This is an overall high quality manuscript in terms of the topic, the techniques of obtaining and analyzing data. With some edits this will be a useful addition to indirect bedload monitoring. Several of the latter parts of this submission are best moved from the manuscript to the supplement.

The present manuscript offers a thorough and data-rich analysis of bedload transport variability in four Swiss mountain streams, drawing upon long-term continuous Swiss Plate Geophone (SPG) monitoring. The study's findings, which are based on an explicit examination of variability at both sub-seasonal (event to multi-week windows) and annual timescales, provide a compelling evidence base demonstrating that the steepness of transport relations (m) and the reference shear stress (τ*ref) are subject to significant temporal and geographical variation. This finding is of significant importance for the field of sediment transport theory as well as for the development of applied models and the management of rivers.

The dataset is exceptional, the methodological approach is largely sound, and the comparison across transport equation families (SEA-1/SEA-2, Recking-type formulations, and MPM variants) is informative. In sum, the submitted manuscript constitutes a substantial contribution and is well suited for EGUsphere.

This manuscript provides a robust and timely contribution to understanding the temporal and spatial variability of bedload transport and highlights fundamental limitations of stationary transport formulations. The manuscript would benefit from minor to moderate revisions, mainly aimed at improving clarity, contextualization, and figure presentation.

Major comments:

1) Sediment availability and non-stationarity

The manuscript plausibly identifies sediment supply as a dominant control on the observed variability of transport parameters and model performance. However, sediment supply is mostly inferred indirectly from bedload transport processes and disequilibrium concepts. The discussion would benefit from a clearer conceptual and, where possible, quantitative framing of sediment supply (e.g. antecedent flow conditions, cumulative transport since major events, or catchment area-related measurement data). This would strengthen the interpretation of m and τ*_ref values and improve reproducibility.

2) Transferability of results

The selected sites are characterized by their pristine alpine mountain streams, which are well-instrumented to facilitate scientific analysis. A more precise articulation regarding the anticipated transferability of the findings to disparate channel categories, such as less precipitous gravel-bed rivers, regulated channels, or wider channels, would enhance the general relevance of the conclusions and facilitate readers' evaluation of where analogous variability can be anticipated.

3) Adjustment options?

The manuscript unequivocally substantiates the constraints imposed by time-invariance parameterizations. The discussion would be strengthened by the provision of more explicit guidance on how these findings could be translated into practical modeling strategies, such as event-based or state-dependent calibration, adaptive parameterizations, or conditional formulations linked to sediment availability.

Minor comments

4) Workflow: Section 2 comprises a multitude of equations and processing steps. A concise workflow schematic that summarizes the analysis would improve accessibility, especially for readers who are less familiar with the individual formulations.

5) Definition of Phase-2 transport conditions: Phase-2 conditions are identified through a visual inspection of smoothed trends and discharge ranges. A concise, precise operational definition (even if site-specific) would enhance clarity and reproducibility. At this juncture, a thorough elucidation of the rationale behind the selection of a static interval of two weeks would be advantageous for the reader's comprehension.

6) Sediment supply and implicit assumptions (Figure 3): The application of transport equations presupposes transport-limited conditions, that is to say, there is no explicit supply limitation. In light of the observed scatter and site-specific trends depicted in Figure 3, it would be beneficial to elucidate the potential influence of variations in sediment availability between sites and years on the comparison between measured and predicted transport rates. This clarification would assist in determining the extent to which deviations might be indicative of supply-limited behavior rather than being solely governed by hydraulic controls.

7) Relation to recent literature: The discussion could be further strengthened by briefly positioning the results within recent publications addressing sediment supply effects based on long-term surrogate monitoring (including plate geophone data), as well as recent transport-equation or comparison studies. This would provide a more accurate placement of the findings within the rapidly expanding body of surrogate-monitoring literature.

8) Figures 2, 3, and 7: layout and interpretability:

a) In Figures 2 and 7, legends partly overlap the plotted curves and, in places, extend beyond the plotting area, reducing readability. Adjusting legend placement (e.g. outside the axes) would improve clarity.

b)Figure 3 shows a Qb–Q relationship that is typical of Alpine environments and comparable to published datasets from Italy and Austria. A brief comparison to such studies could help contextualize the observed scatter.

c) Furthermore, it is not possible to discern individual years in Figure 3. The utilization of distinct colors or symbols to denote different years would significantly enhance interpretability and facilitate a more transparent discussion of interannual variability.

9) Global figure comment

Several figures would benefit from minor layout adjustments (e.g. legend placement, margins, and font sizes), as some legends overlap with plotted data; a consistent re-layout would improve readability without affecting the scientific content.