General Comments

In this paper authors investigate the evolution of wet and dry events collectively in space and time over Upper Jhelum Basin for a period of 1981-2014. They use SPEI index calculated from distribution mapping based corrected ERA5 precipitation estimates and observed temperature data, and locate the hotspot regions for wet, dry and both wet-dry rapid transit events. The idea of the analysis is interesting and the potential for the results is high, however the manuscript remains mostly descriptive.

The paper is well written, with a clear, fluent and concise language and a well organised structure. I think that the manuscript can provide new insights into understanding the evolution of compound extreme events. Hence, my assessment of the manuscript is overall positive. However, some revision is needed before the work can be accepted for publication in the journal. Below detailed comments are listed:

Minor comments:

C1: Figure 1 is not well explained. I suggest that the authors should revise the figure by showing name or number of the gauging stations. I suggest to present a detail figure of study area.

C2: The writing and English need thorough polishing. Numerous grammatical and rhetorical issues too.

C3. I have some concerns about the introduction section. I think that if the authors wish this paper is well considered by experts, more attention should be devoted to discuss the extreme events in the area. Moreover, this section is lacking clarity and sufficient motivations. I suggest to improve it or better explain with realistic examples. Kindly go through the Zaman et al (2020) for extreme events in the UIB.

Zaman, M.; Ahmad, I.; Usman, M.; Saifullah, M.; Anjum, M.N.; Khan, M.I.; Uzair Qamar, M. Event-Based Time Distribution Patterns, Return Levels, and Their Trends of Extreme Precipitation across Indus Basin. Water 2020, 12, 3373

C4: As the data use to carry out a research work is the base of a research work and the most important ingredient. The authors have not provided any detail of the data they have used to carry out their work. I suggest that the authors must provide the complete detail of the data they have used in this research work. Moreover, the authors have applied any homogeneity test on the data to ensure the data quality? In data description section the authors did not mention from where they took observed data and what is the ethnicity of the data. I suggest the authors to go through the Zaman et al 2020 for the data quality and presentation.

Zaman, M.; Ahmad, I.; Usman, M.; Saifullah, M.; Anjum, M.N.; Khan, M.I.; Uzair Qamar, M. Event-Based Time Distribution Patterns, Return Levels, and Their Trends of Extreme Precipitation across Indus Basin. Water 2020, 12, 3373

C5: Line 138, I would strongly suggest adding 2-3 sentences why authors prefer to use distribution mapping method of bias correction of ERA5 precipitation and which frequency distribution was employed/fitted to the precipitation data.

C6: Authors used gridded ERA5 precipitation and observed temperature based potential evapotranspiration for the calculation of SPEI index. Would you please just clarify the reason why authors use gridded and observed data combination instead of use only gridded or observed datasets for both variables?

C7: From line 152 onward. Overall, the explanation of SPEI is very easy to understand and I think it should not be substituted by merely a reference to another publication. However, would it be possible to add basic equations to guide some type of readers?

C8: The authors used monthly time scale to detect floods and flash droughts. What do you mean by flash drought? Please explicitly define somewhere in manuscript.

C9: Figure 8, what are the units of transition time? Kindly mention it.

C10: Geographical coordinates are provided in figure 7 only. It would be better to add geographical coordinates to all figures or remove it from figure 7.

C11: Rapid transition of wet-to-dry or dry-to-wet event refers to the one extreme event is followed by the opposite event. It must not necessarily happen with similar severity level.

C12: Line 261-263, rephrase the sentence.

The paper of Ansari and Grossi provides an exercise where the main features of dry-wet condition transitions are analysed at the monthly time scale in the Upper Jhelum Basin located in between India and Pakistan. The authors use a mixed dataset for the period 1981-2014, including ERA5 derived precipitation and observed temperature, they first calculate the SPEI index and then derive several related indices highlighting both dry, wet and combined dry/wet transition events characteristics.

The main contribution of the paper, besides the specific results achieved for the study area, is the effort of proposing a methodological framework, yet based on well-known approaches and methods. I suggest some improvements detailed below. I hope my comments can contribute to enhancing the quality of the paper.

Major comments

- First, I suggest the authors carefully checking the text to avoid several grammar errors and typos widespread in the manuscript (I list some of them at the end of the review as examples).

- I classify this comment as ‘main’ because it concerns the title. In practice, if the authors agree, it can be easily solved. I don’t agree with the term “wet event” because the expectation of the general audience is for smaller time scales than monthly. Therefore, for the sake of clarity, I suggest different phrasing. Probably “wet-dry months” is a correct, yet simple choice (please refer also to the note at lines 226-227).

- I see several problems with data. First, I can’t read the source of observed temperatures. Then, the reliability of ERA5 precipitation data needs to be accurately checked against available observations. In this regard, the authors provide a reference to a conference abstract (Ansari and Grossi, 2021). It’s not enough, a section about data validation is needed. Finally, I’m not that keen on using the Thornthwaite method, which is very dated. I would suggest using at least a temperature-based model, e.g. Hargreaves-Samani. However, ERA5 provides potential evaporation data, a comparison between such data and the results achieved by the authors with another method would be interesting and could provide useful insights. The authors should discuss their choice of relying partially on datasets and partially on ground observations.

- Overall, I found the results and, mainly, the discussion, not particularly vivid. The authors should strive to emphasize better the added value of their study, avoiding not very fitting comments. E.g., I don’t think the sentence in LL396-398 is very appropriate, because it refers to actual ET, while the method used refers to potential ET (PET).

Minor comments

L30: the authors refer to AR5, maybe they can update considering the brand new AR6

LL80-85: I think this sentence should be better placed in the Conclusions

L93: SSI is cited only here and not explained

L119: a paper under review is cited. I would avoid it. Anyway, it is not in the References

Fig. 1: it is not very clear. Only part of the Kunhar borders is visible. Please flip the colour palette of Elevation (high brown and low green)

L136: basically, a period of 35 years is not enough for such kind of analysis. Please extend the discussion of this issue and hint at the possibility of using an extended (in the past) ERA5 dataset

Table 1 and elsewhere: I guess it’s “extremely wet”, “severely wet”, etc., not “extreme wet”, “severe wet”, etc.

Section 4.4: I suppose that also the number of transitions for each grid cell should be considered. Is it so? If not, why?

L200: alteration --> maybe “rapid transition”?

Fig.3: The year 1980 should not appear here, it’s not within the analysed period

Fig. 4: it’s like AWD and ADD, and MWD and MDD are almost complementary (my feeling)

L279: TDI results are not yet introduced

Fig. 7: only here maps coordinates are made explicit. Please make all maps homogeneous.

L328 and L339: “a greater number”: please quantify

Fig. 8: what are the units? Months?

Typos and English grammar (examples)

L8: “more than” or “rather than”

L24: Extremes weather events

L29: extremes events

L67: standardized indices, which facilitates

L123: The monsoon pattern bring

L161: The severity levels… was classified

L167: Following to Spinoni et al.

L215: not clear, please rephrase

L226: the terms… presents…

L266: “…exhibit two distinct parts of the basin”. Not clear, please rephrase

L313: The higher positive values: I guess “the highest”. Also, in the next line, “highest”

L384: El Nino suppress monsoon rainfall activity over Pakistan