Changing snowmelt and glacier-melt contributions reshape hydrological dynamics and flood generation in a high-mountain headwater basin

Qi, Yu; Zhang, Ting; Kraaijenbrink, Philip D. A.; Liu, Liu; Deng, Xiaoya

doi:10.5194/egusphere-2026-4911

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https://doi.org/10.5194/egusphere-2026-4911

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21 Sep 2026

| 21 Sep 2026

Status: this preprint is open for discussion and under review for Hydrology and Earth System Sciences (HESS).

Changing snowmelt and glacier-melt contributions reshape hydrological dynamics and flood generation in a high-mountain headwater basin

Yu Qi, Ting Zhang, Philip D. A. Kraaijenbrink, Liu Liu, and Xiaoya Deng

Abstract. Climate warming has been reorganizing hydrological dynamics in high-mountain rivers by altering the relative contributions of snowmelt and glacier melt. However, inadequate representation of snow ablation processes can bias runoff-component partitioning and mislead the inferred roles of snowmelt and glacier melt in water availability and flooding. Here, we enhance the representation of snowpack dynamics in a cryosphere-hydrology model (SPHY-S) to quantify how changing snowmelt and glacier-melt contributions reshape flood regimes in the upper Yulongkashi River during historical (1980-2020) and future (2021-2050) periods. Results show that SPHY-S robustly reproduces observed snow-cover dynamics across monthly, seasonal, and interannual timescales, thereby improving the reliability of runoff-component estimates. By 2050, the relative contribution of snowmelt is projected to decrease by 2%-7% whereas that of glacier melt increases by 1%-6%, depending on the climate scenario. Under the high-emission scenario, glacier-melt runoff (increasing from 0.7 to 1.5 billion m³ yr⁻¹) will surpass snowmelt runoff (increasing from 0.9 to 1.2 billion m³ yr⁻¹) to become the dominant contributor. Annual streamflow is projected to increase by 37%-79% and flood frequency is projected to increase by 84%-105% by 2050, depending on the climate scenario, with floods shifting from snowmelt-driven to glacier-melt-driven under the high-emission scenario. The growing contribution of glacier melt to floods creates more sustained high flows, posing challenges for downstream reservoir operation and disaster mitigation. Our findings highlight that improved representation of snow processes is essential for reliable runoff partitioning and diagnosis of shifts in flood regimes, thereby supporting proactive water management strategies in climate-sensitive mountain rivers.

Received: 14 Aug 2026 – Discussion started: 21 Sep 2026

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Yu Qi, Ting Zhang, Philip D. A. Kraaijenbrink, Liu Liu, and Xiaoya Deng

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RC1: 'Comment on egusphere-2026-4911', Santosh Nepal, 10 Oct 2026 reply

Changing snowmelt and glacier-melt contributions reshape hydrological dynamics and flood generation in a high-mountain headwater basin

Yu Qi, Ting Zhang, Philip D. A. Kraaijenbrink, Liu Liu, Xiaoya Deng

The manuscript by Qi et al investigate changes in snowmelt and glacier-melt contributions to streamflow and flood generation in the upper Yulongkashi River Basin using a modified SPHY model that explicitly incorporates snow sublimation. The topic is timely and relevant for HESS, particularly because runoff partitioning remains a major uncertainty in high-mountain hydrology. The staged calibration using snow-cover observations, glacier elevation change, and discharge is also a useful direction.

However, I have some methodological concerns regarding the physical representation and validation of sublimation, future glacier evolution; and, particularly, the interpretation of modeled runoff components as mechanisms of flood generation. These issues affect some of the central conclusions of the manuscript. I therefore recommend major revision.

Major comments

Section 3.1. The central argument is that explicitly representing sublimation improves snowpack dynamics and therefore produces more physically reliable partitioning of snowmelt and glacier-melt runoff. However, snow-related parameters, including the sublimation adjustment parameter are constrained principally using MODIS snow-cover fraction. MODIS provides information about the presence or absence of snow, but not snow water equivalent or the amount of snow lost through sublimation. The model itself converts SWE greater than 2 mm into a snow-covered cell. I recommend that the authors more clearly demonstrate how independent information supports the simulated sublimation magnitude and clarify these details in limitation of the model if needed.
Section 3.1. Line 197: My major concern is SPHY-S constrains sublimation to zero whenever grid-cell air temperature exceeds 0 °C, based on the assumption that melt dominates snow ablation under above-freezing conditions. I find this assumption potentially problematic in a high-elevation, dry and windy environment. In high altitude areas, wind sublimation is relatively high and the occurrence of melt does not necessarily imply that turbulent sublimation ceases. This threshold could also introduce an artificial seasonal transition between sublimation and melt. Since separation between sublimation and melt is fundamental to the conclusions of the paper, the authors should explain the physical basis for switching sublimation completely off
Section 3.1. Since Khanal et al. 2021 also uses snow sublimation in SPHY model for High mountain Asia, it would be helpful for users to specifically indicate how your paper is adding additional processes of snow sublimation than Khanal et al. 2021.
- Khanal, S., Lutz, A. F., Kraaijenbrink, P. D., van den Hurk, B., Yao, T., & Immerzeel, W. W. (2021). Variable 21st century climate change response for rivers in High Mountain Asia at seasonal to decadal time scales. Water Resources Research, 57(5), e2020WR029266.
Flood evaluation: The model is calibrated against discharge using NSE and evaluated mainly using NSE and R2. The manuscript reports NSE of about 0.85 in calibration and 0.78 in validation but also notes that the highest 20% of flows are underestimated by about 12%. Since one of the main objectives is to investigate flood frequency, magnitude and mechanisms, satisfactory overall daily NSE does not necessarily establish satisfactory flood simulation. Research on hydrological model evaluation has shown that calibration metrics can behave differently for high flows and flood peaks. Therefore, I suggest evaluating flood-relevant signatures independently, (eg. peak bias, timing error, high-flow exceedance) behaviour and event volumes. This would make the later flood analysis considerably stronger.
Future glacier evolution: The manuscript projects increasing glacier-melt runoff through 2050, reaching approximately 1.46 billion m³ yr⁻¹ under SSP5-8.5, and this result is central to the projected transition in runoff and flood regimes. However, it is not sufficiently clear from the main text how glacier area, thickness, geometry and available glacier mass evolve during the future simulations. I think this is critical. Increased temperature can increase specific melt, but glacier runoff eventually becomes limited by declining glacier area and ice storage. The credibility of the projected increase therefore depends strongly on whether dynamic glacier geometry and mass conservation are adequately represented. Therefore, I request that the authors elaborate on this section in the main manuscript without referring to other publications.
Figure 2: It is not clear what the difference is between the two validation periods in the manuscript: c) 1984-1989 and e) 2000-2011. Why were two validation periods proposed? Please provide the discharge values of observed streamflow as a line graph (not points). In the scatter plot, please provide r2 value also.
Figure 2: MODIS snow cover data. Cloud screening, gap filling, and topographic effects on MODIS snow detection need to be described, especially given the steep, high-mountain terrain.
Line 246–254: While the authors discuss high flow only (Figure S6), it is important to address low-flow conditions as well, as the model appears to be overestimating low-flow conditions.
Section 3 (L152): I think this section needs to be part of the Results section, as it shows the important result about model improvement processes.
Method: Since snow and glacier melt processes are an important part of your basin hydrology, it is useful to provide the SPHY snow and glacier melt processes and the future evolution of the glaciers in the method section, instead of pointing users to another publication. A short and relevant description should be enough.
Method: Since snowmelt is the dominant runoff in your basin, how does the model take into account snowmelt infiltration? This infiltrated meltwater then comes out as interflow and baseflow; how do you account for this part in the total runoff contribution? Please clarify this.
Section 4.2 – Lines 358-370: The periods for comparing historical and future data seem different: 21 years vs. 30 years. It would be meaningful to keep these periods consistent. For example, 30 years historic vs 30 years future.
Line 555-557: the statement ‘reservoirs in the YRB would require a minimum regulation capacity of approximately 0.5 billion m³’—this is definitely not the outcome of the paper. This is a very specific infrastructure-design conclusion, but the manuscript does not appear to address reservoir operation. Either provide references for this amount or remove it.
Flood attribution to snowmelt and glacier melt is model-derived and needs uncertainty analysis or independent evidence.

Minor comment

Location	Comments/remarks
Figure 1:	Keep elevation unit in the figure itself, write in meter above sea level. Is it possible to indicate westerlies direction in Figure 1 a
Line: 104	Avoid the word -almost exclusively’. Where it falls mostly as snow’ should be sufficient
Line 134	Provide full accornyms, CC, PBAIS
147-151	Please mention the spatial resolution of the selected models here also (although it is in TableS1).
L 189	coefficient 0.353- please provide the source of 0.353 value
Figure 5	The top line graph (yellow color)- what is it? It is not clear from the legend.
Line 482	Snow-cover performance statistics need consistency. The manuscript discusses r2 values of 0.82 and 0.77 for snow cover, while later the Discussion refers to snow-cover dynamics with NSE = 0.77. Figure 2 appears to contain both metrics. Please check values and terminology throughout.

Citation: https://doi.org/10.5194/egusphere-2026-4911-RC1

Yu Qi, Ting Zhang, Philip D. A. Kraaijenbrink, Liu Liu, and Xiaoya Deng

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https://doi.org/10.5194/egusphere-2026-4911-supplement

Yu Qi, Ting Zhang, Philip D. A. Kraaijenbrink, Liu Liu, and Xiaoya Deng

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Short summary

High mountain rivers provide essential water for downstream agriculture and daily life, yet their flow magnitude and composition are changing rapidly as climate warms. By improving snowpack representation in a cryosphere-hydrology model, we show that glacier melt becomes increasingly important while snowmelt contribution declines. This shift increases flood frequency, prolongs high flows, and alters flood generation, creating growing challenges for downstream water management.


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