the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2025
Declining runoff sensitivity to precipitation following permafrost degradation: Insights from event-scale runoff response in the Yellow River source region
Abstract. Frozen ground, including permafrost and seasonally frozen ground (SFG), is a critical component of the cryosphere. Rapid atmospheric warming has accelerated the degradation of frozen ground, profoundly altering hydrological processes in cold regions and affecting downstream water resources. Here we investigate the impacts of frozen ground degradation on event-scale runoff responses to daily precipitation in the source region of the Yellow River (SRYR) on the northeastern Tibetan Plateau using ensemble rainfall-runoff analysis (ERRA). ERRA is a data-driven, nonparametric, and model-independent method that quantifies dynamic, nonlinear, and spatially heterogeneous linkages between streamflow and precipitation. Applying ERRA, we evaluate changes in daily precipitation-streamflow coupling within the permafrost region, where frozen soil depth has decreased by ~0.1 m per decade, and within the SFG region, where frozen soil depth has remained relatively stable, declining by only 0.03 m per decade. Between 1979–1998 and 1999–2018, the permafrost zone experienced a 47 % reduction in peak runoff response per unit precipitation and a 32 % decrease in the 25-day runoff coefficient. By contrast, no substantial changes in runoff response were observed in the SFG region. Rising temperatures and increased active layer thickness in the permafrost zone have substantially reduced streamflow sensitivity to precipitation, particularly under higher precipitation intensities. Specifically, for daily precipitation intensities exceeding 10 mm d-1, peak runoff response in the permafrost zone declined by 73 % and the 25-day runoff coefficient declined by 72 % between the two periods. These changes likely result from increased hydraulic connectivity and water storage capacity within the thawing active layer, facilitating increased infiltration and subsurface storage. Our findings underscore the effectiveness of data-driven methods in capturing hydrological regime shifts and offer critical insights for drought mitigation and flood risk assessment in permafrost-affected regions amid ongoing climate warming.
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Status: open (until 17 Sep 2025)
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RC1: 'Comment on egusphere-2025-3018', Anonymous Referee #1, 21 Aug 2025
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AC1: 'Reply on RC1', Yuting Yang, 26 Aug 2025
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Response to Anonymous Referee #1’s comments:
Anonymous Referee #1 expresses the view that "this study does not find anything that was previously not known or not understood." However, what is "known" and "understood" depends on which side one takes in the ongoing debate over the hydrological impacts of permafrost degradation. On the one hand, several studies suggest that permafrost degradation enhances runoff, as the melting of ground ice releases additional water into the hydrological system (Kuang et al., 2024; Li et al., 2016; Ma et al., 2019; Walvoord and Kurylyk, 2016; Sun et al., 2019; see Manuscript Lines 52–58). On the other hand, other studies argue that permafrost degradation may reduce runoff, primarily due to the thickening of the active layer, which increases soil water storage capacity, enhances evaporation from the active layer, and weakens the barrier function of permafrost, thereby promoting infiltration (Cheng and Jin, 2013; Cheng and Wu, 2007; Guo et al., 2025; Qiu, 2012; Wang et al., 2018; Yang et al., 2023; see Manuscript Lines 58–63). Because this topic remains under debate, with diametrically opposing views in the recent literature, almost any result (either ours, for example, or the exact opposite) could be rhetorically dismissed as nothing "that was not previously known" – at least by somebody, on some side of the current debate.
But of course, it is precisely because of this debate that new analyses and new lines of evidence, like those we present, are particularly important. In particular, the specific focus of our analysis (how permafrost thawing affects streamflow peaks, rather than average streamflow, assessed from real-world data rather than simulation models) has remained, to our knowledge, unexplored in the literature to date.
The referee says that our study "simply quantifies the effect of permafrost loss using recently developed methods." We note that actually quantifying the effect of permafrost loss on whole-basin stormflow response to precipitation – directly from data, not from simulation models with their associated assumptions – is something that has not been attempted before. We further note that doing this is far from "simple", as our analysis shows. In particular, our paper demonstrates how it is possible to separate the effects of precipitation falling in the permafrost region versus in the seasonally frozen ground region, even though these effects are overprinted on one another in Tangnaihai river discharge. The necessary methodology for this was only proposed last year (Kirchner, 2024) and our study represents the first large-scale application of this approach.
Our analysis of hydrological responses to permafrost degradation is based on real-world observational data (i.e., runoff, precipitation, temperature, etc.) rather than hydrological modeling. Ensemble rainfall-runoff analysis (ERRA) is a data-driven approach that operates independently of the predefined assumptions inherent in hydrological models (Gao et al., 2018; Zheng et al., 2018; Yi et al., 2014), thereby enhancing the robustness of our findings. Moreover, compared with conventional statistical methods (e.g., Hu et al., 2011; Ma et al., 2019; Wang et al., 2018; Wu et al., 2020; Chang et al., 2024), ERRA enables analyses at shorter temporal scales (e.g., event scale), across heterogeneous spatial domains, and under different rainfall intensities. This flexibility allows us to design a variety of controlled experiments to quantitatively assess how thawing permafrost alters runoff generation, relying solely on observed data (see Manuscript Lines 63-93).
Our study provides the first event-scale observational evidence that runoff sensitivity to rainfall (as distinct from the sensitivity of total runoff to rainfall) is substantially reduced in degrading permafrost regions. Compared with conventional statistical methods based on observations, ERRA enables analyses at shorter temporal scales, over heterogeneous spatial domains, and across wide ranges of rainfall intensities.
The reviewer points out that our study "does contribute positively to the debate on what happens to river flow with the rise in global temperatures in the presence of permafrost/seasonally frozen ground." The reviewer also "did not find anything that can be improved significantly in this study."
For all of the reasons outlined above, we believe that the reviewer's call for rejection is not supported by a balanced view of our work and its scientific context.
Reference
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Citation: https://doi.org/10.5194/egusphere-2025-3018-AC1
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AC1: 'Reply on RC1', Yuting Yang, 26 Aug 2025
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Dear authors,
Your study deals with the quantification of change in runoff response to rainfall in a river that has partial permafrost and partially seasonally frozen ground using existing techniques.
My impression is that this study does not find anything that was previously not known or not understood. It simply quantifies the effect of permafrost loss using recently developed methods. However, it does contribute positively to the debate on what happens to river flow with the rise in global temperatures in the presence of permafrost/seasonally frozen ground. To be honest, I did not find anything that can be improved significantly in this study, as it is simply an application of existing data and techniques. The authors were meticulous with the application and tested various hypotheses to make sure that their claims are valid. It is very clear that they know what they are doing.
A lot of work went in to this paper, I am not denying that. However, I do not see this study to be at the level that justifies its publication in a prestigious journal like HESS. It is better suited to a journal that deals with regional studies, IMO. I am asking for a rejection.