We used 268 cloud-free Moderate-resolution Imaging Spectroradiometer (MODIS) images from 2003 and 2005-2007 to study the seasonal evolution of supra-glacial lakes in three different regions of the Greenland Ice Sheet. Lake area estimates were obtained by developing an automated classification method for their identification based on 250Â m resolution MODIS surface reflectance observations. Widespread supra-glacial lake formation and drainage is observed across the ice sheet, with a 2-3Â week delay in the evolution of total supra-glacial lake area in the northern areas compared to the south-west. The onset of lake growth varies by up to one month inter-annually, and lakes form and drain at progressively higher altitudes during the melt season. A positive correlation was found between the annual peak in total lake area and modelled annual runoff. High runoff and lake extent years are generally characterised by low accumulation and high melt season temperatures, and vice versa. Our results indicate that, in a future warmer climate Meehl, G. A., Stocker, T. F., Collins W. D., Friedlingstein, P., Gaye, A. T., Gregory, J. M., Kitoh, A., Knutti, R., Murphy, J. M., Noda, A., Raper, S. C. B., Watterson, I. G., Weaver, A. J. & Zhao, Z. C. (2007). Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor & H. L. Miller (eds.), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA., Greenland supra-glacial lakes can be expected to form at higher altitudes and over a longer time period than is presently the case, expanding the area and time period over which connections between the ice sheet surface and base may be established Das, S., Joughin, M., Behn, M., Howat, I., King, M., Lizarralde, D., & Bhatia, M. (2008). Fracture propagation to the base of the Greenland Ice Sheet during supra-glacial lake drainage. Science, 5877, 778-781 with potential consequences for ice sheet discharge Zwally, H.J., Abdalati, W., Herring, T., Larson, K., Saba, J. & Steffen, K. (2002). Surface melt-induced acceleration of Greenland Ice Sheet flow. Science, 297, 218-221.. Â© 2009 Elsevier Inc. All rights reserved.

Evolution of supra-glacial lakes across the Greenland Ice Sheet

Since the early 1990s, the Greenland ice sheet (GrIS) has been losing mass at an accelerating rate, primarily due to enhanced meltwater runoff following atmospheric warming. Here, we show that a pronounced latitudinal contrast exists in the GrIS response to recent warming. The ablation area in north Greenland expanded by 46%, almost twice as much as in the south (+25%), significantly increasing the relative contribution of the north to total GrIS mass loss. This latitudinal contrast originates from a different response to the recent change in large-scale Arctic summertime atmospheric circulation, promoting southwesterly advection of warm air toward the GrIS. In the southwest, persistent high atmospheric pressure reduced cloudiness, increasing runoff through enhanced absorption of solar radiation; in contrast, increased early-summer cloudiness in north Greenland enhanced atmospheric warming through decreased longwave heat loss. This triggered a rapid snowline retreat, causing early bare ice exposure, amplifying northern runoff.

Rapid ablation zone expansion amplifies north Greenland mass loss

Fluvial landscape analysis represents an essential component in geomorphology, hydrology, ecology and cartography. It is traditionally focused on the transition between hillslopes and channel domain, in which the network drainage is represented by static flow lines. However, the natural fluctuations of the processes occurring in the watershed induce lateral and longitudinal expansions and contractions in the drainage patterns and variations of stream surface area. These can be better understood by introducing a two-dimensional (2D) view of catchment hydrography, in which river width and floodplain are included in the analysis. The novelty introduced in this work is the development of a hydrodynamic hierarchical framework (HHF) to analyse the transitions among the geomorphic and hydrographic features of the fluvial landscape, distinguishing hillslope, unchanneled valleys, floodplains, and single/multithreads channels. HHF is based on the estimation of nested inundation pattern domains (IPDs) from digital elevation models and 2D hydrodynamic modeling. IPDs are defined by scaling laws that characterize log–log relations between watershed drainage density and unit discharge thresholds extracted from a 2D direct rainfall method (DRM) approach under steady state solutions. The physical significance of the IPDs is analysed within the context of both the physiographic features of the fluvial landscape, and the rainfall rates employed as input for the modeling approach. Initially, the spatial heterogeneity of the IPDs is initially used to derive stream width metrics as a function of the rainfall rate. Then, a spatial index, representative of the IPDs' heterogeneity, is introduced as a measure of the susceptibility of the drainage network surface area to expand/contract. Finally, the consistency of the results is assessed in comparison to another hydrodynamic-based method for fluvial landscape analysis recently proposed in the literature. The proposed approach is analysed using challenging mountain and low-relief environments, characterized by multithread channels, meander cut-offs, oxbow lakes, and extreme landscapes that feature glacial outwash, permafrost, and peatlands. 

Unravelling spatial heterogeneity of inundation pattern domains for 2D analysis of fluvial landscapes and drainage networks

The spatiotemporal distribution and variability of surface water remained poorly known at the pan-Greenland scale, and its dominant features (supraglacial rivers, lakes, and water-filled crevasses) were rarely studied collectively. We present pan-GrIS surface water extent and volume for a relatively cool (2018) and a relatively warm (2019) summer, using 10 m resolution Sentinel-2 imagery, a semi-automated multi-scale water extraction algorithm, and the Regional Atmospheric Climate Model (RACMO). While the 10 m resolution of Sentinel-2 imagery prevents inclusion of all water-filled crevasses and narrow rivers, our findings include: (1) strong interannual differences are observed in total surface water area (4903km2 vs. 9988 km2), volume (3.5 km3 vs. 6.8 km3), and mean elevation limit (1407 m a.s.l. vs. 1545 m a.s.l.) in response to a low (cool) (265 Gt/yr) and high (warm) (510 Gt/yr) runoff year; (2) large spatial contrasts in surface water extent, volume, elevation limit, and drainage pattern among the eight major GrIS basins; (3) supraglacial rivers dominate GrIS surface water appearance, accounting for 57%/48% of total surface water area/volume, respectively, over the two years (in contrast, water-filled crevasses and supraglacial lakes account for 33%/32% and 10%/20%, respectively); (4) ratio of remotely sensed water volume to RACMO-simulated cumulative surface runoff declines during the cool (2.6%) vs. the warm (1.8%) year; and from north (4.5%) to south (1.1%) Greenland. In summary, this study reveals strong temporal and spatial differences in GrIS surface water extent, volume, and drainage pattern and raises prospects for improved understanding of pan-Greenland Ice Sheet surface hydrology. 

Pan-Greenland mapping of supraglacial rivers, lakes, and water-filled crevasses in a cool summer (2018) and a warm summer (2019)

The dynamic variation in the water surfaces of the river networks within the Qinghai-Tibet Plateau affects the water resource availability for downstream ecosystems and human activities. Small rivers (with a river width less than 30 m) are an important component of this network, but are difficult to map in the Qinghai-Tibet Plateau. Firstly, the width of most rivers is very narrow, at around 20 m, which appears as only one or two pixels in Sentinel-2 images and thus is susceptible to salt-and-pepper noise. Secondly, local mountain shadows, cloud shadows, and snow pixels have spectral characteristics similar to those of rivers, leading to misclassification. Therefore, we propose an automated small river mapping (ASRM) method based on Sentinel-2 imagery to address these two difficulties. A preprocessing procedure was designed to remove the salt-and-pepper noise and enhance the linear characteristic of rivers with specific widths. A flexible digital elevation model (DEM)-based post-processing was then imposed to remove the misclassifications caused by mountain shadows, cloud shadows, and snow pixels. The ASRM results achieved an overall accuracy of 87.5%, outperforming five preexisting remote sensing-derived river network products. The proposed ASRM method has shown great potential for small river mapping in the entire Qinghai-Tibet Plateau.

/pdf/automated-small-river-mapping-asrm-for-the-qinghai-tibet-1btse84g.pdf

Automated Small River Mapping (ASRM) for the Qinghai-Tibet Plateau Based on Sentinel-2 Satellite Imagery and MERIT DEM

The northwest Greenland Ice Sheet (NW GrIS) is rapidly losing mass, and its ablation zone has expanded over the past two decades. Numerous supraglacial streams flowing directly over the NW GrIS surface drain a large lobe of grounded ice at Inglefield Land, into the proglacial Minturn River and the Nares Strait. Owing to the absence of moulins and crevasses, this continuous supraglacial-proglacial drainage system regulates the evacuation of surface meltwater from the ice sheet to the ocean. We examine this Inglefield Land coupled supraglacial-proglacial drainage system during the 2016–2019 melt seasons (July to August), using 137 Sentinel-2 and Landsat-8 visible/near-infrared satellite images. Two surface water metrics (supraglacial meltwater area fraction Am and proglacial river effective width We) are used to track spatio-temporal variations of surface meltwater moving through this drainage system. Satellite-derived Am and We are also compared with daily surface runoff simulations from the MAR v3.11 and MERRA-2 climate/SMB models, to estimate meltwater routing lag times and assess model performance. Satellite-derived Am and We are highly correlated (r2 = 0.85, p < 0.01), indicating that the coupled supraglacial-proglacial drainage system evacuates meltwater directly from the ice surface to the ocean with negligible subglacial storage or delays. Both remotely sensed metrics are positively correlated with modeled runoff, especially MAR (r2 = 0.81 and 0.77 for Am and We, vs. 0.66 and 0.64 for MERRA-2). Lagged MAR runoff (2 days, r2 = 0.87 and 0.82) match both metrics better than simultaneous MAR runoff and the optimal time lag for both metrics are 2 d. We conclude that 1) unlike the southwest GrIS, the coupled supraglacial-proglacial drainage system at Inglefield Land routes surface meltwater runoff directly off the ice surface to the proglacial zone, with virtually no subglacial capture of runoff by moulins; 2) most of the ∼2 d transit time occurs on the ice surface rather than in the proglacial zone; and 3) multi-temporal satellite imaging facilitates holistic, source-to-sink tracking of NW GrIS meltwater from the ice surface to the global ocean. 

Surface meltwater runoff routing through a coupled supraglacial-proglacial drainage system, Inglefield Land, northwest Greenland

Response of supraglacial rivers and lakes to ice flow and surface melt on the northeast Greenland ice sheet during the 2017 melt season

Small rivers (width <30 m) are crucial components of Arctic terrestrial river networks. Yet to date, spatial resolution limitations of commonly used satellite imagery have inhibited quantification of their hydrography. By merging newly available Sentinel-2 multispectral satellite imagery with 2-m ArcticDEM digital elevation data, we created a continuous actively-flowing river network map of the Colville (Kuukpik) River Basin (∼36 000 km2) in Alaska, which includes small rivers as narrow as 10 m. We quantified the river hydrography (stream order and river width, length,surface area, velocity, slope, sinuosity, and catchment area) of the Colville river network in detail, revealing the dominant role of small rivers. Our results show that: (1) small rivers occupy >80% of total river length and surface area of the Colville river network and drain >90% of the catchment area; (2) including numerous small rivers increases the peak of hillslope-channel travel time distribution (TTD) by ∼4 times and shortens the mean hillslope-channel travel time by at least an order of magnitude compared to coarser-resolution river hydrography products; and (3) 87% of the Colville River Basin’s carbon dioxide is emitted from small rivers. In sum, we show that small Arctic rivers greatly influence streamflow TTD and carbon cycle. These findings expand our understanding of Arctic river hydrography to a 10-m spatial resolution and raise prospects for tracking dynamic surface water processes with high-resolution satellite observations.

High-resolution satellite-derived river network map reveals small Arctic river hydrography

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Surface meltwater runoff routing through a coupled supraglacial-proglacial drainage system, Inglefield Land, northwest Greenland

Response of supraglacial rivers and lakes to ice flow and surface melt on the northeast Greenland ice sheet during the 2017 melt season

High-resolution satellite-derived river network map reveals small Arctic river hydrography