This manuscript presents a case study in understanding rainy season dynamics in semi-arid regions. The study is intended to develop and validate a new framework for calibrating rainy season metrics. The paper is generally clear.

Although CMIP5 data seem to be applicable in your research, I will still suggest you using the newer CMIP6 data, which are accessible in many data portals provided by organizations such as ESGF Copernicus. CMIP5 data has been more than ten years since developed, while CMIP6 incorporate numerous updates and incorporate more features, let alone it may address the issues you do mentioned in your text (lines 345-350). In addition, the implications or significance for this novel method in evaluating rainy season and to be applied in future research could be discussed further. A brief comment on the potential transferability of this framework to other semi-arid regions would be valuable as well.

Overall, I will give this paper the suggestion of moderate revisions

Specific Comments:

Line 17: I suggest change it to “only a slight delay in rainy season end, but no consistent trends in rainy season onset,” to stress the later parts with the bad results

Lines 40-45: The review of existing metrics could be more comprehensive, particularly regarding their applications in similar geographical contexts. Some results from the former study can be put here.

Line 123: maybe you can mention why you choose these two scenarios

Lines 254-269: I think most of these can be moved into methods, just leave some brief descriptions here in the results

Line 320: "Feb."

Lines 345-367: Most of these can be moved into conclusions part. They are discussions from my perspective

Figure 1: you could move the index plot of Rio Santa Basin with NDVI a little further away from the basic topography map. The Basin looks like it’s a big part located within the eastern part of the map

Figures 2 and 3: you could add a legend indicating different data sources and also different methods

Figures 6 and 7: maybe add a legend showing different data sources and different scenarios

Some paragraphs have indentation while others don’t, please keep them consistent

The manuscript presents an interesting and well-structured approach to understanding rainy season dynamics in semi-arid regions. It contributes meaningfully to hydrology and earth system sciences by introducing a novel framework for calibrating rainy season metrics with vegetation data, addressing their applicability under current and future climate scenarios. However, there are areas where improvements can enhance the manuscript’s scientific rigor and presentation quality.

While the manuscript provides valuable insights using CMIP5 data, incorporating CMIP6 would likely enhance the study due to historical forcings, and more comprehensive future scenarios. As noted by another reviewer, it would be appropriate to perform the study with CMIP6 to align with current advancements in climate modeling. Otherwise, the authors should provide a solid justification for relying on CMIP5, addressing how its limitations might affect the results and conclusions of the study.

Here are some areas of improvement:

Line 14: Introducing the bucket-type metric is a significant contribution but could be more prominently emphasized earlier in the abstract. Currently, it feels buried in the middle.

Line 15: The term "sensitivities" might need elaboration—does it refer to responsiveness, instability, or another issue?

Lines 40–45: The discussion about the lack of strategies to validate rainy season metrics based on independent data is important but somewhat abrupt. Also, the emphasis on uncertainties in precipitation measurements is valid but could be expanded with specific examples of how these uncertainties affect the metrics or decision-making.

For example, tie the need for validation directly to the challenges of applying metrics in real-world scenarios. Add a sentence elaborating on the practical implications of these uncertainties, especially for agricultural or water management applications.

Lines 57: The transition to the Rio Santa Basin context is slightly abrupt, and the text does not adequately establish why this region is particularly suitable for testing the proposed framework.

Line 75: The phrase “and two other precipitation datasets for comparison” is vague and could lead to confusion. It is unclear what “other” refers to—whether additional datasets are used for validation, alternative sources of precipitation data, or datasets of different spatial/temporal resolution.

Lines 78: While using NDVI for calibration is well-justified, it might help to briefly highlight why NDVI is a robust proxy for water availability in this region compared to alternatives.

Lines 95: The final objective includes exploring past and future changes in rainy season dynamics, but it does not specify the importance of these changes for the broader context of climate change. It should emphasize how understanding these changes can inform adaptation strategies in similar semi-arid regions.

Figure 1: The climograph at the bottom is helpful but not explicitly referenced in the text. Discuss the precipitation seasonality and temperature trends shown in the climograph and connect them to the rainy season dynamics discussed in the study.

Recommendation: Add a sentence or two explicitly linking the Rio Santa basin's unique hydroclimatic and socioeconomic characteristics to the study's focus on rainy season metrics. Reference specific features of Figure 1 (e.g., NDVI, climograph) in the study area description to better integrate the figure with the text.

Line 112 (Data): The datasets have varying spatial resolutions. How do you account for or interpret these differences in your study?"

Line 150-160: The differentiation between onset-only metrics and metrics that address the onset and end of the rainy season is crucial but is introduced abruptly.

Recommendation: Include a transitional sentence to guide the reader through this distinction (e.g., “While some metrics are focused exclusively on the onset of the rainy season, others provide a more comprehensive approach by also addressing the season’s end”).

Line 179: The formula is clear, but the conditional structure might be challenging for some readers to interpret.

Recommendation: Add a sentence explaining the formula in simpler terms: “The bucket water content is updated daily based on precipitation input and constant evapotranspiration. It is constrained between a minimum and maximum value, ensuring realistic water balance limits.”

Section 3.3 Past and Future: Transitioning from historical trends to future projections feels abrupt. Add a bridging sentence to guide readers, e.g., “Having established the variability in historical trends, we now turn to the projected changes in rainy season metrics up to 2100.”

Figures 6, 7, A7, and A8 are referenced, but their key findings are not fully summarized in the text.

Line 361: While uncertainties are acknowledged (e.g., in evapotranspiration rates), the potential impact on results is not fully explored. Provide more detail on how these uncertainties might affect the interpretation of trends.

The conclusion summarizes key findings well but could improve with clearer transitions, a detailed discussion of the bucket metric's strengths, and actionable practitioner recommendations.

1. While ENSO is mentioned, its significance to the study's findings could be elaborated further. Expand on how ENSO-related precipitation anomalies might influence the calibration and robustness of rainy season metrics.
2. The flow between discussions of calibration, the bucket metric, and future projections could be smoother. Use transitional phrases to connect paragraphs, e.g., "Building on these findings, we introduced the bucket metric to address limitations in existing methods."
3. The conclusions acknowledge uncertainty in projections but do not fully explore its implications for the study’s findings. Discuss how these uncertainties might affect the interpretation of trends or the applicability of metrics in other regions.
4. The reliance on vegetation proxies is discussed, but alternative calibration strategies are only briefly mentioned. Elaborate on potential alternative data sources, such as soil moisture or runoff measurements, and their advantages or challenges.
5. The conclusion mentions practitioner needs but could provide more actionable recommendations. Include a sentence like, “Practitioners can use the bucket metric to better predict water availability in semi-arid regions, particularly during ENSO-driven precipitation anomalies.”
6. The call for more robust climate models is appropriate but could be tied more explicitly to the study’s findings. Link this need to specific challenges identified in the study, e.g., “The limitations of CMIP5 models, such as their inability to resolve ENSO impacts at finer scales, underscore the importance of developing high-resolution convection-permitting models.”