the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Quantifying the migration rate of drainage divides from high-resolution topographic data
Chao Zhou
Yiduo Liu
Feng Shi
Abstract. The lateral movement of drainage divides is co-influenced by tectonics, lithology, and climate, and therefore archives a wealth of geologic and climatic information. Several methods have been proposed to determine the direction of drainage-divide migration. However, how to quantify the migration rate of drainage divides remains challenging. Here, we propose a new approach to calculate the migration rate of drainage divides from high-resolution topographic data. The new method is based on the cross-divide comparison of channel-head parameters, including the critical upstream drainage area and the gradient of channel head, both of which are used to calculate the normalized channel steepness at the channel head. We then apply the new method to an active rift shoulder (Wutai Shan), and a tectonically stable area (a mountain range in the Loess Plateau) in North China, to illustrate the calculation of drainage-divide migration rates. The northward migration rates at the Wutai Shan range from 0.10 to 0.13 mm/yr. The migration rates are approximately zero at the mountain range in the Loess Plateau. This study demonstrates that the migration rate of drainage divides can be determined more accurately once the cross-divide differences in uplift rate are taken into account.
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Chao Zhou et al.
Status: open (until 23 Oct 2023)
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RC1: 'Comment on egusphere-2023-1428', Thomas Bernard, 18 Sep 2023
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General comments:
Zhou et al. present in their manuscript a framework to determine the migration rate of drainage divide and apply the methods to two study cases in the Wutai Shan and Loess Plateau. The authors developed new methods of drainage divide migration rate estimation using high-topographic data based on channel-head parameters, values of channel segments, and erosion rate parameters. They argue that by determining the exact location of the channel heads, the migration rate of drainage divide can be accurately calculated. They determined drainage divide migration rate of about 0.10-0.13 mm.yr-1 and closed to zero for the Wutai Shan and Loess Plateau study cases. Finally, they suggest that the difference in uplift on both sides of the drainage divide have to be considered in order to calculate drainage divide migration rate with this method.
The topic of drainage reorganisation by drainage divide migration or river capture is receiving increasing attention these years. Therefore, this contribution, which determined the rate at which drainage divide migrate, is timely and should be of interest to the EGUsphere journal. This study also presents a nice follow-up of drainage divide migration estimation to the study by Zhou et al., 2022 (although I find the two manuscripts really close). The manuscript is interesting and overall well-written. The provided model, methods and equations in the manuscript are sound and well-used in the study area cases. There are a few awkward sentences that I have indicated below. Finally, I found the figures well realized and easy to understand.
My main comments/concerns regarding this manuscript are the use of strong assumptions made in the calculation of the drainage divide rate migration which have not been mentioned in the main text. The first one is the use of erosion rate estimate from low-temperature thermochronology data, which cannot correspond to a modern rate for the Wutai Shan study case. The second is the systematic use of a “standard” concavity index for both study cases. I feel like these assumptions have to be addressed. Since the main message of the study is to quantify migration rate from high-resolution topographic data, estimation of the equation parameters needs to follow the same logic.
More specific comments tied to line number:
Specific comments:
- Line 37-39: “The evolution of topography is fundamentally coupled with changes in drainage systems, including river’s vertical and lateral movements”. Can you be more precise about vertical and lateral movements? Maybe add river capture as another important process for drainage changes.
- Line 70-71: “For example, Willett et al. (2014) developed the χ method to map the dynamic state of river basins”. I don’t think Willet et al (2014) developed the χ method. Please change the term “developed” by “applied” or cite “Royden et al., 2000; Perron and Royden, 2013” instead.
- Line 79-81: “Zhou et al. (2022a) developed a technique to calculate the migration rate through the cross-divide χ ratio of high base-level channel segments”. This statement is in contradiction with line 78 “No rates have been obtained”.
- Line 89-90: “to obtain the high-resolution topography data of these two areas”. Can you precise the resolution of your topography data here or somewhere in the manuscript.
- Line 98: “Moreover, benefiting from the detailed tectonic research”. Please reformulate or precise what is this “tectonic research”.
- Line 127-128: “A large erosion coefficient also creates a high channel-head erosion rate”. This sentence is unclear. What does the term “erosion coefficient” refer to?
- Equation 4: This equation is correct only if the erodibility is the same on both side of the drainage divide. Correct the equation or precise this assumption in the text. This is important, especially, since you demonstrate in your Figure 1 that the erodibility affect the channel-head erosion rate.
- Line 184-185: integral function of channels’ upstream area (A) to horizontal distance (x) (Perron and Royden, 2013; Willet et al., 2014)”. Replace the reference “Willet et al., 2014” by “Royden et al., 2000”.
- Line 241: “If we assume the rock uplift rate decreases linearly from 0.25 to 0 mm/yr from northwest to southeast of the Wutai Shan horst”. If I understood correctly the 0.25 mm/yr rate come from the low-temperature thermochronology study. Even if this rate is predicted for the late Cenozoic, it cannot reflect the modern erosion rate (the method is not sensible) needed to accurately calculate the modern drainage divide. It assumes that the erosion rate stayed constant. This assumption is not reported in this paragraph or the discussion.
- Line 246: “We assume n = 1 and m = 0.45 in the calculation following previous studies (Wobus et al., 2006; DiBiase et al., 2010; Perron and Royden, 2013; Wang et al., 2021)”. This is a reference concavity of 0.45 that you are using for your calculation. How does this concavity really reflect the geomorphology of your study area? There is available framework to calculate the channel concavity of your catchment, check Mudd et al., 2018. This is even more important since you can have different concavities on both side of your drainage divide and strongly affect your migration rate calculations based on your equations.
- Line 313: “We also assume n = 1 and m = 0.45 in the calculation”. Same as previous comment.
- Line 294: “An unnamed mountain range in the Loess Plateau”. The term “unnamed” sounds strange. I suggest to just remove it or find another solution.
- Figure 1: It might be better to directly indicate the value of in the different panel instead of the ratio. You can also precise the erodibility for each panel in the figure description since you did it for the slope and area coefficients.
- Figure 2: Precise the meaning of the white rectangle in the description like for the back rectangles.
- Figure 4: The text of Eq. 4 and 8 could be coloured in black and blue respectively in order to directly identified which arrows correspond to which equations and rates. Or you could add small arrow legends on the left of the text.
Technical corrections:
- Line 60-62: “However, these techniques are usually based on samples collected from an outlet that is several kilometers away from the drainage divide and thus may not represent the erosion rates close to the drainage divide”. Change “an outlet” to “a catchment outlet”.
- Line 65-67: “Hence, it would be ideal to find an accessible and efficient method that can be applied to the entire landscape and cross-checked to make full use of the 10Be-derived erosion rates”. The term “cross-checked” is unclear in this sentence.
- Line 68-69: “The advancement of remote sensing technology has promoted the development of geomorphic analysis theory”. Remove the term “theory”.
- Line 77: “No rates have been obtained”. This short sentence might be combined with another one.
- Line 86: “an unnamed mountain range in the Loess plateau”. Remove the term “unnamed”.
- Line 128-129: “The results indicate that the side with a higher Acr or Sch can have a higher erosion rate than the other side of the drainage divide”. Move the terms “of the drainage divide” to the first occurrence of the term “side”.
- Line 134-136: “when one uses the cross-divide erosion rates … one should also consider the influence of differential uplift rates”. I suggest to remove the terms “when one” and “one should” and reformulate the sentence.
- Line 144: “Combining Eqs. 2 and 3, one can derive the equation”. Same as previous comment, replace “one can” by “allow to” for example.
- Line 216-217: Correct to “The bedrock of the Wutai Shan area consists mainly of metamorphic and igneous basement rocks”.
- Line 263: Change to “Figure 3. Topography (A) and normalized channel steepness (ksn) (B) distribution”.
- Line 264-265: The black dashed curve shows the location of the main drainage divide”. Change the term “curve” to “line”.
- Line 272-273: “The topography swath profile along E-E’ in Fig. 3A. (D) The ksn swath profile along F-F’ in Fig. 3B”. Change the terms “Fig. 3” to “panel”.
- Line 273-274: “The red dotted line shows”. Correct to “The red dotted lines show”.
- Line 279-292: Replace the terms “curve” by “line”.
- Line 292: Correct to “the channel-head (Eq. 4) and channel-head-segment (Eq. 8) methods respectively”.
- Line 296: “(Yin, 2010 Su et al., 2021)”. There is a missing coma between the two references.
- Line 306: “the slope-area plots (Figs. 5 B, E, H) and the χ values”. Change “and” by a coma.
- Line 324-334: Replace the terms “curve” by “line”.
- Line 346-347: “In the tectonically active area”. Change “the” by “a”.
- Line 350-351: Zhou et al. (2022a) combined the advantages of the χ and Gilbert metrics methods, proposed to use the χ contrast with a high base level”. This sentence does not sound right. Maybe change “combined” by “by combining”.
- Line 361: “(Forte and Whipple, 2018; Zhou et al., 2022a; this study)”. Remove “this study” to the references.
- Line 363-366: “In this study, we advocate the use of high-resolution DEM to determine a more accurate position and related parameters of the channel head, given that the use of UAVs to obtain the local DEM has become highly efficient”. I suggest to break this sentence in two (maybe around the coma).
- Line 368-369: “one can obtain the sub-meter resolution topography data of drainage divides”. Replace “one can” by “it is possible to”.
- Line 396-397: Change “Consider an extreme example: when the main drainage divide” by “If we consider an extreme example where the main drainage divide”.
Citation: https://doi.org/10.5194/egusphere-2023-1428-RC1
Chao Zhou et al.
Chao Zhou et al.
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