Important advances have been made in the development of field-deployable analysers and sensors that enables in-situ, high-temporal-resolution measurements at a reasonable cost without the requirement involving sample collection, transport and analysis to and in the lab. Such continuous measurements using either deployable sensors to look at for example nutrients and/or dissolved organic matter, or semi-continuous measurements using devises that pump water to the analytical instrument for biogeochemical, physical and stable water isotopes analysis are increasing our ability to provide process-based understanding. Hence, the further development of such lab-in-the-field devices will be important in the years to come.

The manuscript provides another dimension of this such a lab-in-the-field approach, namely being mobile. While this is a rather obvious development, it has to my knowledge not previously been formalized into a paper. This is an advantage since others can more easily follow by using the detailed instructions or coming up with new ideas on how to improve it further.  

The mobile laboratory allows for relatively advanced measurement, while the authors also are honest with not only the pros, but also the cons for others to develop alternative solutions.

While I generally applaud this, I think that the data presentation could be improved. It is difficult to make much sense of the data. Perhaps use better colour scheme, try to help the reader with some lines between points and make clear what data is what.  But these are minor issues.

This new manuscript is a strong and valuable contribution to water quality monitoring research. It combines technical innovation with practical field validation and addresses an important challenge in hydrology by obtaining reliable, high-frequency and multi-source water quality data. The article is well-structured but the choice of the colours and symbols in the graph could be improved for the reader’s understanding. Despite some minor limitations, the study demonstrates real potential for advancing catchment science and management of water quality in mixed structured landscapes.

The manuscript focuses primarily on technical implementation rather than on the scientific interpretation of the collected data. Although this is acceptable for a technical note, additional examples of hydrological insights derived from the measurements would have given some added value to the manuscript. Strongest interpretations on the relationship between hydro-climatological records and water isotopic or nitrate composition could have been done.

I think that the economic dimension would require more details that could be valuable for research institutions or public authorities interested in this recent way of monitoring water quality. Additional explanations and estimations about installation costs, operational expenses and long-term maintenance would have been welcome.

Despite the nice presentation of the considerations when operating the WATER system, showing flood event cases for which the results were not as promising would have given more realistic indication of WATER operational limitations. This is of importance because such monitoring tool may not always be as straight forward in its use and in the results it may provide.

Finally, I wonder about the 5 µm threshold of the filter selected in this analytical chain. I do not really see its relevance according to the standard filtration scheme that are generally used by environmental agencies and research institutions: 1 µm to separate suspended particles from colloids and real dissolved fractions; 0.45 µm for water quality assessment. How the data collected with WATER could be used as a comparison in places where historical data already exist if the filtration is so different? More detailed information for the selection of the filter system could be done as well.

General comment:

This manuscript is a relevant contribution presenting a new lab-in-the-field set-up for high frequency natural water analysis and its application over a 6-month period in a research observatory. To my opinion this original set-up is promising and presents several relevant specificities: 1) it analyses water stable isotopes that are not possible to analyze with in situ probes , 2) it allows for analyzing several different water sampling points which is of major interest, and 3) it sounds easy to move from one catchment to another. I think the authors should detail several aspects so that readers can fully understand: 

• i) about multiple water sampling locations :
  • how far could be the sampling locations from the WATER? How deep? Are there any constraint for the pumping ? for example, would it be possible to sample deep groundwater? What if the well gets dry or if the stream gets frozen? 
  • would it be possible to manage multiple event-triggered sources? e.g. to capture rain + stormflow dynamics or to investigate the spatial variability in rainwater composition with multiple collectors?
• ii) time recording: 
  • how long does it take to the water to travel from sampling point to the WATER? and to the reservoir to be rinced and filled? 
  • what it the time associated to the recorded analysis values? is it sampling time + 20 min for isotopic results? is it less for NO3 and EC? 
  • Did you compare with in situ EC probes in stream sources and piezometers?

Specific comments: 

No detail about filtration is given in the abstract, would it be difficult to have stronger filtrations (e.g. 0.45 µm)? Is this threshold of 5 µm resulting from preliminary tests or any other strategy? 

lines 101-104 : temperature fluctuations (+/- 5°C for low temperature, +/-1° with air conditionner) : what is the range of temparature for which these ranges of fluctuations are guaranteed? 

line 153: please explain the different parameters proposed as measurement of organic content 

line 175 : Why do you mean by "code aligns with FAIR principles"? is that because the code is open? or is the code directly reusable by algorithms and based on interoperability standards ?

section 2.4 : If someone had to be autonomous for changing acquisition parameterization or adding new devices : what skills will it required? My understanding is that it is implemented in the PLC software, then it should be modified in the CODESYS scripts, is that right? Same question for changing sampling locations or measurements actions: my understanding is that this part required "only" to know Python coding, is that right? this needs to be clearer. 

lines 225-229 : there is no additionnal time for filling reservoir and pumping from sources? 

lines 277-278:  does it need someone in charge of watching the alarms and solving eventual issues? 

line 286: pumping set-up for Groundwater is not described. What is the range of water table depth that is supported by this set-up? 

line 328 : can you explain what is your criteria of quality? (high NO3 concentration? high turbidity? others?) 

line 339: It would be helpful to know how often consumables need to be replaced or the approximate monthly costs associated with this.