrology directly (Pourrier et al. 2014).
Rock glaciers may be recharged from various sources, including precipitation, surface runoff, snowmelt, avalanches, groundwater, or melting of permafrost and/or glacial ice from surrounding areas (Giardino et al. 1992; Krainer and Mostler 2002; Krainer et al. 2007; Harrington et al. 2018). These sources may be incorporated into the perennially frozen permafrost core of the rock glacier (Fig. 2), an upper layer which melts seasonally called the active layer, or pool within the rock glacier forming aquifers or discharge directly without freezing (Barsch 1996; Haeberli et al. 2006). It is important to note that not all of these water sources (e.g., snow cover) will contribute to surface or groundwater flow since water may be lost via evaporation. Mass loss from evaporative processes may be large in the SA. In the Pascua-Lama region model, results show that sublimation accounts for 71–73% of snow ablation (Gascoin et al. 2013) and up to 81% on glacier surfaces (MacDonell et al. 2013). Ayala et al. (2017) showed an elevation dependence for sublimation along the SA, such that close to 3000 m a.s.l., summer sublimation rates were close to 0, and above 5500 m a.s.l., sublimation rates corresponded to over 85% of summer ablation. Near the summit of Cerro Tapado, sublimation accounted for 46% of the total precipitation between 1962 and 1999 (Ginot et al. 2006).
Rock glacier discharge can exhibit diurnal variations (Krainer and Mostler 2002; Berger et al. 2004; Krainer et al. 2007) but fluctuates less than debris-free glaciers (Corte 1976; Bajewsky and Gardner 1989; Pourrier et al. 2014). For example, Tapado Glacier in the Elqui Province consists of a debris-free glacier, debris-covered glacier, and a rock glacier that are hydrologically connected (Fig. 3). Measurements of discharge from the debris-free glacier show relatively large daily oscillations, while measurements from the rock glacier are approximately constant (Pourrier et al. 2014). Discharge from rock glaciers varies between ~ 0.005 and ~ 0.25 m3 s−1 (Bajewsky and Gardner 1989; Barsch 1996; Krainer and Mostler 2002; Harrington 2017).
Seasonal variations in discharge occur, with the lowest flows observed in winter, consisting mostly, if not completely, of baseflow (Burger et al. 1999; Krainer et al. 2007). During winter, the rock glacier is recovering mass from the refreezing of groundwater and snow that is easily trapped on the relatively rough (e.g., boulder-sized material) rock glacier surface (Corte 1976; Croce and Milana 2002). During late spring and summer, this snow trapped near the surface melts, resulting in peak discharge. At this time, a seasonal aquifer often forms perched on top of the frozen core (Burger et al. 1999; Krainer and Mostler 2002; Schrott 2002; Krainer et al. 2007). During the late summer, water is derived mainly from the melting of the permafrost core of the rock glacier and snow patches in depressions on the surface (Burger et al. 1999). In autumn, water still mainly comes from the permafrost core and groundwater, but the drop in temperature causes minimal or even no melting at this time (Burger et al. 1999; Krainer et al. 2007).
Rock glaciers have the capability to store water over relatively long periods of time (decades to centuries) due to the insulation provided by the debris within the active layer, which maintains the core cooler, even on hot days (Burger et al. 1999; Krainer and Mostler 2002; Harrington et al. 2018). Discharge measurements at three rock glaciers in the Austrian Alps support the idea of an insulated core. After the winter snow melts, discharge and diurnal variations decrease significantly there, and there is no correlation between air temperature and discharge, indicating that these glaciers are relatively insensitive to temperature changes (Krainer and Mostler 2002). Perhaps related to the insulating layer, the specific annual mean discharge of active rock glaciers is lower than glaciers (Bajewsky and Gardner 1989; Krainer and Mostler 2002; Monnier and Kinnard 2015b). The presence of an insulating debris layer and lower discharge compared to debris-free glaciers suggests that rock glaciers might be less sensitive to changes in temperature over decadal timescales. Changes in elevation measured on the Tapado glacier complex between 1956-2010 support this hypothesis, as the maximum surface lowering for the rock glacier portion of 10 m is far lower than that of the debris-covered and debris-free glaciers above (40-60 m). However, more studies on ice volume change and long-term studies of rock glacier discharge are needed to verify or disprove this hypothesis. In Chile, discharge measurements span a maximum of three months (Pourrier et al. 2014). In Argentina, measurements at one rock glacier in the Andes of San Juan span one complete year (1990/91; Schrott 2002). Given the limited number of studies available, it is currently not possible to evaluate the impact of climate change on rock glacier discharge.
Modeling the response of rock glaciers to future climate warming
In order to model how the ice content of rock glaciers and, therefore, hydrological contribution will change over time, their sensitivity to changes in climate must be known. It is possible that Andean rock glaciers are very sensitive to changes in climate. Studies in the northern Andes of Chile and Argentina have found that rock glaciers are present near or even below the 0 °C isotherm (Trombotto et al. 1997; Geostudios 1998; Brenning 2005b; Azócar and Brenning 2010; Azócar et al. 2017). However, most rock glaciers around the world are found in areas near or above the − 2 °C isotherm (Humlum 1998). This suggests that Andean rock glaciers are vulnerable and out of balance with the current local climate, as temperatures above 0 °C lead to permafrost degradation (Monnier and Kinnard 2013). Measurements of active layer change in the Argentinian SA (32–33°S), on the rock glacier Morenas Coloradas, provide an indication of the sensitivity to climate change. Here, the active layer has increased by ~ 25 cm/year between 1992 and 2007 (Trombotto and Borzotta 2009). Two studies in the European Alps showed that rock glacier velocities increased over time, which suggests a decrease in ice content associated with an increase in liquid water within rock glaciers (Kääb et al. 2007; Bodin et al. 2009). The same may be true in SA, but there are no data available to verify this.
As there is essentially no information on the response of rock glaciers in the Chilean SA to recent changes in climate, possibilities for modeling their response are very limited. The only future projection in Chile, computed by Nicholson et al. (2009), assumes that the zero-degree isotherm altitude (ZIA) coincides with the lower limit of rock glaciers. This relationship was verified for in the present day by comparing the ZIA to an inventory of glaciers in the upper Huasco valley (Nicholson et al. 2009). It was assumed that rock glaciers will not exist below the annual ZIA in the future, allowing the loss of rock glaciers in the valley to be computed. Under the IPCC scenarios B2 and A2 for 2071–2100, the ZIA projections suggest a loss of 4.4 km2 (70%) and 5.7 km2 (91%) of the current rock-glacier area, respectively. Only active rock glaciers were considered in this study, and these projections do not account for changes in precipitation or rock glacier geometry. A similar study in the Bolivian Andes concludes that all currently active rock glaciers in Bolivia will be at or below the ZIA by 2050 (IPCC A1B scenario (Rangecroft et al. 2016).
Limits of existing studies and research agenda to quantify the hydrological contribution of semiarid Andes rock glaciers
In this section, we discuss the limitations of existing studies and propose research ideas to fill knowledge gaps. We start with a discussion on discharge measurements, then quantifying ice volume at one point in time, changes in ice volume over time, and conclude with rock glacier modeling.
There are currently no published measurements on rock glacier discharge for the SA of Chile or studies that use rock glacier discharge measurements to calculate their contribution to streamflow. Examples of unpublished discharge data for the Elqui Valley include measurements made periodically between 2011 and 2016 (Fig. 1; Table 3) at high elevations in a catchment where several rock glaciers exist (e.g., CEAZA 2012, 2015). In this study, we utilized the unpublished discharge data to calculate the contribution to streamflow for the La Laguna Basin (the “Hydrological importance of glaciers and rock glaciers in the Chilean semiarid Andes” section). These rough approximations, indicating a rock glacier contribution of 9-20%, compare well with rock glacier contributions of 13–30% calculated for basins in the USA and Argentina (Schrott 1996; Brenning 2005b).
In the absence of continuous discharge measurements, it is possible to identify flow sources and approximate contributions from sub-catchments using tracers including isotopes and dissolved solids. For example, a study focused on the Juncal River determined that the Juncal Norte Glacier (a debris-free glacier) headwater sub-catchment contributed at least 50% of summer flows to this river (Rodriguez et al. 2016). Reliable contribution estimates could be obtained because a significant portion of the area of this sub-catchment was covered by debris-free glaciers, but the technique was not successful in discriminating between sub-catchments with rock glaciers covering a significant portion of the area. Unique chemical patterns have been observed for the outflow of Green Lake 5 rock glacier, Colorado Mountains, USA (Williams et al. 2006), and in the Agua Negra drainage basin in the Andes of Argentina (Lecomte et al. 2008), providing a starting point for the discrimination of outflow from rock glacier-dominated basins. Changes in rock glacier ice volume over time could also be used to determine the hydrological contribution, but such data are unavailable in the SA.
A first step to determining changes in ice volume is to calculate the ice volume at one point in time. Rock glacier ice volume has been calculated for the SA of Chile using an inventory of the location of rock glaciers from 1996 to 2000 or 1955 to 1956 aerial photographs (27–35°S; Brenning 2003, 2005b; Azócar and Brenning 2010). These results are a valuable first approximation but are rough estimates. The areal extent is calculated using statistical methods rather than delineating each glacier; it is assumed that ice occupies 50% of the total volume and that the ice debris layer thickness is 30 m or it is calculated from an area-to-volume relationship developed for debris-free alpine glaciers (Chen and Ohmura 1990). Other studies have been completed in the Huasco valley (Nicholson et al. 2009), the Aconcagua River basin (Janke et al. 2017), and within six catchments in the Andes near Santiago (Bodin et al. 2010) using actual rock glacier areas. These studies are an improvement over the statistical method used to estimate rock glacier areas based on the mean area from a sample population in Azócar and Brenning (2010) but make the same assumptions for ice thickness and percentage of ice content. These assumptions are based on measurements from rock glaciers outside of Chile (see the “Hydrological importance of glaciers and rock glaciers in the Chilean semiarid Andes” section) and could therefore be inaccurate. In the SA of Argentina, there is one study which uses thickness and percentage of ice content, determined from geophysical measurements, along with measured areal extent to calculate the ice volume (Croce and Milana 2002). Ideally, similar studies based on local measurements could be carried out in the SA of Chile to obtain more accurate estimates of rock glacier ice volume.
To obtain more realistic ice volume estimates at a regional scale, the areal extent of rock glaciers could be used, ideally derived from recent imagery. Rock glacier areas mapped from satellite images (2000/03) are already available for the SA (Barcaza et al. 2017), and areas mapped from recent imagery (2004 or later) for the Atacama and Huasco valleys are available as described in the “Hydrological importance of glaciers and rock glaciers in the Chilean semiarid Andes” section. In order to efficiently map rock glaciers for the entire SA using recent satellite imagery, it would be useful to develop automatic delineation techniques. Thus far, methods to automatically determine the location of rock glaciers in the landscape with the identification of characteristic ridges and furrows in optical satellite imagery have been developed (Brenning et al. 2012), but there are no methods, to our knowledge, to automatically delineate rock glacier areas. Techniques currently used to delineate debris-covered glaciers (e.g., Atwood et al. 2010; Frey et al. 2012; Robson et al. 2015), landslides (e.g., Strozzi et al. 2010), or permafrost (e.g., Hauck et al. 2004; Kääb et al. 2005) could be modified for this purpose.
Direct measurements of rock glacier thickness and percentage of ice content would also be very valuable but are time-consuming and costly. Indirect geophysical methods verified with borehole measurements at some locations would be a more practical approach to improving ice thickness and percentage of ice content estimates in the SA. GPR has been used successfully to determine the ice thickness for three rock glaciers in Chile (CEAZA 2012; Monnier and Kinnard 2013, 2015a), and resistivity measurements have been used for a rock glacier in Argentina (Croce and Milana 2002). These geophysical techniques (GPR and resistivity) are well-established. GPR measurements on rock glaciers began in 2000 and more than a dozen studies have been published on this specific subject (e.g., Degenhardt et al. 2003; Musil et al. 2006; Hausmann et al. 2007; Monnier et al. 2011). Resistivity measurements on rock glaciers began in the 1970s, and there are, likewise, more than a dozen publications (e.g., Francou et al. 1999; Maurer and Hauck 2007; Bodin et al. 2009; Leopold et al. 2011). However, it is important for measurements of ice thickness to be verified with two or more methods at the same location as GPR and resistivity measurements almost always involve some subjective interpretation and/or interpolation to fill data gaps.
Geophysical data has also been used to calculate the fraction of ice content of three rock glaciers in the Elqui Province (Milana and Güell 2008; Monnier and Kinnard 2015a). Milana and Güell (2008) calculated the percentage of ice content using seismic data velocities by applying a modified version of an empirical formula developed by Müller (1947). The original formula was developed for permafrost areas in the northern hemisphere and provides completely unrealistic ice content percentage values in SA; so, Milana and Güell (2008) modified the formula for the Tapado rock glacier. However, they express serious doubt regarding their modified version and strongly suggest that borehole measurements be completed in SA to properly calibrate the Müller formula. Monnier and Kinnard (2015a) used a novel approach to calculate the percentage of ice content utilizing the density of diffracting points in the radargram (likely boulders), which have a strong negative correlation with the electromagnetic velocity. They assumed that the rock glacier was made up of ice, water, air, and rock debris and calculated the fraction of each component. Rock debris content was determined from the diffracting points, water content from the Topp’s equation (Topp et al. 1980), and air content based on measurements in the Swiss Alps, allowing the percentage of ice content to be calculated. It is similar to techniques applied in the Austrian Alps (Hausmann et al. 2007, 2012). Other published techniques require drilling boreholes which may not be feasible (e.g., Arenson et al. 2002; Musil et al. 2006). Ideally, such geophysical techniques would be applied to many more glaciers in SA and verified with borehole data. In the absence of borehole data, two or more geophysical methods could be applied at each glacier to obtain confidence in the estimated percentage of ice content. Maurer and Hauck (2007) provide a thorough review and of geophysical techniques for alpine rock glaciers and recommend the most economical and effective techniques.
Once enough measurements of rock glacier thickness and percentage of ice content have been obtained, these data could be used to develop models to quantify ice thickness and the percentage of ice content at a regional scale. The only model available for ice thickness is the Chen and Ohmura (1990) relationship, an area-to-volume relationship developed for alpine glaciers. The accuracy of this relationship needs to be tested at sites where rock glacier thickness is measured to evaluate its usefulness. A model that accounts for other factors that may influence rock glacier thickness could also be developed. Factors such as the type of rock glacier (active versus passive), velocity, how it was formed, topographic variables (e.g., elevation, slope, aspect), and talus accumulation could be included.
As the percentage of ice content has an impact on velocity (Moore 2014), mapping of rock glacier rates of movement using satellite imagery could provide a first-order approximation for the percentage of ice content at a regional scale. However, the relationship between the percentage of ice content and movement is complex. At low temperatures (e.g., <− 4 °C), increased percentage of ice content is associated with increased velocities up to an ice content of ~ 85% (Arenson and Springman 2005). Near the melting point, increased debris content, associated with decreased percentage of ice content and an increase in water content, leads to a reduction in strength, increased deformation, and, therefore, velocity (Moore 2014). A numerical flow model, calibrated with known surface velocities and the internal composition of Llano de las Liebres rock glacier, supports this theory. For this rock glacier, with an average ice content of 66%, viscosity is inversely related to debris and water content (Monnier and Kinnard 2016). Monitoring data from rock glaciers in the Swiss Alps and modeling work on these glaciers show that rock glaciers with ground temperatures close to 0 °C generally creep faster than colder rock glaciers, also supporting this theory (Kääb et al. 2007). This relationship is likely to hold true for many rock glaciers, since rock glaciers in mountain environments are often near their melting points (Haeberli et al. 2006). Once the percentage of ice content reaches a critically low level, motion is assumed to decrease with reduced ice content (Janke et al. 2015). Eventually, the rock glacier may become stagnant. This state with zero or near-zero motion has been associated with low or no ice content as observed in ice cores on a rock glacier in the Swiss Alps (e.g., Kääb and Vollmer 2000).
These initial studies provide a good foundation for understanding the complex relationship between percentage of ice content and rock glacier velocities. While additional studies are needed, this method of deriving percentage of ice content at a regional scale is promising. Mapping of rock glacier velocities from synthetic aperture radar (SAR) or optical imagery has already been proven successful (Kääb and Vollmer 2000; Rignot 2002; Kääb et al. 2003; Kääb et al. 2005; Strozzi et al. 2010; Monnier and Kinnard 2017). Optical imagery has also been used to classify rock glaciers into velocity groupings. Those with pronounced transverse ridges and furrows and a steep front are assumed to be actively flowing (25–45% ice content), rock glaciers with ridges and furrows that appear linear in the direction of flow are assumed to have reduced flow or to be stagnant (10–25% ice content), and those with subdued surface topography are assumed to be stagnant (< 10% ice content; Janke et al. 2015, 2017). The rock glacier origin can additionally be used as an independent indicator of the percentage of ice content since rock glaciers with a glaciogenic origin have been found to contain more ice than those with a cryogenic origin (Milana and Güell 2008).
Our knowledge of how rock glaciers have changed in recent decades and ability to predict how they will respond in the future is limited. The two publications on rock glacier change indicate that the areal coverage of selected rock glaciers has not changed significantly between 1975 and 2007 in the Laguna Negra Catchment (~ 33.5°S; Bodin et al. 2010) and between 2000/03 and 2015 throughout the SA of Chile (Barcaza et al. 2017). However, this does not mean that the ice volumes have not changed. An analysis using existing aerial photographs and satellite images could be carried out to evaluate changes in geomorphology or activity status, providing a measure of changes in ice volume. It is evident that the basic information required to develop and run a model of rock glacier evolution in the SA over time is currently lacking. Perhaps because of this paucity of data, the only projections of rock glacier change in the SA use a basic relationship where the lower limit of active rock glaciers is assumed to coincide with the ZIA (described in the “Limits of existing studies and research agenda to quantify the hydrological contribution of semiarid Andes rock glaciers” section; Nicholson et al. 2009; Rangecroft et al. 2016). These model results provide a valuable first approximation of the response of rock glaciers to climate change in the area. Perhaps the model could be improved if multiple inventories of rock glacier areas and activity status were available (e.g., 1950s and present day) so that the assumed invariance of this relationship through time could be tested. Models predicting changes in rock glacier velocities have been developed elsewhere (e.g., Müller et al. 2016), but these do not account for melting and therefore are not useful for predicting changes in the discharge of rock glaciers through time. To adequately model rock glacier contribution to streamflow now and in the future, models of rock glacier ice volume change and hydrology will likely need to be developed. However, our current knowledge and available data of rock glacier ice volume change and hydrology is insufficient to develop such models. For the SA, we have no records of ice volume change from individual rock glaciers and we are aware of only one study published in Chile (Pourrier et al. 2014) and one in Argentina (Schrott 2002) on rock glacier hydrology. Insight into the temporal variability in discharge and cryospheric processes may be gained from studies outside of the SA, but these studies may not be entirely applicable to the SA given the drier climate with significant sublimation (Favier et al. 2009; Gascoin et al. 2013).
There is insufficient knowledge available to adequately quantify the hydrological contribution of rock glaciers to streamflow within the SA. Determining the current and future hydrological contribution of rock glaciers is a critical step toward assessing their importance for water resources in the region and necessary to make informed decisions regarding glacier protection. Such information could provide the scientific backing needed to better formulate a glacier protection and preservation law and to adequately evaluate EIAs. Without knowing rock glacier contributions and/or the amount of water contained within rock glaciers, the resulting EIAs are at risk of being inaccurate, making it impossible to predict the severity of mining or other impacts on water resources in a region where rock glaciers are the dominant glacier type (Azócar and Brenning 2010). In the future, temperatures are expected to rise, precipitation is expected to decline, and debris-free glaciers are expected to decrease in size in the SA; thus, rock glaciers will likely become an increasingly important water resource (Bodin et al. 2010; Vicuña et al. 2011). It is therefore important to quantify the hydrological importance of rock glaciers in order to provide adequate protection to ensure sufficient water availability for future generations. In order to address the lack of information available, we recommend the following strategy:
Undertake ground-truthing of developed inventories.
Arrive to a scientific community consensus regarding definitions of rock glacier classifications and construct related indices for ice content based on region-specific characteristics.
Undertake more measurements of rock glacier ice volume and percentage of ice content to enable improved extrapolation of ice volume at a regional scale.
Undertake inventories of changes in rock glacier geomorphology (e.g., elevation change) and activity status to provide measures of ice loss over time.
Implement long-term temperature monitoring on the surface and at depth on selected rock glaciers.
Setup long-term monitoring of rock glacier discharge and undertake measurements of thickness change to provide baseline data for determining contributions to streamflow.
Evaluate different methods for determining rock glacier contribution and hydrological role.
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We thank Cristián Campos, Sebastián Vivero, Sébastien Monnier, and Rodrigo Ponce for their helpful discussions and/or assistance with this manuscript.
This work was supported by CONICYT + Programa Regional + Fortalecimiento (R16A10003) and FIC-R (2016) Coquimbo (BIP: 40000343). Nicole Schaffer was supported by CONICYT + FONDECYT + Postdoctorado (3180417).
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Schaffer, N., MacDonell, S., Réveillet, M. et al. Rock glaciers as a water resource in a changing climate in the semiarid Chilean Andes. Reg Environ Change 19, 1263–1279 (2019). https://doi.org/10.1007/s10113-018-01459-3
- Rock glacier
- Water resource
- Semiarid Andes
- Ice content
- Glacier area