The paper points out increasing conflicts between water demand and water availability (with “Day Zero” as the most extreme case) in regions facing water scarcity. This triggers extensive ground water (GW) extraction, to some extent, unsustainably. The authors choose central-north Chile as a testbed to unveil the phenomena, i.e., increasing water stress and unsustainable GW use. The authors also highlight the inadequate water management strategy and policies and give their recommendations.

The findings are practically important, but lack of scientific significance. The caveats in water management sections read informative and structured. However, the introduction is not organized in a balanced way and the conclusions are not well structured. See below for details:

Major comments:  

• The word “conceal” in the title and throughout the manuscript is confusing. I can not understand the meaning and the reasoning behind.
• The scientific significance needs to be well sharpened. I can not find out substantial new concepts or methods. In L15, the authors claim they based on “novel” estimates. However, I can not tell what the novel estimates are? Why they are novel? What problems previous studies have? The introduction is not in a balanced way, The author talk about the water scarcity condition with some local examples, GW extraction and consequences (with no details), and then skip to what this study do. No mention about current gaps or inconsistencies in the field.
• Some brief descriptions of data and methods are needed. For instance, what is the format, length and spatial-temporal resolution of the water availability and water use data? What’s new about the data (e.g., first/better estimate water availability/use?)? How to calculate the trend of GW level in Figure 3 (linear regression? Sen’s slope?).
• The results are generally presented well but the conclusions are a bit lengthy and unstructured. I would recommend to make it more concise and well structed by firstly, briefly reporting main findings of this study, then implications, and giving recommendations to different authorities (researchers, government., etc.). Clearer ways would be a simple sentence at the start of each paragraph or give sequential numbers (like Section 4).

Specific comments:

• L28: why “curve”? both water availability and demand are single values. Are there time series?
• L37: why desalinated water “non-drinkable”? Is this policy positive or negative? Here comes “conceal”, does it mean “relief” or “save”?
• L43: some exact number of withdrawal/natural recharge rate would be better.
• L45: unsustainable GW use brings “challenges and risk”, what exactly are they?
• L47: what “situation”?
• L78: what is “water use right”?
• L88: is “15 m” to identify surface and underground water a consensus or only in this paper? why “15 m”?
• L122: please be precise about “high” and “low” emission scenarios (RCP xx?).
• L107-108, L149-151 and L224-226 read very similar
• Figure 2b: why use 1980-2010 as baseline period? Around 1980 there is little data, seems problematic.
• Figure 3b: for each basin, each well can have a GW level trend, then the bar can have ranges. Therefore, I would suggest adding error bars.

The manuscript presents “day zero” as a critical event of unsustainable groundwater use. As an opinion paper, the script identifies potential problems with water management and future planning. To a large extent, the document calls for quantifying groundwater recharge and withdrawals before aquifers dry up. "Day Zero" is considered a relevant topic for scientific research and public concern, but the framework should be explained further.

Minor comments

Line 27. Is it possible that a decrease in water availability may cause "Day Zero” through climate change, keeping water demand stable?

Line 78. Why not include precipitation at the top of graph 2a to relate the reduction in precipitation since 2010?

Line 88. Is there a reason for the 15 m threshold between shallow and deep wells?

Line 97. How was the D₀ time frame calculated?

Line 119. Why do permanent water uses rely on depleting GW storage? A reduction in surface water due to water management can also trigger overexploitation of groundwater. A cascade effect from surface to groundwater is observed.

Line 177. Is there any information about fossil groundwater flow within the aquifer?

Line 186. With a neglected legal uncertainty about the economy, should the water management legal context (unregulated or weak) be explained?

Line 200: When does the irreversible impact occur?