Inventory of rock glaciers in Himachal Himalaya, India using high-resolution Google Earth imagery

Geomorphology 340 (2019) 103–115

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Inventory of rock glaciers in Himachal Himalaya, India using high-resolution Google Earth imagery Pratima Pandey Geosciences Department, Indian Institute of Remote Sensing, 4-Kalidas Road, Dehradun 248001, India

a r t i c l e

i n f o

Article history: Received 10 October 2018 Received in revised form 8 May 2019 Accepted 8 May 2019 Available online 10 May 2019 Keywords: Rock glaciers Inventory Permafrost Periglacial processes Himachal Himalaya

a b s t r a c t Little information about the number, spatial distribution, and characteristics of rock glaciers in Himachal Himalaya is available. This information is crucial to assess the hydrological contribution of permafrost regimes and to understand its response to changing climate. Employing high-resolution satellite data freely accessible through Google Earth, the first comprehensive rock glacier inventory of Himachal Himalaya is presented in this work. The inventory reports 516 rock glaciers in the study area corresponding to an estimated area of 353 km2 of which 59% have glacier origin and 41% have talus origin. The frontal elevation of the lowest rock glacier in the Himachal Himalaya is 3052 m above sea level which was significantly low, whereas the highest rock glacier occurs at 5503 m above sea level. The mean minimum elevation of rock glaciers was 4484 m above sea level and the maximum was 4900 m above sea level. The majority of the rock glaciers have a northerly aspect (N, NE, NW) followed by westerly aspect signifying that slopes with lower potential incoming solar radiation favour the formation of rock glaciers. The mean annual surface temperature of the rock glaciers derived from Moderate Resolution Imaging Spectroradiometer (MODIS) Land surface temperature (LST) product (MOD11B3) was −1.5 °C, with glacier-derived rock glaciers having colder surfaces than talus-derived rock glaciers. The topographical and climatological parameters greatly influenced the formation and development of rock glaciers. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The cryosphere plays a major role in regulating the global climate system through its feedback mechanism and crucial linkage with surface energy exchange, hydrology, precipitation and global circulation (Goodison et al., 1999). Perennially frozen ground, known as permafrost is an integral component of the cryosphere and is very sensitive towards climate warming. Permafrost refers to any ground material that remains at or below 0 °C for at least two consecutive years (van Everdingen, 1998).Rock glaciers occurrences are direct and visible indicators of mountain permafrost (Barsch, 1996; Haeberli et al., 2006; Azócar et al., 2016; Schmid et al., 2015), and are used as indicator for continuous permafrost and are also important elements of the periglacial process-system. The rock glaciers are described as a group of landform that has origins ranging from mountain creeping permafrost, glaciers and rock avalanche (Haeberli, 1985; Barsch, 1996; Humlum, 1988, 1996; Johnson, 1984). Berthling (2011) defined rock glaciers as the visible expression of cumulative deformation by long-term creep of ice/debris mixtures under permafrost conditions. Permafrost is the essential requirement of maintenance of the ice core of the rock glaciers (Owen and England, 1998), however, they also require other climatic,

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https://doi.org/10.1016/j.geomorph.2019.05.001 0169-555X/© 2019 Elsevier B.V. All rights reserved.

hydrological and geomorphological conditions for their development (Haeberli et al., 2006). Rock glaciers can take several thousands of years to form and are the best geomorphic indicator of permafrost in the steep mountainous regions, therefore the knowledge of their distribution can provide reliable information about the past incidences of permafrost and related climatological conditions (Humlum, 1998; Konrad et al., 1999; Millar et al., 2015; Sorg et al., 2015). Rock glaciers are tongue-like or lobate shaped landforms consisting of unconsolidated rock debris and ice in an alpine environment, creeping downslope due to the deformation of internal ice and frozen sediment (Owen and England, 1998; Schmid et al., 2015; Wang et al., 2017). The longitudinal furrows and the transverse ridges visible on the surface of the rock glaciers are the visual expressions of the internal ice deformations (Barsch, 1996; Frehner et al., 2015). The rock glaciers are characterized by 0.5 to 5 m thick seasonally frozen active layer that thaws each summer (Bonnaventure and Lamoureux, 2013; Pourrier et al., 2015). Despite the surface fluctuations in the snow surface and the air temperature reaching above 0 °C, the cold and dense air settled between the interstitial spaces of rocks helps to maintain the permafrost cool and frozen irrespective of the warmer mean annual temperature at the lower elevations. Hanson and Hoelzle (2004) have discussed the role of snow cover and snow depth in influencing the thermal regime of rock glacier in the Swiss Alps and concluded that timing of development of snow cover thicker than a particular thickness

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(depending on local topographical, climatological factors) would be most important single factor controlling the thermal state of the active layer of rock glaciers. These characteristics of rock glacier enable them to represent as evidence of lower extent of permafrost in the mountainous regions (Humlum, 1997; Bolch et al., 2012; Lilleøren and Etzelüller, 2011; Lilleøren et al., 2013; Scotti et al., 2013). According to the dynamic activity and presence of ice, rock glaciers can be subdivided as active, inactive and fossils/relict type rock glacier (Barsch, 1996). Active rock glaciers creep downslope by few centimeters to few meters under gravity (Berger et al., 2004; Kääb et al., 2003; Necsoiu et al., 2016). Inactive rock glaciers still contain permafrost but are stagnant and relict rock glaciers devoid of both movement and ice, indicating the former presence of permafrost (Haeberli, 1985). The active and inactive rock glacier collectively can be called as intact rock glaciers (Barsch, 1996). In view of the current glacier recession scenario when most of the glaciers are losing ice and are the major source of water, the intact rock glaciers could be considered as a water reservoir as they preserve and increase their ice content (Azócar and Brenning, 2010). Due to the thick insulating debris cover, unlike temperate glaciers, rock glaciers are not sensitive to annual and seasonal fluctuations of temperature and precipitation, thus preserving ice in it which can be released in a warmer climate in the future (Schrott, 1996; Millar and Westfall, 2008). A vast number of rock glacier inventories have been compiled in various mountain regions with maximum inventories compiled for European Alps (Kellerer-Pirklbauer et al., 2012; Krainer and Ribis, 2012; Scotti et al., 2013; Guglielmin and Smiraglia, 1998), Greenland and Scandinavia (Humlum, 2000; Lilleøren and Etzelüller, 2011), Iceland (Etzelmüller et al., 2007), Central Andes (Trombotto et al., 2012; Forte et al., 2016); Chilean Andes (Trombotto et al., 1999; Brenning, 2005), Bolivian Andes (Rangecroft et al., 2014), North America (Ellis and Calkin, 1979; Janke, 2007; Janke et al., 2015; Legg, 2016; Clark et al., 1994), British Columbia (Charbonneau and Smith, 2018), Argentina (Falaschi et al., 2014, 2015, 2016), Asia (Ishikawa et al., 2001; Bolch and Marchenko, 2006; Bolch and Gorbunov, 2014; Schmid et al., 2015; Wang et al., 2017; Jones et al., 2018). Despite the presence of abundant rock glaciers in the Hindu-Kush-Himalaya (HKH) regions, very limited research has been conducted on them particularly within Indian Himalaya. About 1500 rock glaciers have been inventoried in the Kazakh and Kyrgyz Tien Shan (Gorbunov et al., 1992, 1998), 261 active rock glaciers in Northern Tien Shan (Wang et al., 2017), 295 rock glaciers in Daxue Shan, south-eastern Tibetan Plateau (Ran and Liu, 2018), 702 rock glaciers in the HKH region (Schmid et al., 2015), and 6000 rock glaciers in the Nepalese Himalaya (Jones et al., 2018). Very sporadic research has been conducted on the rock glaciers of Karakoram, Pakistan and Indian Himalaya (Hewitt, 2014; Owen and England, 1998; Shroder et al., 2000; Allen et al., 2016). Based on field observations, Owen and England (1998) reported an abundance of rock glaciers in the India, Pakistan Karakoram and Himalaya with a lower limit of about 4000 m above sea level (m asl). Allen et al. (2016) have identified 60 rock glaciers in the Kullu region of Himachal Himalaya. Mayewski and Jeschke (1981) have observed and described an active rock glacier in Jammu and Kashmir with snout elevation of 4055 m asl. In Karakoram range, for the northern regions of India and Pakistan, lowermost elevations of active rock glaciers vary between 3850 and 5100 m asl (Hewitt, 2014; Schmid et al., 2015). In the HKH region, the minimum elevation of rock glaciers varied between 3500 and 5500 m asl (Schmid et al., 2015) whereas in Nepalese Himalaya the rock glaciers were located between as low as 3225 and as high as 5675 m asl (Jones et al., 2018). So far no systematic inventory of rock glaciers in the Indian part of Himalaya has been conducted. In this context, the primary aim of the study was to map and document the distribution and general characteristics of rock glaciers in the Himachal Himalaya employing high resolution Google Earth imageries. The intent of this work was to constitute a first step towards understanding the spatial and altitudinal distribution

of rock glaciers and their topographical and climatological characteristics in the parts of western Himalaya, specifically within the boundary of the Himachal Pradesh, which is a Himalayan state in India. 2. Study area Himachal Pradesh is the north-western Indian Himalayan state located between 30° 22′ 40″ to 33° 12′ 20” N latitudes and 75° 45′ 55“ to 79° 04′ 20″ E longitudes with an area of about 55,000 km2 ranging between 350 m asl and 7000 m asl in altitude (Fig. 1) (Allen et al., 2016). About 80% area of the state has mountain terrain with 10% areas covered with permanent snow and ice (Dobhal and Kumar, 1997). There are N600 glaciers in the Himachal Pradesh concentrating in four major river basins namely Chenab, Ravi, Beas, and Sutlej. All the four rivers in Himachal Pradesh drain their water to Indus River. There is a great variation in the climatic conditions of Himachal Pradesh due to extreme variations in elevation. The climate varies from hot and sub-humid tropical in the southern tracts to cold, alpine and freezing in the northern and eastern mountain ranges with greater elevation. Broadly the state experiences three marked seasons; hot weather season, cold weather season and rainy season (Geological Survey of India, GSI, 2012). The average rainfall in Himachal Pradesh is 1111 mm, varying from 450 mm in Lahaul and Spiti to over 3400 mm (GSI, 2012). The Himachal Pradesh falls in the monsoon arid transition zone and is alternately influenced by Indian Summer Monsoon during summer and mid-latitude westerlies during winter (Bookhagen and Burbank, 2010). The southern part of the state falls in the windward side of Indian Summer Monsoon and the northern part lies in the leeward side of it. An extrapolated study (Azam et al., 2014) has reported a high humid, warm and calm summer monsoon from June to September and cold and windy winter from December to March. Analysis of observed data report rise of temperature more than the global average in this part of Himalaya (Bhutiyani et al., 2007; Shekhar et al., 2010). 3. Material and methods The rock glacier inventory were completed using high-resolution remote sensing data available through Google Earth. The Google Earth data with quality of aerial photographs have been applied for a range of scientific research areas (Butler, 2006; Nourbakhsh et al., 2006; Ballagh et al., 2007, 2011; Chang et al., 2009; Sheppard and Cizek, 2009; Yu and Gong, 2012). The images used in Google Earth are SPOT Images or products from Digital Globe (e.g., Ikonos, QuickBird) with a resolution close to aerial photographs. The images were georectified with a DEM based on the Shuttle Radar Topography Mission SRTM) data, which have a 90 m resolution in the research area. Further, Google Earth supports user-friendly GIS tools facilitating the formation of a user-defined database which can be exported as KML files and converted into shapefiles for further analysis in ArcGIS environment (Schmid et al., 2015; Jones et al., 2018). Google Earth has been used previously as a platform for mapping of rock glaciers in British Columbia, the Bolivian Andes, the Hindu Kush-Himalayan region and Nepalese Himalaya (Charbonneau and Smith, 2018; Rangecroft et al., 2014; Schmid et al., 2015; Jones et al., 2018; Ran and Liu, 2018). In the absence of any spectral and spatial information of the used images, quantification of uncertainty in the inventory was difficult. However, in a similar location (Schmid et al., 2015) the accuracy of the images was found to be sufficient for the purpose. Based on the distinct flow and structural patterns of rock glaciers, visual identification and pinning were done on the cloud and snow-free summer images of Google Earth. The prominent transversal (ridges and furrows) or longitudinal flow features, steep frontal slopes, convex shape body and the texture difference of the rock glacier from the surrounding were the main criteria for visual identification of the rock glaciers on Google Earth imageries. A polygonized inventory with detailed spatial attributes was created within Google Earth by manually

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Fig. 1. Spatial distribution of rock glaciers in Himachal Himalaya shown on SRTM DEM. The four river basins (e.g. Chenab, Satlej, Beas and Ravi) have been marked on the map.

digitizing the located and pinned rock glaciers. The historical imageries, the navigation key and the pseudo -3D viewer in Google Earth were extensively used to minimize the ambiguity associated with clouds, snow cover and shadow effects. The methodology of Stumm et al. (2015) and Jones et al. (2018) was followed for the inventory. The landform characteristics such as geographic coordinates (centroid), minimum and maximum elevation, length of the rock glacier parallel to the flow, the width of the rock glacier perpendicular to the flow, the main aspect, and origin (glacier-derived or talus-derived) were extracted and recorded for each rock glaciers. The general aspect of each rock glaciers was manually derived according to the main flow direction and assigned into 8 classes. The lowermost point of the rock glacier also known as toe has been taken as the best first estimate for identification of rock glacier and considered discrete boundary between the rock glacier and the surrounding terrain. The elevation at which the front slope of the rock glacier meets the slope beneath it was considered as the minimum elevation of the rock glacier (Scotti et al., 2013). For each rock glacier, the whole landform was mapped from the rooting zone to the foot of the front slope (Barsch, 1996). However, defining the upper boundary of the rock glacier has been found to be the most critical part of the mapping (Krainer and Ribis, 2012) and consistent judgment was made on deciding the upper boundary. Upper boundary of some of the rock glacier might not be delineated correctly owing to the delimitation of the upper boundary of rock glaciers. Since, the whole analysis was carried out by only one researcher, the uncertainties associated with identification and delineation were supposed to be consistent. The delineation of complex rock glaciers where multiple rock glaciers coalesce into single rock glacier is inherently subjective (Scotti et al., 2013; Schmid et al., 2015). Nonetheless, it was challenging to distinguish the rock glaciers from similar landforms (debris covered glaciers, debris flow,

gelifluction) however, owing to the high resolution Google Earth imageries, it was possible to differentiate between other landforms and rock glaciers. Delineation of separate or complex rock glaciers was done by following the criteria of Scotti et al. (2013) and Jones et al. (2018). After digitization within Google Earth, the polygons were exported to shapefiles in ArcGIS and re-projected to WGS 84-UTM Zone 43 N for further analysis. Fig. 2 demonstrates the digitization of rock glacier boundary within Google Earth, measurement of length and width of the glacier. The elevation, slope, aspect, and area of the polygons were revisited in ArcGIS using SRTM30DEM. Table 1 summarizes the attributes used in the inventory. Fig. 3 shows the various landforms similar to rock glaciers. In terms of activities, rock glaciers can be classified into active, inactive and relict rock glaciers. An active rock glacier was recognized as a landform with a steep front near the angle of repose with visible longitudinal and transverse ridge/furrow representing internal deformation (Barsch, 1996; Haeberli, 1985; Charbonneau and Smith, 2018). Inactive rock glaciers also contain an ice core but are no longer mobile due to the melting of most of the upper layers within the frontal slope (Barsch, 1996; Scotti et al., 2013). Active and inactive types are designated together as ‘intact rock glaciers’ (Haeberli, 1985). Relict or fossil rock glaciers lack both movement and ice and indicate the former presence of permafrost. Since direct assessment of the kinematic status of rock glaciers was difficult, the distinction between active and inactive rock glaciers become challenging (Onaca et al., 2016; Sattler et al., 2016) and erroneous classification between active and inactive rock glaciers was obvious. In view of the difficulties involved in the differentiation of active and inactive rock glaciers due to lack of data regarding the flow, and to avoid any misclassification, the active and inactive rock glaciers were treated as one group and in absence of any ground observations,

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Fig. 2. Example of measurement of (a) maximum length and (b) width of rock glacier using Google Earth's measurement tool. MEF = Minimum elevation of the front, MaxE = Maximum elevation of the rock glacier.

only rock glaciers with visible intact morphology were considered. In future therefore, the research could be benefitted from the additional information of rock glacier movement from field observation and SAR interferometry. A lot of confusion exists towards genesis and formation of rock glaciers (Berthling, 2011), however, conventionally they have been regarded as the transitional features formed by the interaction between ice of glacial and periglacial origin (Haeberli et al., 2006; Charbonneau and Smith, 2018). Rock glaciers can be classified on the basis of their origin and ice presence. Depending on the visual interpretation techniques on high-resolution images, the most suitable rock glacier classification was to distinguish them in terms of dynamics i.e. rock glaciers influenced by slope dynamics and rock glaciers influenced by glacier dynamics (Charbonneau and Smith, 2018). They can be called as talus-derived and glacier-derived respectively (Fig. 4). The talusderived rock glaciers have an established place in the literature and they are considered as true rock glaciers (Clark et al., 1998). Rock glaciers formed by glacial activities (glacier-derived) have also been discussed previously (Humlum, 1996, 1998), however, they have not been considered to have a glacigenic origin (Berthling, 2011; Clark et al., 1998). Glacier-derived rock glaciers were defined as the landforms situated directly underneath a moraine indicating former glacier (Fig. 5a and b) or an existing glacier (Humlum, 1988). Similar features have been discussed in western Greenland (Humlum, 1996, 1997), Wyoming (Clark et al., 1998), the Andes of central Chile (Brenning, 2005), and French Alps (Monnier et al., 2013). Humlum (1997, 1998) have provided an argument for these landforms to be considered as permafrost as they display active layer dynamics. According to Haeberli et al. Table 1 Attributes derived for the inventory of the rock glaciers. Attribute

Explanation

RG ID

No given to rock glaciers from 1 to 516, roughly following clockwise direction Latitude of the centroid of rock glacier Longitude of the centroid of rock glacier Total area of the rock glacier Minimum elevation of the front of the rock glacier

Lat Lon Area (km2) Min Ele(m asl) Max Ele(m asl) Length (m) Width (m) Aspect class Genesis

Maximum elevation of the rock glacier Maximum length of the rock glacier Maximum width of the rock glacier Major aspect of the rock glacier (N, NE, E, SE, S, SW, W, NW) Whether glacier-derived or Talus-derived

(2006) and Berthling (2011), it is not possible to differentiate the rock glaciers into purely glacial or periglacial without considering the permafrost-glacier interactions which explain these landforms. 4. Results A total of 516 active rock glaciers covering an area of about 353 km2 with an average lower limit at 4484 m asl, an average area of 0.68 km2, length and width of about 1.6 km and 375 m respectively have been identified and inventoried in the Himachal Himalaya. The largest mapped rock glacier in the area was 4.3 km2 whereas the smallest rock glacier was of about 0.03 km2. The longest rock glacier was of 6 km length and shortest was of about 300 m lengths. The glacierderived rock glaciers were more abundant than the talus-derived rock glaciers in the study region. About 59% of the rock glaciers in the Himachal Himalaya were glacier-derived and the remaining 41% were talus-derived. A number of rock glaciers with multiple episodes of activity were also found in the study area where newer lobes have overridden the older lobes. Compound rock glaciers with more than one root zones were very common in the study area. Fig. 6 displays examples of rock glaciers in the study area. The distribution of the rock glaciers in the study area was very heterogeneous with very few number of rock glaciers in the central and northern part of the Himachal. The north facing slope were found to be more conducive for the formation of rock glaciers than the south facing slopes. Lesser solar radiation in the northern facing slopes have favoured the development of rock glaciers (Figs. 7, 9). The number of rock glaciers inventoried in this study can be considered as a conservative estimation. Table 2 lists the key characteristics of the rock glaciers obtained from the analysis. 4.1. Rock glacier elevation distribution and topography The rock glaciers in Himachal Himalaya occurred between altitude ranges of 3052 to 5503 m asl. The mean minimum elevation of rock glaciers was 4484 m asl whereas the mean maximum elevation was 4900 m asl. The majority of rock glaciers with 42% were located between 4000 and 4500 m asl elevation ranges, 35% between 4500 and 5000 m asl, 12% between 5000 and 5500 m asl, 10% between 3500 and 4000 m asl and 1% between 3000 and 3500 m asl (Fig. 8a). A total of 4 rock glaciers were found to be below 3550 m asl. Two parameters viz. relief and compactness ratio of each rock glaciers were used to demonstrate the topographical control on their distribution in the region. The rock glaciers formed at lower altitudes were found to have higher

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Fig. 3. Examples of landforms similar to rock glaciers (a) an avalanche fed fully debris covered glacier (Hamtah glacier); (b) Debris flow; (c) Gelifluction process.

relief than that formed at higher altitude (Fig. 8b). The compactness ratio is the measure of glacier morphometry and has been derived from the formula(4 × pi × Area) / (Perimeter) 2 following DeBeer and Sharp (2007) and Way et al. (2014). The compactness ratio of a rock glacier specifies potential mass inputs (rock/debris/ice/snow) on glacier surface from surrounding avalanching zones. A rock glacier with a low compactness ratio is viable to receive lesser mass from its surrounding upslope than the glacier with higher compactness ratio. As a reference, the compactness ratio of Chhota Shigri and Hamtah glaciers in this zone are 0.06 and 0.24 respectively where Hamtah is considered to be an avalanche fed glacier with a high compactness ratio (Pandey et al., 2016). The compactness ratio of the rock glaciers in these zones was also derived to understand the possible contribution of the mass inputs from the surroundings of the rock glaciers. It was observed that the rock glaciers in this region have in general high compactness ratio, which could be indicative of large supply of mass from the surrounding, making the catchment suitable for the development and sustenance of the rock glaciers (Fig. 8c). Also, a positive relation was found between the

compactness ratio of rock glaciers and their minimum elevation, illustrating that the rock glaciers formed at lower elevations were wider than those formed at higher elevations (Fig. 8d). In the context of basins, the highest rock glacier was located in Satlej basin at 5503 m asl and the lowest was found in Chenab basin at 3052 m asl. The mean minimum elevation of rock glacier in Satlej basin was also highest at 4720 m asl, followed by Chenab at 4325 m asl, Beas at 4280 m asl with lowest in Ravi basin at 4105 m asl. A correlation analysis was carried out between the minimum elevation of rock glacier and latitude and longitude to understand the influence of geographical locations on the altitudinal distributions of rock glaciers. The analysis revealed that the minimum altitudes of rock glaciers generally increased from west to east and decreased from south to north, favouring larger rock glaciers to develop in the eastern side than the western side (Fig. 9). This could be attributed to humidity brought by monsoon which has more strength on the eastern side than the western side. The similar geographical pattern of spatial distribution of rock glaciers was reported in Tien Shan by Wang et al. (2017).

Fig. 4. Examples of (a) glacier-derived rock glacier and (b) talus-derived rock glacier in the study area.

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Fig. 5. Examples of (a) originating from moraine (b) rock glacier apparently connected to a glacier without a distinct separation between glacier ice and rock glacier body.

4.2. Rock glacier aspect The analysis exposed that the north-facing slopes would have promoted the formation of rock glaciers in the region. About 48% of the rock glaciers were developed within north facing aspects with 27% N, 12% NE and 9% NW aspects, whereas a significant 17% of the rock glaciers have developed at western flanks. Only 8% of rock glaciers, covering 6% of total rock glacier area have developed in the south-facing slopes. On the south-east-facing slopes, 3% of rock glaciers have formed (Figs. 10a and 11). The glacier-derived rock glacier demonstrated a predominant north face while talus-derived glaciers displayed a broader distribution in aspect with the majority facing northern slopes (Fig. 10c). An overall analysis showed that rock glaciers developed at lower elevations in the northern aspects while at higher elevation in the southern aspects (Fig. 10b). A significant variability was observed in the aspect wise distribution of rock glacier area. The northern slopes have larger rock glacier area followed by western slopes. A total of more 30% rock glacier area was located on the northern slopes (Fig. 10a). Rock glaciers in Chenab and Satlej basins have predominating north aspects, while Ravi and Beas basins displayed significant formation of rock glaciers with western orientations also (Fig. 10d).

4.3. Potential incoming solar radiation (PISR) To understand the meteorological settings of rock glaciers, the potential incoming solar radiation from 30 m SRTM DEM for the noon of 30th September (end of ablation season) using ArcGIS was extracted.

The PISR influences the ground temperature of rock glaciers and can be taken as a proxy to explore the subsurface thermal conditions of the rock glaciers. The distribution of PISR is predominantly influenced by surface orography, elevation, slope and relief and plays a crucial role in the formation of permafrost (Wang et al., 2017) by influencing the microclimate and hence ground temperature. A positive relation between the minimum altitude and relief of rock glaciers with PISR was observed, signifying that higher rock glaciers with greater relief were likely to receive more solar radiation than rock glaciers located at lower altitudes (Fig. 12a, b). Again, the south-facing rock glaciers have higher PISR than north-facing rock glaciers (Fig. 12c) which was obvious. Since, in the study region, most of the rock glaciers were northfacing, it can be inferred that slopes with lower PISR favoured the development of rock glaciers. 4.4. Ground surface temperature The ground thermal conditions of the rock glaciers considered to be the controlling factor for their formation and distribution (Humlum, 2000; Serrano and López-Martínez, 2000; Frauenfelder, 2005; Haeberli et al., 2006; Kääb, 2007; Janke et al., 2015). A close relationship governed by topography exists between ground surface temperature and air temperature and radiation. To comprehend the thermal control of the ground on rock glacier distribution, the MODIS-Land surface temperature (LST) product (MOD11B3) were used. The MOD11B3 version 6 product provides average, monthly per pixel land surface temperature (LST) in a 1200 × 1200 (km) tile with a pixel size of 5600 m (m). Each Land Surface Temperature (LST) and emissivity pixel value in the MOD11B3 is a simple average of all the corresponding values from the

Fig. 6. (a) A typical glacier-derived rock glacier; (b) Rock glacier showing overridden lobes; (c) a rock glacier spilled till valley touching river; (d) a rock glacier near Zing-zing Bar, Patsio region reaching till NH road.

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Fig. 7. The potential incoming solar radiation of the study area derived from 30 m SRTM DEM (derived for the ablation season i.e. 30th September) showing locations of rock glacier and zoomed in image depict the example of rock glaciers located with northern aspects with lesser incoming radiation.

LST values from the MOD11B1 collected during the month period (Wan et al., 2015). The arithmetic mean of day and night time temperature was considered as the mean surface temperature. From the monthly average LST, mean annual LST was obtained. The average of the mean annual air temperature for the years 2017 and 2018 was used to represent the Mean Annual Air temperature (MAAT). Niu et al. (2018) have compared mean annual MODIS LST with the field measurement of the air temperature and concluded that mean annual MODIS LST can well represent the MAAT spatially. The mean annual surface temperature (MAST) at each rock glacier surface was recorded. The MAST of the rock glaciers as observed from MODIS data found to be varied between −10 °C to 10.54 °C with a mean of −1.5 °C. The MAST of about 67% of rock glaciers was b0 °C, however, 33% of the rock glaciers had a MAST N0 °C. The MAST of glacier-derived rock glaciers was −2 °C and that of talus-derived rock glaciers were −1 °C, demonstrating that the surface of glacier-derived rock glaciers was cooler than the surface of the talus-derived rock glaciers. Fig. 13a and b show the frequency distribution of rock glaciers with respect to MAST. An inverse relationship was observed between MAST and the minimum altitude and area of rock glaciers (Fig. 13c, d). The lesser surface temperature thus can be considered as facilitating the formation of larger rock glaciers.

4.5. Comparison with the Permafrost Zonation Index (PZI) The mapped rock glaciers have been compared with the Global Permafrost Zonation Index (PZI) given by Gruber (2012). The PZI is an index representing a comprehensive spatial pattern of permafrost occurrence at 1 km resolution. However, PZI does not provide actual permafrost extent or probability of permafrost (Schmid et al., 2015). The PZI has been compared with mapped rock glaciers for the HKH region (Schmid et al., 2015) and Nepal (Jones et al., 2018). PZI values b0.1 attributed to the PZI fringe of uncertainty – “the zone of uncertainty over which PZI could extend under conservative estimates”, and PZI values ≥0.1 form the permafrost region (PR). In this study, 18% of rock glaciers were situated within the PZI fringe of uncertainty, with the remaining 82% rock glaciers in the permafrost region (Fig. 14 a, b). It should be noted here that the PZI is strictly controlled temperature however, rock glaciers entail other meteorological, hydrological, topographical and geomorphological conditions for their development in addition to permafrost (Haeberli et al., 2006; Allen et al., 2016). Allen et al. (2016) have argued that in this region, the rock glacier zone can extend down below currently expected permafrost limits owing to their slow downslope movement.

Table 2 Basic mean characteristics of rock glaciers. Region

No of RG

GL (%)

TL (%)

Av area km2

MEF m asl

MaxE m asl

Length km

Width m

Aspects

Total

516

59

41

0.68

4484

4900

1.6

375

Chenab Ravi Beas Satlej

91 82 89 254

66 46 19 73

34 54 81 27

0.92 0.52 0.46 0.73

4352 4105 4280 4720

4829 4502 4616 5149

1.9 1.3 1.2 1.7

388 355 392 365

N (138) NE(60) E(59) SE(16) S(39) SW(58) W(96) NW(50) N&W N&W N N

RG: Rock Glacier; GL: Glacier derived; TL: Talus-derived; MEF: Minimum elevation of the front; MaxE: Maximum elevation.

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Fig. 8. Topographical characteristics of rock glaciers (a) rock glacier frequency with respect to their minimum elevation; (b) relief of rock glacier versus minimum elevation; (c) compactness of rock glaciers; (d) compactness ratio versus minimum elevation of rock glaciers.

5. Discussion An abundance of rock glaciers existed in the Himachal Himalaya as identified by high resolution Google earth imageries. The present inventory has identified and analyzed 516 rock glaciers in the Himachal Himalaya. However, in view of the limitations involved with the remote sensing data, the presence of more number of rock glaciers in this zone cannot be ruled out. All the rock glaciers were classified as talus-derived, originating from the talus below a steep headwall and glacier-derived, existing either below a cirque or a normal glacier or a moraine. Presence of a significant number of rock glaciers outside permafrost (PZI) zones ascribed the fact that the formation of rock glaciers needs other climatic, topographical, hydrological and geomorphological conditions in addition to permafrost. Northern slopes with lesser PISR have found to be the more favourable locations for the development of rock glaciers. Rock glaciers in this region were characterized with higher compactness ratio showing the potential of abundance mass supply from surrounding slopes. Rock glaciers of lower altitude were observed to be wider than their counterparts at higher altitude. From the scatter plot (Fig. 10a, c) it was inferred that the shaded locations receiving lesser PISR can present a favourable condition for the formation of rock glaciers even at lower altitudes. Allen et al. (2016) have mentioned that for the Himalayan region, the basic rule of defining permafrost boundary may not be applicable in view of the geographically variable climatic settings of Himalaya. This part of Himalaya falls in the climatically transition zones, with the south-east part influencing predominantly by Indian summer monsoon

(ISM) and north-west part by western disturbances. Therefore, this region has unique climatic settings. Larger proportion of rock glaciers was observed to be glacier-derived (59%) than talus-derived (41%). The dominance of glacier-derived rock glaciers in the region was in agreement with the other studies (Jones et al., 2018; Berthling, 2011). Spatial analysis showed that the majority of rock glaciers (74%) of Satlej and Chenab (66%) basins were glacierderived, whereas the most of the rock glaciers (81%) in Beas basin were of talus-derived. Ravi basin has an almost homogeneous occurrence of glacier-derived (56%) and talus-derived (44%) of rock glaciers. The minimum elevation of rock glacier was situated between 3052 and 5503 m asl which was broadly consistent with the minimum elevation reported for Nepalese region (3225–5675 m asl, Jones et al., 2018), for the KHK region (3500–5500 m asl, Schmid et al., 2015) and for Tien Shan region (3174 m asl, Wang et al., 2017). The average length and width of the rock glacier were about 1.6 km and 375 m respectively which was in agreement with that reported by Owen and England for this region (1998). The average area of the rock glacier was about 0.68 km2. The lower limit of the rock glaciers in Himachal Himalaya was found to be lower than reported by Owen and England, 1998 and also than the lowermost elevation of permafrost in Kullu region suggested by Allen et al. (2016). Existence of rock glacier below the lower limit of permafrost was explained by Allen et al. (2016) attributing to their slow dynamical downslope motion. The lowest rock glacier was identified near the Udaipur region of Chenab basin with a minimum elevation of 3052 m asl and a maximum altitude of about 4000 m asl and having an area of about 2.6 km2 with glacier-derived origin. Among the

Fig. 9. Relation between latitude and longitude with the mean minimum elevation and area covered by rock glaciers.

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Fig. 10. a) Aspect wise distribution of total area covered by rock glacier; b) Mean minimum elevation for each aspect class; c) major aspect of glaciers including Gl: glacier-derived and Tl: talus-derived; d) the aspects wise distribution of glaciers in the four major river basins.

four major basins of the Himachal Himalaya, Satlej being the largest has hosted a maximum number of rock glaciers followed by Chenab, Beas, and Ravi. The analysis of rock glaciers in the Himachal Himalaya demonstrated the northern aspect as the most favourable orientation for the development of rock glacier followed by western aspect. This was in agreement with other findings in the HKH region (Wang et al., 2017; Jones et al., 2018). Corroborating with the others' findings, the rock glaciers with north and western aspects have lower minimum elevations than that of south and east aspects glaciers (Seppi et al., 2012; Scotti et al., 2013; Jones et al., 2018). On an average, the lowermost elevation of rock glacier with northern aspect was 4420 m asl (including N, NE, and NW), with the western aspect as 4502 m asl and that of southern aspect as 4643 m asl (including S, SE and SW). Also, the northern aspect has hosted the maximum area (31%) of rock glaciers with south aspect as minimum (6%). A total of 49% (22%) glacier-derived and 36% (29%) talus-derived rock glaciers were with northern aspect (southern)

Fig. 11. Aspect wise distribution of rock glaciers as shown in rose diagram.

including north-eastern (south-eastern) and north-western (southwestern). From the derived potential incoming shortwave radiation from SRTM DEM, it was evident that the development of rock glaciers preferred areas with lower incoming solar radiations. It can be therefore coined that the reduced insolation of northern and western aspects have enabled the formation and conservation of rock glaciers as also noticed by Jones et al. (2018) for Nepal region. The average area of rock glacier was found to be largest in the Chenab basin at 0.9 km2 and smallest for Beas basin at 0.46 km2. In general, the rock glaciers were found to occur in cluster, however, few rock glaciers were also observed to exist sporadically in the study area. Many of the rock glaciers in the Himachal Himalaya were morphologically complex and with the congregation of lobes resulted from differential movement of the landform. As reported also by Owen and England, 1998, some of the rock glaciers have overridden older rock glacier suggesting a distinct interval of rock glacier formation. A number of rock glaciers were observed to be advancing from cirques with supplies from surrounding peaks by rockfalls and rock avalanches. Many rock glaciers visually appeared to be advancing. Also, there were a number of rock glaciers which spilled up to the Valley and touched the river downstream (Fig. 4c). Very less abundance to no occurrence of rock glaciers was found near upper Chandra valley and to the north and south of Spiti Valley. The topographical analysis of rock glaciers showed that rock glaciers in this region have suitable topographical settings in terms of mass (debris) supply from the surrounding as indicated by the higher compactness ratio of rock glaciers in the region. Rock glaciers at lower altitudes were wider than the rock glaciers formed at higher altitudes. MODIS LST was used to understand the climatological settings of the rock glaciers. The mean surface temperature of rock glaciers was −1.5 °C with glacierderived rock glaciers having lesser surface temperature (−2 °C) than talus-derived rock glaciers (−1 °C). One of the reason for lower surface temperature of glacier-derived rock glacier could be due to the fact that more percentage of glacier-derived rock glaciers were located at northern slopes than the talus-derived rock glaciers. Additionally, the origins of the rock glaciers i.e. glacier and talus might also have control on their surface temperature. However, the analysis was done with relatively coarser

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Fig. 12. (a) The potential incoming solar radiation (PISR) with respect to the minimum elevation of the rock glacier; (b) PISR versus PZI of rock glaciers; (c) the aspect wise distribution of rock glaciers.

MODIS LST data which require further investigation with high-resolution surface temperature data and field validation. The decrease in surface temperature with altitude was obvious. It was noted from the analysis that larger rock glaciers have lesser surface temperature than smaller rock glaciers. From the inverse relationship between rock glacier area and surface temperature, it can be inferred that colder surfaces have aided in the formation of larger rock glaciers. A total of 33% of rock glaciers were found to have a surface temperature above 0 °C. Further analysis of rock glaciers with surface temperature N0 °C revealed that these rock glaciers were mostly located at northern slopes with lesser PISR. It could be inferred that in a few cases if topographical conditions are favourable, rock glaciers can be developed even at locations with above 0 °C. This was in agreement with the

suggestions given by Allen et al. (2016) for Himalayan regions and by Hoelzle (1992) for the Alps region. An attempt has been made to compare the ratio between the area occupied by rock glaciers and that of glaciers in the study region (Fig. 15). The glacier area of the region has been obtained from the inventory published by Geological Survey of India (2011). This ratio can be taken as a representative for the dominance of glacial and periglacial activities in the region. It was noteworthy to observe that the ratio between rock glacier area and glacier area was significantly higher for Ravi basin, even though the basin has the least number of rock glaciers. The ratio was lowest for the Chenab basin. It could be inferred that the periglacial activities in the Ravi basin are prevalent than the glacial activities. However, this analysis was just an effort to understand the ratio and

Fig. 13. (a) Frequency distribution of rock glacier as per temperature; (b) distribution of rock glaciers with temperature lesser than 0 °C; (c) relationship between rock glacier temperature and minimum elevation; (d) relationship between rock glacier temperature and area.

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Fig. 14. (a) Spatial distribution of rock glaciers shown on PZI map of Himachal Pradesh, the cyan area represents the fringe of uncertainty; (b) the frequency of rock glaciers as per PZI values.

should be treated cautiously in lack of comprehensive field observation and validation.

Fig. 15. The distribution of the ratio between rock glacier area and total glacier area.

6. Conclusions The study presented the first comprehensive documentation of characteristic of rock glaciers in the Himachal Himalaya. A total of 516 active rock glaciers occupying an estimated area of 353 km2 was inventoried and analyzed. The minimum lower elevation of rock glaciers in this part of Himalaya found between 3052 m asl (ranging between 3052 and 5503 m asl) which was lower than previously reported minimum elevations. About 59% of the rock glaciers have glacier origin and 41% have talus origin. The mean annual temperature of the rock glacier was −1.5 °C. The glacier-derived rock glaciers have lesser surface temperature than the talus-derived rock glaciers. Majority of them had northerly (N, NE, NW) to westerly aspects suggesting the important role of solar insolation in their formation and preservation. Slopes with lower PISR favoured the development of rock glaciers. Additionally, the rock glaciers with northern to western aspects have lower elevations than those with the south to east aspects. The altitudes of rock glaciers generally increased from west to east and decreased from south to north indicating influence of latitude and longitude on

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the rock glacier altitude. The present inventory provided a baseline dataset for the further research on rock glaciers as a water reservoir and also for permafrost study for slope instability, water resources and emission of greenhouse gases. Acknowledgements The author is very grateful to the three anonymous reviewers for their careful and meticulous reading of the paper and critical comments which notably improved the manuscript. Thank is due to Dr. Prakash Chauhan, Director, IIRS and Dr. P.K. Champti ray, Group Head, G&DMS, IIRS for their support and encouragement. I also acknowledge Ms. Vasudha Chaturvedi and Mr. Raza Ataullah Khan for their various helps during the preparation of the revision of the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.geomorph.2019.05.001. References Allen, S.K., Fiddes, J., Linsbauer, A., Randhawa, S.S., Saklani, B., Salzmann, N., 2016. 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