Hydrology of debris-covered glaciers in High Mountain Asia

Abstract

The hydrological characteristics of debris-covered glaciers are known to be fundamentally different from those of clean-ice glaciers, even within the same climatological, geological, and geomorphological setting. Understanding how these characteristics influence the timing and magnitude of meltwater discharge is particularly important for regions where downstream communities rely on this resource for sanitation, irrigation, and hydropower, such as High Mountain Asia. The hydrology of debris-covered glaciers is complex: rugged surface topographies typically route meltwater through compound supraglacial-englacial systems involving both channels and ponds, as well as pathways that remain unknown. Low-gradient tongues that extend several kilometres retard water conveyance and promote englacial storage. Englacial conduits are frequently abandoned and reactivated as water supply changes, new lines of permeability are exploited, and drainage is captured due to high rates of surface and subsurface change. Seasonal influences, such as the monsoon, are superimposed on these distinctive characteristics, reorganising surface and subsurface drainage rapidly from one season to the next. Recent advances in understanding have mostly come from studies aimed at quantifying and describing supraglacial processes; little is known about the subsurface hydrology, particularly the nature (or even existence) of subglacial drainage. In this review, we consider in turn the supraglacial, englacial, subglacial, and proglacial hydrological domains of debris-covered glaciers in High Mountain Asia. We summarise different lines of evidence to establish the current state of knowledge and, in doing so, identify major knowledge gaps. Finally, we use this information to suggest six themes for future hydrological research at High Mountain Asian debris-covered glaciers in order to make timely long-term predictions of changes in the water they supply.

Introduction

Debris-covered glaciers have gained increased research attention over recent years, partly in recognition of their role as water sources for large parts of the world’s population (Immerzeel et al., 2020; Scherler et al., 2011) and partly because they host a range of distinctive features, driven by processes that are largely absent from their clean-ice counterparts. Definitions of what constitutes a ‘debris-covered glacier’ vary widely (e.g. Anderson, 2000; Kirkbride, 2011), but here we define them to be glaciers with a largely continuous layer of supraglacial debris over most of the ablation area, typically increasing in thickness towards the terminus (Fig. 1). Debris can be supplied to such glaciers by snow avalanches, rockfalls, and landslides from mountainsides onto the glacier surface (Fig. 2, Fig. 3A), melt-out of englacial debris, thrusting from the glacier bed, dust blown from exposed moraines, or solifluction from (ice-cored) moraines (Dunning et al., 2015; Gibson et al., 2017b; Hambrey et al., 2008; Kirkbride and Deline, 2013; Rowan et al., 2015; van Woerkom et al., 2019). The surface debris layer can range in thickness from scattered particles to several metres, including large rocks and substantial boulders (Fig. 3C and D) (Inoue and Yoshida, 1980; McCarthy et al., 2017; Nicholson et al., 2018).

Debris-covered glaciers are present in nearly all of Earth’s glacierised regions, with a particularly large concentration in High Mountain Asia (Bolch et al., 2012; Kraaijenbrink et al., 2017; Scherler et al., 2018); sub-regional variability in the debris cover of which is presented by Brun et al. (2019) (their Fig. 1). Debris-covered glaciers therefore contribute an important proportion of streamflow used for drinking water, irrigation, and hydroelectric power; this streamflow is particularly effective in reducing seasonal water shortages (Bolch et al., 2019; Immerzeel et al., 2020; Immerzeel et al., 2010; Pritchard, 2019; Scott et al., 2019). Glacier mass loss in response to climate warming is currently increasing river discharge and contributions to sea level (Hock et al., 2019; Lutz et al., 2014; Radić et al., 2014; Shea and Immerzeel, 2016), but studies simulating future scenarios universally project long-term reductions in flow, perhaps as soon as 2050 in central Asia (Barnett et al., 2005; Bolch et al., 2012; Huss and Hock, 2018; Lutz et al., 2014; Ragettli et al., 2016b; Rounce et al., 2020; Sorg et al., 2012). Passing of ‘peak water’ threatens future water security in many regions, particularly across High Mountain Asia (Bolch et al., 2019; Eriksson et al., 2009; Hannah et al., 2005; Huss and Hock, 2018; Immerzeel et al., 2010; Winiger et al., 2005). A decrease in discharge from the Indus and Brahmaputra rivers alone is estimated to affect 260 million people (Immerzeel et al., 2010).

The long-term response of debris-covered glaciers to changing climatic conditions is non-linear and results from complexities relating to spatial variability in debris concentration and climatic controls integrated over at least several decades (Benn et al., 2012; Vaughan et al., 2013). A multidecadal trend of surface lowering, stagnation, and glacier mass loss has already been observed on many debris-covered glaciers across High Mountain Asia (Bolch et al., 2012; Bolch et al., 2011; Brun et al., 2017; Dehecq et al., 2019; Hock et al., 2019; Kääb et al., 2012; Pellicciotti et al., 2015; Scherler et al., 2011) as a result of warmer air temperatures and weaker monsoons (Pieczonka et al., 2013; Thakuri et al., 2014). However, predictions of mass loss from individual glacierised regions vary considerably. For example, in the Everest region of the Himalaya, estimates of ice mass loss by 2100 vary from ~10% (Rowan et al., 2015), through 50% (Soncini et al., 2016), to 99% in extreme scenarios (warming of ~3°C) (Shea et al., 2015). Model outputs also vary spatially at a regional scale (e.g. Chaturvedi et al., 2014; Kraaijenbrink et al., 2017; Zhao et al., 2014). Such projections depend on the future climate scenario used, but a number of key knowledge gaps also exist concerning the character of debris-covered glaciers and the processes influencing their varied geometrical response to climate change (Benn et al., 2012; Bolch et al., 2012; Huss, 2011; Scherler et al., 2011).

Understanding how meltwater is produced, transported, and stored within High Mountain Asian debris-covered glaciers is therefore imperative. There is growing recognition that the configuration and efficiency (i.e. bulk system transit velocity) of water routing across and through debris-covered ice is distinctively different from that of clean-ice glaciers, even within the same glacial system. This was first shown by a recent study on Miage Glacier, a debris-covered glacier in the Italian Alps (Fyffe et al., 2019b). Debris-covered glacier surfaces are complex, particularly those in High Mountain Asia, the ablation areas of which are often characterised by hummocky, rugged topography atop a shallow (or even reversed) longitudinal surface gradient (Fig. 1, Fig. 2). This commonly results from an inverted mass-balance regime, where the greatest ablation rates are experienced in the middle, rather than lower, ablation area (King et al., 2017). Debris-covered ablation areas also exhibit bare ice cliffs and supraglacial ponds – depressions capable of storing meltwater for both short and long periods within nested catchments of varying spatial scales (Section 2) – and these glaciers frequently terminate in proglacial lakes (Section 5). Other unique characteristics of High Mountain Asian debris-covered glaciers include the accumulation areas often being at extremely high elevations, with a steep surface gradient (often an icefall) transporting ice into the ablation area (Fig. 1). These features provide a setting that strongly influences the nature of hydrological systems in this region (Benn et al., 2017; Miles et al., 2019), but has restricted hydrological research due to the remoteness and inaccessibility of such glaciers.

In this review, we consider the current state of knowledge of debris-covered glacier hydrological systems in High Mountain Asia. Four hydrological domains are considered in turn: supraglacial (Section 2), englacial (Section 3), subglacial (Section 4), and proglacial (Section 5). Within each section, we summarise existing research and understanding of debris-covered glacier hydrological systems and then address key remaining knowledge gaps. Fig. 2 provides a reference conceptual diagram of a (High Mountain Asian) debris-covered glacier, with each hydrological feature encompassing both known and unknown elements of each domain. Finally, in light of the review, we propose six future research themes concerning the hydrology of debris-covered glaciers (Section 6). This review is intended to complement existing reviews of clean-ice valley glacier hydrology (e.g. Fountain and Walder, 1998; Hubbard and Nienow, 1997; Irvine-Fynn et al., 2011; Jansson et al., 2003). We note that there are a number of differing climatic regimes across High Mountain Asia, with precipitation in particular varying closely with topography (Bookhagen and Burbank, 2006); these climatic regimes will influence the thermal regime, geometry, mass balance, and thus hydrology of the glaciers in each of these sub-regions. While our review draws on research carried out across High Mountain Asia, much of that research has been carried out in the monsoon-influenced Himalaya, particularly Nepal, from where the review and our illustrations of many of the key elements draw strongly.