Rock glaciers represent hidden water stores in the Himalaya
Graphical abstract
Introduction
The cryosphere of High Mountain Asia (HMA), which comprises the Tibetan Plateau and its surrounding mountain ranges (including the Himalaya, Karakoram, Tien Shan, and Pamir), forms water towers that are integral for ecosystem services provision, and for servicing the multiple societal needs of ~800 million people living in the mountains and surrounding lowlands (Pritchard, 2019). These mountain water towers (e.g., Indus and Ganges-Brahmaputra) are among the most important globally. However, most are also highly vulnerable as they are “transboundary, densely populated, heavily irrigated basins and their vulnerability is primarily driven by high population andeconomic growth rates and, in most cases, ineffective governance” (Immerzeel et al., 2020). Furthermore, considerable and continued glacier mass loss is projected throughout this century (Kraaijenbrink et al., 2017; Hock et al., 2019; Shannon et al., 2019). A high-end climate change scenario (Representative Concentration Pathways [RCP] 8.5) is projected to lead to a HMA glacier volume loss of ~95% relative to the present-day (Shannon et al., 2019). Volume losses are driven by an average temperature change of +5.9 °C and a +20.9% rise in average precipitation, the latter increasingly of rain rather than snow (Fig. 1). Indeed, reductions in snow water equivalent have been reported for a number of catchments in HMA, particularly during spring and summer (Smith and Bookhagen, 2018). For the RCP4.5 scenario, most basins fed by HMA glaciers are projected to reach peak water by ~2050: 2045 ± 17 years (Indus), 2044 ± 21 years (Ganges) and 2049 ± 18 years (Brahmaputra), for example (Huss and Hock, 2018).
Given the need for strong climate adaptation in HMA, a clearer understanding of all components of the hydrological cycle in the high-mountain cryosphere is required (Jones et al., 2019). Recent research shows that rock glaciers constitute globally significant long-term water stores (Jones et al., 2018a). Rock glaciers are masses of poorly sorted, angular-rock debris bound together by massive ice or an ice-cemented matrix, which creep slowly downslope (Martin and Whalley, 1987; Barsch, 1996; Haeberli et al., 2006; Berthling, 2011). Typically, rock glaciers are characterised by distinctive flow-like morphometric features, including spatially organised transverse and longitudinal ridge-and-furrow assemblages, and steep (approx. >30–35°; gradients of >40° have been observed [Krainer et al., 2012]) and sharp-crested frontal and lateral slopes (Wahrhaftig and Cox, 1959; Baroni et al., 2004; Kääb and Weber, 2004) (Fig. 2). They are further characterised by a continuous, thick seasonally frozen debris layer (known as the active layer [AL]) – owing to the insulating and damping properties of the AL, rock glaciers are considered to be climatically more resistant than debris-free and debris-covered glaciers. Consequently, their relative hydrological importance vs glaciers will increase under future climate warming (Jones et al., 2018a; Jones et al., 2019).
Yet, to date, with a few notable exceptions (Jones et al., 2019; Schaffer et al., 2019), the hydrological role of rock glaciers globally has been afforded little attention compared to both debris-free glaciers (Fountain and Walder, 1998; Jansson et al., 2003; Irvine-Fynn et al., 2011) and debris-covered glaciers (Fyffe et al., 2019, and references therein). In the Himalaya, a recent impactful report synthesised and evaluated the state of current scientific knowledge regarding changes in the high-mountain cryosphere; however, rock glaciers received no critical attention (Bolch et al., 2019). Furthermore, while systematic rock glacier inventory coverage has increased globally, HMA is comparatively data-deficient (Jones et al., 2018a). Across HMA, with few exceptions (Jones et al., 2018b; Blöthe et al., 2019; Pandey, 2019; Baral et al., 2020), rock glacier inventories have been conducted at localised sites, over relatively small spatial scales or using non-spatially explicit methods (Regmi, 2008; Bolch and Gorbunov, 2014; Schmid et al., 2015). As a result, the distribution and hydrological value of rock glaciers remains unknown. In HMA, Pritchard (2019) notes that “detailed and comprehensive assessments of the future water availability in the region are only possible once the present hydrological regime is better quantified (Miller et al., 2012)”. Therefore, we argue that quantifying rock glacier WVEQ across HMA is a critical requirement to quantify the present, and future, hydrological regime of the region.
Consequently, our primary objective was to calculate the first estimation of rock glacier WVEQ across the Himalaya. To do this we compiled the first systematic rock glacier inventory for the Himalaya, from which rock glacier WVEQ was quantified. Subsequently we assessed their comparative importance vs glaciers (i.e. rock glacier: glacier WVEQ ratio) across a range of spatial scales – west Himalaya, central Himalaya, and east Himalaya.
Section snippets
Rock glacier inventory compilation
In the Google Earth Pro platform (version 7.1.8.3036), we used publicly available current and archived satellite image data, including fine spatial resolution CNES/Airbus (e.g., SPOT and Pleiades) and DigitalGlobe-derived imagery (e.g., Worldview-1 and 2, and QuickBird), to generate a systematic rock glacier inventory for the Himalaya region. Large-scale geomorphological surveys have been facilitated by the Google Earth Pro platform, including several systematic rock glacier inventories (e.g.,
Results
A total of 24,968 rock glaciers were identified across the Himalaya. Intact (features containing ice) and relict (features not containing ice) rock glaciers accounted for ~65% (n = 16,334) and ~35% (n = 8, 634) of the total, respectively. Most are located within the central Himalaya (~40%, n = 10,060) with ~30% situated in the east Himalaya and ~29% in the west Himalaya (Fig. 3). Across the Himalaya, rock glacier estimated areal coverage is 3747 km2 (i.e. intact and relict), representing ~16%
Discussion
We have developed the most extensive systematic rock glacier inventory generated to date, addressing the need for information in critical data-deficient regions (Central Asia, South Asia East, and South Asia West) previously defined as research priorities (Jones et al., 2018a). The previous estimate of rock glacier WVEQ across HMA (Randolph Glacier Inventory [RGI] regions: South Asia East, South Asia West, and Central Asia) significantly underestimated rock glacier WVEQ in this region (see
Conclusion
Here, we present the first systematic assessment of rock glacier WVEQ across the Himalaya range. Our Himalayan-wide analysis illustrates that the ~25,000 rock glaciers identified constitute hydrologically valuable long-term water stores. The ongoing climatically-driven glacier recession and mass loss across the high mountains of Asia has rightly attracted much research attention due to the potential impacts upon ~800 million people living downstream. Yet, mountain water resources are nuanced
CRediT authorship contribution statement
D.B. Jones: Methodology, Data curation, Formal analysis, Writing – original draft. S. Harrison: Conceptualization, Writing – review & editing, Supervision. K. Anderson: Writing – review & editing, Supervision. S. Shannon: Writing – review & editing, Visualization. R.A. Betts: Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
DBJ work was supported with a research grant by the Natural Environment Research Council (Grant No. NE/L002434/1) and the Royal Geographical Society (with IBG) through a Dudley Stamp Memorial Award. RAB was supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra.
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2022, Science of the Total EnvironmentCitation Excerpt :Periglacial mountain environments are important areas of water supply, especially in regions without glacial reservoirs. Rock glaciers and other related periglacial landforms, (solifluction lobes, solifluction slopes, debris slopes, etc.) distributed in water recharge areas of mountain regions, constitute water storage and distribution units (Scapozza et al., 2011; Jones et al., 2018; Jones et al., 2019; Jones et al., 2021; Hilbich et al., 2021). There are few studies in other regions of the world with emphasis on the hydrological role of relict and inactive rock glaciers and other morphosedimentary units, being these mainly related to slope deposits or moraines (Clow et al., 2003; Winkler et al., 2012; Millar et al., 2013; Hood and Hayashi, 2015; Winkler et al., 2016a, Janke et al., 2017).
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2021, Science of the Total EnvironmentCitation Excerpt :Despite negative global glacier mass budget anticipated by global climate change for rest of the world (Farinotti et al., 2019; Nie et al., 2021; Huss and Hock, 2015; Huss and Hock, 2018), in the Karakoram region, most of the cryospheric studies have found glacier mass balance anomalies, with insignificant losses of glacier mass or balanced glacier mass budgets in recent years (Azam et al., 2018; Berthier and Brun, 2019; Kääb et al., 2012; Muhammad et al., 2019b; Yao et al., 2012; Muhammad and Tian, 2016). An improved understanding of the cryospheric changes (including permafrost and rock glaciers) within the HKH region is important because of its large area (3746.77 km2 (Jones et al., 2021)) and dense population (Schmid et al., 2015). Global climate change is likely to impact rock glaciers destabilization and permafrost, which could affect slope destabilization, landslides, debris flows, vegetation changes, run-off patterns, and water qualities.