Supraglacial rivers on the northwest Greenland Ice Sheet, Devon Ice Cap, and Barnes Ice Cap mapped using Sentinel-2 imagery

Supraglacial rivers set efficacy and time lags by which surface meltwater is routed to the englacial, subglacial, and proglacial portions of ice masses. However, these hydrologic features remain poorly studied mainly because they are too narrow (typically <30 m) to be reliably delineated in conventional moderate-resolution satellite images (e.g., 30 m Landsat-8 imagery). This study demonstrates the utility of 10 m Sentinel-2 Multi-Spectral Instrument images to map supraglacial rivers on the northwest Greenland Ice Sheet, Devon Ice Cap, and Barnes Ice Cap, covering a total area of ∼10,000 km2. Sentinel-2 and Landsat-8 both capture overall supraglacial drainage patterns, but Sentinel-2 images are superior to Landsat-8 images for delineating narrow and continuous supraglacial rivers. Sentinel-2 mapping across the three study areas reveals a variety of supraglacial drainage patterns. In northwest Greenland near Inglefield Land, subparallel supraglacial rivers up to 55 km long drain meltwater directly off the ice sheet onto the proglacial zone. On the Devon and the Barnes ice caps, shorter supraglacial rivers (up to 15–30 km long) are commonly interrupted by moulins, which drain internally drained catchments on the ice surface to subglacial systems. We conclude that Sentinel-2 offers strong potential for investigating supraglacial meltwater drainage patterns and improving our understanding of the hydrological conditions of ice masses globally

Direct Measurements of Meltwater Runoff on the Greenland Ice Sheet Surface

Meltwater runoff from the Greenland Ice Sheet surface influences surface mass balance (SMB), ice dynamics and global sea level rise, but is estimated with climate models and thus difficult to validate. We present a way to measure ice surface runoff directly, from hourly in situ supraglacial river discharge measurements and simultaneous high-resolution satellite/drone remote sensing of upstream fluvial catchment area. A first 72-hour trial for a 63.1 square kilometer moulin-terminating internally drained catchment (IDC) on Greenland's mid-elevation (1207-1381 meters above sea level) ablation zone is compared with melt and runoff simulations from HIRHAM5, MAR3.6.1 (Modele Atmospherique Regionale 3.6.1), RACMO2.3 (Regional Atmospheric Climate Model 2.3), MERRA-2 (Modern Era Retrospective-analysis for Research and Applications-2) and SEB climate/SMB models. Current models cannot reproduce peak discharges or timing of runoff entering moulins, but are improved using synthetic unit hydrograph theory (SUH). Retroactive SUH applications to two older field studies reproduces their findings, signifying that remotely sensed IDC area, shape, and river-length are useful for predicting delays in peak runoff delivery to moulins. Applying SUH to HIRHAM5, MAR3.6.1, RACMO2.3 gridded melt products for 799 surrounding IDCs suggests their terminal moulins receive lower peak discharges, less diurnal variability, and asynchronous runoff timing relative to climate/SMB model output alone. Conversely, large IDCs produce high moulin discharges, even at high elevations where melt rates are low. During this particular field experiment models overestimated runoff by plus 21 percent to plus 58 percent, linked to overestimated ablation and possible meltwater retention in bare, low-density ice. Direct measurements of ice surface runoff will improve climate/SMB models, and incorporating remotely sensed IDCs will aid coupling of surface mass balance with ice dynamics and subglacial systems

Seasonal evolution of the supraglacial drainage network at Humboldt Glacier, northern Greenland, between 2016 and 2020

Supraglacial rivers and lakes are important for the routing and storage of surface meltwater during the summer melt season across the Greenland Ice Sheet (GrIS) but remain poorly mapped and quantified across the northern part of the ice sheet, which is rapidly losing mass. Here we produce, for the first time, a high-resolution record of the supraglacial drainage network (including both rivers and lakes) and its seasonal behaviour at Humboldt Glacier, a wide-outlet glacier draining a large melt-prone hydrologic catchment (13 488 km2), spanning the period 2016 to 2020 using 10 m spatial resolution Sentinel-2 imagery. Our results reveal a perennially extensive yet interannually variable supraglacial network extending from an elevation of 200 m a.s.l. to a maximum of ∼ 1440 m a.s.l. recorded in 2020, with limited development of the network observed in the low-melt years of 2017 and 2018. The supraglacial drainage network is shown to cover an area ranging between 966 km2 (2018) and 1566 km2 (2019) at its maximum seasonal extent, with spatial coverage of up to 2685 km2 recorded during the early phases of the melt season when a slush zone is most prominent. Up-glacier expansion and the development of an efficient supraglacial drainage network as surface runoff increases and the snowline retreats is clearly visible. Preconditioning of the ice surface following a high-melt year is also observed, with an extreme and long-lasting 2019 melt season and over-winter persistence of liquid lakes, followed by low snow accumulation the following spring, culminating in earlier widespread exposure of the supraglacial drainage network in 2020 compared to other years. This preconditioning is predicted to become more common with persistent warmer years into the future. Overall, this study provides evidence of a persistent, yet dynamic, supraglacial drainage network at this prominent northern GrIS outlet glacier and advances our understanding of such hydrologic processes, particularly under ongoing climatic warming and enhanced runoff

Seasonal evolution of the supraglacial drainage network at Humboldt Glacier, North Greenland, between 2016 and 2020

Supraglacial rivers and lakes are important for the routing and storage of surface meltwater during the summer melt season across the Greenland Ice Sheet (GrIS), yet remain poorly mapped and quantified across the northern part of the ice sheet, which is rapidly losing mass. Here we produce, for the first time, a high-resolution record of the supraglacial drainage network (including both rivers and lakes) and its seasonal behaviour at Humboldt Glacier, a wide-outlet glacier draining a large hydrologic catchment (13,488 km2), spanning the period 2016 to 2020 using 10 m spatial resolution Sentinel-2 imagery. Our results reveal a perennially extensive yet interannually-variable supraglacial network extending from an elevation of 200 m a.s.l to a maximum of ~1440 m a.s.l recorded in 2020, with limited development of the network observed in the low melt years of 2017 and 2018. The supraglacial drainage network is shown to cover an area ranging between 965.7 km2 (2018) and 1566.3 km2 (2019) at its maximum seasonal extent, with spatial coverage of up to 2685 km2 recorded during the early phases of the melt season when a slush zone is most prominent. Up-glacier expansion and the development of an efficient supraglacial drainage network as surface runoff increases and the snowline retreats is clearly visible. Preconditioning of the ice surface following a high melt year is also observed, with the earlier widespread exposure of the supraglacial drainage network in 2020 compared to other years; a finding that may become representative with persistent warmer years into the future. Overall, this study provides evidence of a persistent, yet dynamic, supraglacial drainage network at this prominent northern GrIS outlet glacier and advances our understanding of such hydrologic processes, particularly under ongoing climatic warming and enhanced runoff

Supraglacial drainage efficiency of the Greenland Ice Sheet estimated from remote sensing and climate models

Supraglacial stream/river catchments drain large volumes of surface meltwater off the southwestern Greenland Ice Sheet (GrIS) surface. Previous studies note a strong seasonal evolution of their drainage density (Dd), a classic measure of drainage efficiency defined as open channel length per unit catchment area, but a direct correlation between Dd and surface meltwater runoff (R) has not been established. We use 27 high-resolution (∼0.5 m) satellite images to map seasonally evolving Dd for four GrIS supraglacial catchments, with elevations ranging from 1100 m to 1700 m. We find a positive linear correlation (r2 = 0.70, p<0.01) between Dd and simulations of runoff production from two climate models (MAR v3.11 and MERRA-2). Applying this R-Dd empirical relationship to climate model output enables parameterization of spatial and temporal changes in supraglacial drainage efficiency continuously throughout the melt season, although temporal and spatial skewness of Dd observations likely affects the application of this R-Dd relationship on crevasse fields and snow/firn surfaces. Incorporating this information into a simple surface routing model finds that high runoff leads to earlier, larger diurnal peaks of runoff transport on the ice surface, owing to increased Dd. This effect progressively declines from low (∼1100 m) to high (∼1700 m) elevation, causing a roughly order-of-magnitude reduction in diurnal runoff variability at the highest elevations relative to standard climate model output. Combining intermittent satellite Dd mapping with climate model output thus promises to improve characterization of supraglacial drainage efficiency to the benefit of supraglacial meltwater routing and subglacial hydrology models

Seismologically Observed Spatiotemporal Drainage Activity at Moulins

Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross-correlation techniques. We locate the noise sources by backprojecting the amplitude of the cross correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2 km station separation has the ability to detect moulins within the array with a precision of 50 m. We confirm that seismic activity is correlated with air temperature, and thus, melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross-correlation backprojection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor

Near-surface hydraulic conductivity of Northern Hemisphere glaciers

The hydrology of near‐surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this near‐surface glacial weathering crust, we describe the design and operation of a capacitance‐based piezometer that enables rapid, economical deployment across multiple sites and provides an accurate, high‐resolution record of near‐surface water‐level fluctuations. Piezometers were employed at 10 northern hemisphere glaciers, and through the application of standard bail–recharge techniques, we derive hydraulic conductivity (K) values from 0.003 to 3.519 m day−1, with a mean of 0.185 ± 0.019 m day−1. These results are comparable to those obtained in other discrete studies of glacier near‐surface ice, and for firn, and indicate that the weathering crust represents a hydrologically inefficient aquifer. Hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative short‐wave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance. The large range of K observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity of near‐surface ice hydrology than hitherto appreciated and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production. The conductivities reported here, coupled with typical supraglacial channel spacing, suggest that meltwater can be retained within the weathering crust for at least several days. Not only does this have implications for the accuracy of predictive meltwater run‐off models, but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, impurities, microbes, and nutrients to downstream ecosystems. Because continued atmospheric warming will incur rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions, and consequently, detailing weathering crust hydraulics represents a research priority because the flow path it represents remains poorly constrained

UAV-based investigations into the hydrology and dynamics of the Greenland Ice Sheet

Variation in the rate of meltwater input into the subglacial system of the Greenland Ice Sheet can force dynamic responses on a range of scales from hourly to interannual. Observations of the ice sheet dynamic response are commonly made either through ground-based Global Navigation Satellite System (GNSS) measurements, which can provide continuous and accurate point measurements, or through satellite remote sensing, which can provide regional-scale observations but at coarse temporal resolutions. This thesis investigates the potential of Uncrewed Aerial Vehicles (UAVs) to provide intermediate-level observations of the interactions between ice sheet hydrology and dynamics at a fast-flowing, marine terminating glacier in West Greenland. I first describe the development of a low- cost UAV suitable for deriving ice sheet velocity fields from Structure-from-Motion photogrammetry. In order to geolocate products without using ground control, image locations are determined directly using an on-board L1 GNSS receiver. I validate this method, showing that accuracies are sufficient for producing velocity fields in the ice sheet interior. Next, this method is used, alongside in-situ geophysical observations, to characterise the causes and dynamic influence of a rapid supraglacial lake drainage. I show that rapid drainage can induce a significant dynamic response up to 4 km away from the lake itself, and that fracture history can exert controls on interannual lake drainage behaviour. Finally, I upscale UAV ob- servations using satellite datasets over a ~3,000 km² area, exploring dynamic controls on crevasse hydrology. I find that in compressive mean stress compressive regimes, crevasses are more likely to display ponding and rapid hydrofracture than in extensional regimes, where continuous slow drainage is typical. Continued high-resolution observations are necessary to further identify key controls on the hydrological influences of Greenland Ice Sheet dynamics.Funded by a Natural Environment Research Council studentship awarded through the Cambridge Earth System Science Doctoral Training Partnership (Grant NE/L002507/1). Research logistics funded by the European Research Council as part of the RESPONDER project under the European Union’s Horizon 2020 research and innovation program (Grant 683043)

Storage and export of microbial biomass across the western Greenland Ice Sheet

The Greenland Ice Sheet harbours a wealth of microbial life, yet the total biomass stored or exported from its surface to downstream environments is unconstrained. Here, we quantify microbial abundance and cellular biomass flux within the near-surface weathering crust photic zone of the western sector of the ice sheet. Using groundwater techniques, we demonstrate that interstitial water flow is slow (~10−2 m d−1), while flow cytometry enumeration reveals this pathway delivers 5 × 108 cells m−2 d−1 to supraglacial streams, equivalent to a carbon flux up to 250 g km−2 d−1. We infer that cellular carbon accumulation in the weathering crust exceeds fluvial export, promoting biomass sequestration, enhanced carbon cycling, and biological albedo reduction. We estimate that up to 37 kg km−2 of cellular carbon is flushed from the weathering crust environment of the western Greenland Ice Sheet each summer, providing an appreciable flux to support heterotrophs and methanogenesis at the bed