This is a comprehensive manuscript which discuss the environmental flows and droughts in the entire Indus basin in Pakistan. Overall manuscript is well written, figures and results are well presented and conclusions are valuable. However, the quality of manuscript needs to be further improved,here are few comments which may be useful in this regards.

1. In abstract section authors concluded " The alterations are subject to several factors, including climate change,

seasonality of the river flow, land use changes, topography, and anthropogenic activities" which type of analysis have been performed to reach these conclusions which seems to be generic.

2. In the Introduction section authors stated "The Indus River basin is one of the typical basins facing substantial climate and land use changes, resulting in limited water  availability". However, no references have been added to support this statement authors should state which part of the Indus basin has faced serious land use and climate changes?

3. How did the authors catagorised the flow data into extreme low flow and low flow?

4. Various data qualities issues have been reported regarding the hydrological and meterological datasets of the Indus basin. How authors addressed the missing datasets and which type of analysis have been performed to check the data quality?

5. On which basis authors have done the demarcation of the indus basin into UIB, MIB and LIB?

6. Various studies have been performed to understand the drought in the Indus basin Authors can open the scholar and search from key word droughts in Indus basin. However, no discussion has been performed to compare the results of this study with literature. Discussing the results with previous studies will be useful for readers and this manuscript has potential to be extended for a brief discussion.

7. I have some minor comments regarding use of abbreviations which are unnecessary , should be reduced and must be explained at first use e.g. In abstract section authors should explain EFs before first use.

General comments:

In this study, Rahman et al. explore the impact of drought on environmental flow (EF) in 27 catchments in the Indus basin, focusing on the period 1980-2018, using the Indicators of Hydrologic Alterations (IHA). The authors use SPEI to quantify drought at various timescales. Drought impact on low EFs is quantified using RVA. Their results show that the Lower Indus Basin (LIB) is more vulnerable to drought than the Upper Indus Basin (UIB) and that drought is related to extreme low flow (ELF) and low flow (LF).

The study on drought is in scope of the NHESS journal and is a good contribution to the field of drought and environmental flow. The figures are  well presented. However, the method and analysis need some elaboration, especially the explanation on the IHA. The text is well written although some paragraphs can be shortened because of repetition. The Discussion need some elaboration to discuss the findings with existing literature. Below find my comments.

Specific comments:

• Abstract

The abstract is very concise, I suggest to provide a bit more information on the Indicators of Hydrologic Alterations (IHA) in relation to the Range of variability analysis (RVA). In addition, line 29-30 sound a bit too generic and is not directly studied here, please be more specific.

1. Introduction

Line 33) “to the quality, timing and quality of freshwater flows”: The word “quality” is repeated.

Line 37-39) The authors mention here that 65% of the discharge in rivers poses a moderate to severe threat to biodiversity; in what way, in relation to water quantity or quality? In addition, since when are those numbers altered? Please be more specific.

Line 46) “the alterations in flow regime”: which alterations?

Line 55-56) “Eckstein et al. (2018) that …”: this part does not fit into the sentence, what is meant here?

Line 58) Note that soil moisture drought is more specific than agricultural drought, see Van Loon (2015).

Line 70) Where refers “this” to?

1. Study area

Figure 1) Where do the colors refer to in Figure 1d?

Line 96-97) This is mentioned before already in the same section.

In general, there is some overlap in text between the Introduction and Study area as the Indus basin has been addressed in the Introduction already. I suggest to better align those texts to avoid overlap and to address the urgency and the knowledge gap of studying EF in relation to drought in the Introduction.

1. Methodology

Line 141) Please provide more information about using PCA in this regard? Has it done before in this way etc.?

Line 180-181) How are the IHA used to compute the EFC and how are ELF and LF defined?

Line 184) What are the EFC parameters?

Line 191) “widely used to assess hydrological alterations”. Please include references.

Line 197) “major steps in implementing RVA include..”. However, the authors are not considering all those steps, which steps do the authors use and why?

Line 201-202) Why are the drought years considered a post-impact period and the whole period a pre-impact period? The impacts of drought are felt during drought and afterwards. Please explain this division.

Line 217-223) The authors explain here that threshold regression differs from change-point analysis. Why is this explained and why is threshold regression chosen instead of change-point analysis? Please elaborate on this better.

Line 225) How do the authors define drought severity in this study? Please explain this either in the Introduction or Methodology. For example, are specific gradations of SPEI used to consider drought severity (moderate drought, extreme drought etc.)?

1. Results and Discussion

Line 241) Where are the authors referring to with “Representative catchments”? Furthermore, this sentence is probably not necessary.

Line 245) The authors mention here “extreme, severe, and moderate drought events”, coming back to my earlier comment, how are those defined?

Line 283-284) How are ELF, LF, high flow pulses, small floods, and large floods defined?

Table 2) Indicate in the table that the first values are related to ELF and the second to LF.

Line 347) “three and four”, is meant here “three locations”?

Line 395) The 18 EFC components is coming a bit out of the blue, what are those 18 components?

Line 397) “Overall, environmental flow … catchments in LIB”: why is this the case?

Figure 7) Please include more information in the caption; is it averaged over which time period, which years etc.?

There is no discussion (either in the Introduction or Discussion) about drought literature in the Indus basin in general; did other authors use other methods to look at flows in the Indus basin in relation to drought and what makes this study so innovative? Please elaborate on this in the Introduction and/or Discussion (to compare it with the results of this study). In addition, it is probably interesting to address catchment memory in relation to drought as it plays a huge role in drought impact on river flows and the prediction of hydrological drought, see for example Sutanto & Van Lanen (2022).

1. Conclusion

Line 456) “The analyses have … occurrence of droughts”: how did the authors show that temperature plays an important role in the occurrence of drought? This study does not compare drought indicators or look specifically at evapotranspiration or other definitions of drought.

Line 465) “In other words” does not really seem appropriate here as the text is not summarizing what is said before.  

Line 475-476) “In addition, the … in specific catchments”: the SPEI coefficient increases with increasing drought; do the authors mean the accumulation periods? At the moment, this sentence is not clear.

References:

Sutanto, S. J., & Van Lanen, H. A. (2022). Catchment memory explains hydrological drought forecast performance. Scientific Reports, 12(1), 2689.

Van Loon, A. F. (2015). Hydrological drought explained. Wiley Interdisciplinary Reviews: Water, 2(4), 359-392.