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the Creative Commons Attribution 4.0 License.
| 23 Apr 2025
Climatic and Tectonic Forcing Lead to Contrasting Headwater Slope Evolutions
Abstract. Landscapes evolve through the coupled effects of tectonics and surface processes. Previous studies have shown that uplift rate changes generate upstream-migrating erosion waves, altering downstream slopes while upstream ones remain constant until the wave arrives. However, the distinctive differences between landscape responses to uplift versus climatic changes, particularly rainfall rate changes, remain incompletely described. This study uses a numerical model to investigate landscape responses to changes in both rainfall and uplift rates. Results show that, unlike the simple upstream-migrating erosion waves from uplift rate changes, rainfall rate changes generate more complex responses. Specifically, rainfall rate changes cause transient slope change reversals at the headwaters due to differential erosion between the divide and its adjacent areas, a pattern not observed in uplift-induced evolution. These reversals are more pronounced when hillslope diffusion plays a dominant role (i.e., high diffusion coefficient). While both tectonic and climatic forcing drive landscape change, they produce recognizably different signatures in river profiles. If these distinctive signatures can be identified from river profiles or inferred from erosion rate measurements, they can help disentangle climatic and tectonic influences on landscape evolution.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1585', Anonymous Referee #1, 13 May 2025
- AC1: 'Reply on RC1', Yinbing Zhu, 02 Jul 2025
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RC2: 'Comment on egusphere-2025-1585', Anonymous Referee #2, 20 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1585/egusphere-2025-1585-RC2-supplement.pdf
- AC2: 'Reply on RC2', Yinbing Zhu, 02 Jul 2025
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RC3: 'Comment on egusphere-2025-1585', Anonymous Referee #3, 24 May 2025
Summary
In this contribution the authors aim to examine how topography responds to changes in uplift versus climate using the streampower+diffusion landscape evolution model. They specifically focus on how diffusion modulates the response of channel profiles to changes in rainfall rate in the streampower model. They find a nonmonotonic response of channel slope near ridges when precipitation is increased or decreased, which doesn’t appear in the absence of diffusion, or when the uplift rate is changed. They propose that this occurs because diffusion-driven erosion is unaffected by change in precipitation, inducing a local change in the balance of advection versus diffusion processes near channel heads. They suggest that this could be a diagnostic feature of response to changes in climate.
While this is an interesting signature that I don’t think has been described before, the authors have missed a few important points that call into question the usefulness of the feature they have described. First, and most importantly, their method relies on the idea that the diffusion term is insensitive to changes in climate. This seems highly unlikely, given that slope stability is sensitive to hydrological processes (Bogaard & Greco, 2016), and other biophysical processes that drive soil production and creep are almost certainly climate-sensitive (Andersen et al., 2015; Gabet, 2000; Gabet & Mudd, 2010). While theories that clearly link hillslope processes to climate are still needed, it is generally accepted that both soil production and the diffusion coefficient increase with mean annual precipitation (Perron, 2017).
A second related issue is distilling the effects of climate change down to a linear increase in average precipitation. Climate change is manifested in changes to the not just the mean, but also the distribution of event magnitudes and the phase (snow, rain) of precipitation due to changing temperature, which will be especially important in mountainous settings such as those considered (Meira Neto et al., 2020). Settings respond to precipitation changes differently depending on the dominant runoff generation mechanisms (Uhlenbrook et al., 2005), which are further modulated by erosion thresholds (DiBiase & Whipple, 2011). Such thresholds are especially important in headwaters, where the authors report their slope effect. Furthermore, geomorphic models that consider vegetation response to climate change suggest that the erosion response to precipitation change could even be reversed due to dynamic feedbacks with vegetation cover and evapotranspiration (Yetemen et al., 2019). None of these processes are mentioned in the present paper.
While I’m unsure that the streampower+diffusion model is the right tool to answer questions of climate sensitivity, I understand the tendency to stick with it in the name of interpretable simplicity. One of the reasons to stick with this model is because of its well-developed nondimensional forms (Bonetti et al., 2020; Litwin et al., 2025; Perron et al., 2008; Theodoratos et al., 2018), which provide clear methods for understanding fundamental process competition. The authors run into the problem of non-uniqueness in process competition when they change the streampower coefficient and diffusion coefficient but maintain their ratio. However, they do not provide any explanation of the fundamental scaling between the two, which is well understood (e.g., Perron et al., 2008).
Overall, I think this paper needs substantial work to become a valuable contribution. My main recommendation would be to engage with models that link changes in climatic, hydrological, and geomorphic processes in some more realistic level of detail. If not, they could describe the deficiencies of the streampower+diffusion model and provide a more comprehensive description of the effect they observe, using available nondimensional frameworks and acknowledging that the diffusion coefficient likely is not constant in response to climate change.
Line-by-line comments
- Transient slope change reversals are not yet defined.
- Needs more description of how changes in climate actually yield changes in runoff production. Q=PA is a little too simple.
48-49. Needs better description of what causes diffusion processes, how they might be linked to climate as well.
62-63. Part of the reason this hasn’t been explored is because we don’t have adequate theory describing how the diffusion coefficient changes with climate, although it almost certainly does.
84-86. Needs citation.
91-94. Maybe just describe the processes that are relevant. No marine? Source to sink?
Table 1. Those are really large values of the diffusion coefficient! Usually, find values on the order of 0.001-0.01 m2/yr using hilltop curvature and erosion rates. The sensitivity to the value used is dependent on the grid size, and there are already well-established nondimensional forms that can help describe this (Bonetti et al., 2020; Litwin et al., 2025). It might be useful to consider those.
- Again, this would be evident if you used established nondimensionalizations.
- You have not explained why topographic roughness is a useful or interesting metric, or how you are calculating it.
3.2 “rivers’ channel”
Fig. 4 Here it sounds like the effect of diffusion is unimportant, but in subsequent figures, it clearly is important. You could just explain Figure 4 as the kind of null case.
- “Monotonously”
- Needs to be more specific.
190-191. Not all channels experience this effect? Is there a threshold where it starts to occur?
192-199. Just a copy of the previous text.
Works cited
Andersen, J. L., Egholm, D. L., Knudsen, M. F., Jansen, J. D., & Nielsen, S. B. (2015). The periglacial engine of mountain erosion; Part 1: Rates of frost cracking and frost creep. Earth Surface Dynamics, 3(4), 447–462. https://doi.org/10.5194/esurf-3-447-2015
Bogaard, T. A., & Greco, R. (2016). Landslide hydrology: from hydrology to pore pressure. WIREs Water, 3(3), 439–459. https://doi.org/10.1002/wat2.1126
Bonetti, S., Hooshyar, M., Camporeale, C., & Porporato, A. (2020). Channelization cascade in landscape evolution. Proceedings of the National Academy of Sciences, 117(3), 1375–1382. https://doi.org/10.1073/pnas.1911817117
DiBiase, R. A., & Whipple, K. X. (2011). The influence of erosion thresholds and runoff variability on the relationships among topography, climate, and erosion rate. Journal of Geophysical Research: Earth Surface, 116(F4). https://doi.org/10.1029/2011JF002095
Gabet, E. J. (2000). Gopher bioturbation: field evidence for non-linear hillslope diffusion. Earth Surface Processes and Landforms, 25(13), 1419–1428. https://doi.org/10.1002/1096-9837(200012)25:13<1419::AID-ESP148>3.0.CO;2-1
Gabet, E. J., & Mudd, S. M. (2010). Bedrock erosion by root fracture and tree throw: A coupled biogeomorphic model to explore the humped soil production function and the persistence of hillslope soils. Journal of Geophysical Research: Earth Surface, 115(F4). https://doi.org/10.1029/2009JF001526
Litwin, D. G., Malatesta, L. C., & Sklar, L. S. (2025). Hillslope diffusion and channel steepness in landscape evolution models. Earth Surface Dynamics, 13(2), 277–293. https://doi.org/10.5194/esurf-13-277-2025
Meira Neto, A. A., Niu, G.-Y., Roy, T., Tyler, S., & Troch, P. A. (2020). Interactions between snow cover and evaporation lead to higher sensitivity of streamflow to temperature. Communications Earth & Environment, 1(1), 1–7. https://doi.org/10.1038/s43247-020-00056-9
Perron, J. T. (2017). Climate and the Pace of Erosional Landscape Evolution. Annual Review of Earth and Planetary Sciences, 45(Volume 45, 2017), 561–591. https://doi.org/10.1146/annurev-earth-060614-105405
Perron, J. T., Dietrich, W. E., & Kirchner, J. W. (2008). Controls on the spacing of first-order valleys. Journal of Geophysical Research: Earth Surface, 113(4), 1–21. https://doi.org/10.1029/2007JF000977
Theodoratos, N., Seybold, H., & Kirchner, J. W. (2018). Scaling and similarity of a stream-power incision and linear diffusion landscape evolution model. Earth Surface Dynamics, 6(3), 779–808. https://doi.org/10.5194/esurf-6-779-2018
Uhlenbrook, S., Didszun, J., & Leibundgut, C. (2005). Runoff Generation Processes on Hillslopes and Their Susceptibility to Global Change. In U. M. Huber, H. K. M. Bugmann, & M. A. Reasoner (Eds.), Global Change and Mountain Regions: An Overview of Current Knowledge (pp. 297–307). Dordrecht: Springer Netherlands. https://doi.org/10.1007/1-4020-3508-X_30
Yetemen, O., Saco, P. M., & Istanbulluoglu, E. (2019). Ecohydrology Controls the Geomorphic Response to Climate Change. Geophysical Research Letters, 46(15), 8852–8861. https://doi.org/10.1029/2019GL083874
Citation: https://doi.org/10.5194/egusphere-2025-1585-RC3 - AC3: 'Reply on RC3', Yinbing Zhu, 02 Jul 2025
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