the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Sediment transport in Indian rivers high enough to impact satellite gravimetry
Thorsten Warneke
Heinrich Bovensmann
Matthias Weigelt
Jürgen Müller
Tim Rixen
Justus Notholt
Claus Lämmerzahl
Abstract. Satellite gravimetry is used to study the global hydrological cycle. It is a key component in the investigation of groundwater depletion on the Indian subcontinent. Terrestrial mass loss caused by river sediment transport is assumed to be below the detection limit in current gravimetric satellites of the Gravity Recovery and Climate Experiment Follow-On mission. Thus, it is not considered in the calculation of terrestrial water storage from such satellite data. However, the Ganges and Brahmaputra rivers, which drain the Indian subcontinent, constitute one of the world's most sediment rich river systems. In this study, we estimate the impact of sediment mass loss within their catchments on gravimetric estimates of trends in the local mass equivalent water height (EWH). We find that for the Ganges-Brahmaputra-Meghna catchment, sediment transport accounts for (4±2) % of the gravity decrease that is currently attributed to groundwater depletion. The sediment is mainly eroded from the Himalayas, where correction for the sediment mass loss reduces the decrease in EWH by 0.22 cm yr-1, which is about 14 % of the EWH trend observed in that region. However, with sediment mass loss in the Brahmaputra catchment resulting to be more than twice that in the Ganges catchment and sediment mainly being eroded from mountain regions, the impact on gravimetric EWH data within the Indo-Gangetic plain, the main region identified for groundwater depletion, results to be comparatively small.
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Alexandra Klemme et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2023-37', Maxime Mouyen, 21 Apr 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2023-37/hess-2023-37-RC1-supplement.pdf
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RC2: 'Comment on hess-2023-37', Anonymous Referee #2, 28 Jul 2023
The paper “Sediment transport in Indian rivers high enough to impact satellite gravimetry” by Klemme et al. examines how sediment transport can affect trends in gravity fields observed by the GRACE satellites in several river basins in India. This is an important study as accounting for sediment transport can directly affect how we interpret GRACE derived terrestrial water storage changes. The paper is concise and well written. But the manuscript is too focused on impacts on trends. I think the manuscript can be improved substantially if the authors can add some analyses on sediment data. My major comments are:
- As the most important data for this study, sediment data are not well analyzed and presented, making it difficult to assess the quality of the research. At minimum, there should be an analysis on seasonal and interannual variability of sediment data and their correlations with precipitation and GRACE EWH in each basin. There is a figure on seasonal variation of sediments but it is buried in the Supplementary file. In addition, an analysis on how temporal variability of sediments varies from one basin to another would be helpful to understand their climate and environmental controls. If sediments eventually end up at the Bay of Bengal, do sediment data collected at the Bay of Bengal show higher seasonal maximum and lagged correlations with sediments at each basin? These analyses will establish the basis for the need to consider the impact of sediment loss on gravity changes. To accompany these analyses, I suggest a paneled figure that shows time series of sediments, GRACE EWH and precipitation data for each of the basins and for all basin average.
- Seasonality (i.e., monthly mean) needs to be removed from the GRACE EWH time series before computing any trends. Strong seasonality as in GRACE data may affect computing long-term trends. Related to this issue, seasonal cycles should be removed from Figs.4-6 to make the differences in trends more discernable. Seasonal variations can be shown in the figure suggested above.
- At the end of reading section 3.3, those numbers no longer register with me. Since all the numbers are provided in tables, there is no need to state them in the text. Instead, the manuscript should highlight the largest impacts or patterns of impacts that may be interesting to readers. A scatter plot showing TWS trends without correction for sediments vs those with the corrections would be useful for identifying patterns and for accompanying the manuscript.
- Given the coarse scale of GRACE data and the lack of detailed sediment data, section 3.3.4. is flimsy. If included, the authors need to show their calculations and provide justification for assumptions made in Lines 184-187 and line 188-192.
Minor comment:
Fig.1. The white color for high elevation is invisible.
Line 10-14: The sentence is too long and difficult to understand. Please revise.
Line 20: e.g. is not correctly used here. Replace “on e.g.” with “such as”
Line 23: explain it -> explained it
Line 36: annual -> interannual?
Line 56: the clause after whereat needs to be revised for clarity.
Line 120: EWH increase and EWH decrease may be replaced by “high EWH values” and “low EWH values”, respectively.
Line 138: please delete “, with”.
Line 150: reduction in GRACE EWH ->decreasing trend in GRACE EWH?
Figs.4&5 contain references to σ-environment which is not explained anywhere else in the manuscript.
Citation: https://doi.org/10.5194/hess-2023-37-RC2
Alexandra Klemme et al.
Alexandra Klemme et al.
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