| 16 Jun 2026
Simulating jökulhlaups from an ice-marginal lake within a 2D model of subglacial drainage and basal sliding
Abstract. Ice-marginal lakes are an increasingly common feature of glacierised landscapes, and their sudden drainage beneath glaciers (a jökulhlaup) can threaten downstream communities and infrastructure. Numerous efforts to model jökulhlaups have been made, however, because these models are 1D representations of a single channel connected to a lake, they cannot simulate lateral jökulhlaup propagation through the subglacial system. Here, to simulate jökulhlaups within a 2D subglacial drainage system, we use a fully coupled model of subglacial hydrology and basal sliding with a time-evolving ice-marginal lake located at its boundary. In experiments on a synthetic domain, the model produces stable, recurrent jökulhlaup cycles, and glacier acceleration during flood onset followed by abrupt slowdown at peak flood discharge. Sensitivity testing highlights the efficiency of the subglacial hydrology system as a key control on flood timing, peak discharge, and the basal sliding response. We also explore our model’s ability to represent an observed record of jökulhlaups by applying it to Isunnguata Sermia, West Greenland. The model successfully reproduces variability over a 17-year period, but underpredicts peak flood discharges, likely because its formulation omits ice uplift and lake temperature variability. These results establish the coupling of a lake to 2D subglacial hydrology and ice dynamics as a viable approach for multi-decadal jökulhlaup simulation.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
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This is a interesting and well written paper that presents the results of a coupled conduit–sheet model of jökulhlaups based on the subglacial hydrology model GLaDS for both a synthetic glacier and for the Isunnguata Sermia glacier in W-Greenland, which is known for semi-regular jökulhlaups every 2–4 years. The model includes coupling with the flow of the overlying glacier implemented with the ISSM ice-flow model. The model represents a significant advance over earlier models of similar type. The simulated jökulhlaups from Isunnguata Sermia are nevertheless longer in duration (by more than an order of magnitude!) and have too small maximum discharge compared with observations. The authors attribute this failure of the model to its inability to capture large-scale, elastic lifting of the glacier as there is "no physical representation of elastic uplift of ice once water pressure exceeds ice overburden pressure". This makes sense and underlines the need to include this physics in future work on jökulhlaup dynamics. The coupling with ice flow and basal sliding reproduces some features of the observed response of the Isunnguata Sermia glacier to jökulhlaups but the lack of quantitative agreement regarding the magnitude and spatial patterns of anomalies in surface ice velocity shows that jökulhlaup models are still not able to faithfully represent the underlying dynamics in the coupled subglacial hydrology – ice flow problem, but the presented model is, nevertheless, a significant step forward.
Comments:
1. There is a brief model description on p. 5–6 and in Appendix B. This includes explanations of model model parameters and variables that are studied later in the paper. However, the variable l_c (channel sheet width) and M_r (basal melt rate) are not explained although they are part of the sensitivity analysis shown in Figure 3. Consider adding a short description of these parameters/variables.
2. In l. 102/103 you say "As in the model of Kingslake and Ng (2013), we do not account for frictional heating in our solution." If you are just explaining that the model assumes the water to be at the pressure melting point, it would be better to just say that. The assumption that the water is maintained at the pressure melting point means that potential energy is dissipated through friction in the water flow so that frictional "heating" is in fact represented in the model although the water cannot be said to be heated. Perhaps clarify this statement?
3. In Eq. (2), the y-component of the basal sliding velocity (v_b) appears in the numerator but the x-component does not. This seems strange. Consider whether this is as intended. Also, you might briefly explain the difference between the formulation of the Schoof-type sliding law in ISSM and the more well-known original formulation by Schoof.
4. In lines 375–377 you state that "Ice near to the lake accelerates markedly" but in fact the acceleration affects a much larger area. Clarify. You also mention that "Such a velocity pattern has been observed during jökulhlaups at ... Skaftá by Magnússon et al. (2007). The velocity pattern observed by Magnússon affects almost the entire Skaftárjökull outlet glacier, not just an area near the subglacial lake. Clarify this also. You might call this location "Skaftá cauldrons" as Skaftá is the name of the river outside the glacier, whereas the jökulhlaups in question are released from cauldrons in W-Vatnajökull called Skaftá cauldrons.
5. In l. 399 you mention melt rates of 1 m a^{−1} to 5 m a^{−1}. These seem very high. Explain.
6. In l. 437 you explain that "Of these floods, seven are hindcasted by our model (Fig. 5f), closely agreeing with observations in terms of timing and the amplitude of change in l_h." This is impressive but you might explain that this success is mainly a consequence of the fitting of the simulated inflow to the lake with a degree−day model and the capacity of the lake which depends on easily measured quantities and continuity (assuming that the lake will eventually drain at a high stand for more or less any subglacial hydrology model configuration). The success of the impressive fitting to the observed variations in the lake level is only indirectly caused by the coupled subglacial hydrology − ice flow model, which is the main subject of the paper. Explain briefly how the successful fitting to the lake level variation is achieved and that it does not necessarily reflect successful simulations of the subglacial hydrology.
7. Did Flowers et al. (2004) really work with sheet thicknesses exceeding tens of metres (l. 486) (?). This seems excessive. In the 2004 GRL paper, she mentions 7–11 m sheet thickness. If you are referring to this range you should say "sheet thicknesses on the order of ten metres" or something like that. This sheet thickness magnitude refers to Björnsson's (1997) interpretation of observations of the catastrophic 1996 jökulhlaup from Grímsvötn, which you might find worth referencing in this context.
In the following sentence (l. 486−488) you state "Recent repeat satellite observations identified metres of uplift along a jökulhlaup pathway ..." quoting Magnússon et al. (2026). The lifting reported by Magnússon et al. was close 1 m. Please clarify. Maybe say "identified appoximately 1 metre of uplift" or "identified ~1 metre of uplift" about Magnússon's observations.
8. In l. 518-519, you state: "... and recognizing that our model successfully reproduces flood timing across a multidecadal record without tuning to individual events, we view the discrepancies in duration and peak as a tractable modelling challenge rather than a fundamental limitation." The successful reproduction of the flood timing is interesting, but as I mention above does not, in my opinion, have much to do with the coupled subglacial hydrology − ice flow model developed in the paper. It seems to me that in order to reproduce the short flood duration and higher maximum discharge, a substantial improvement in the model is necessary, such as the implementation of large-scale (elastic) lifting of the ice from the bed, as stated in the paper at several places. This would be a fundamental improvement, so the sentence that I quote above is perhaps too optimistic or describes this to lightly, although I hope this problem is indeed "tractable".
Minor and editorial comments: