Review of egusphere-2024-1678

Nasta et al.

HESS Opinions: Towards a common vision for the future of hydrological observatories

Review by Andrew J. Guswa, Smith College, aguswa@smith.edu

This editorial, which grew out of discussions at the 8^th Galileo Conference held in Napoli, Italy in June 2023, raises the question of how funding and resources for hydrological observatories could best be deployed, considering two scenarios: 1) a large number of moderately instrumented sites or 2) a small number of highly instrumented “super sites” (the dynamic-response approach notwithstanding).  The question is a provocative and important one.  With the current version of the manuscript, I also found myself wishing for more details of the two models, more discussion of the scientific tradeoffs between them, and perhaps considering ideas beyond the two.  Additionally, I think it would be interesting and valuable to discuss potential differences in governance structures, data access, and equity issues.  To make room for such detail and discussion, I think some of the preamble/motivation could be shortened and one or two of the figures removed/replaced.

Specifically, perhaps Figure 1 could be expanded into two – one that presents the instruments and their distribution associated with a site for Scenario 1, and a second figure that presents the vision for a super-site associated with Scenario 2.  Would they have the same types of instruments, but with different levels of intensity and resolution?  Or would there be additional variables and characteristics measured in the super-sites?  Providing more concrete detail – even if purely hypothetical – will help the reader better understand the two models and the differences between them.

Then, given those differences, which of the UPH would be more amenable to Scenario 1 versus Scenario 2?  The Appendix provides a list of important and intriguing questions – which would be better addressed by which Scenario?  The manuscript already calls out differences in representativeness: e.g., Scenario 1 can cover more of “existing gradients of geology, climate, and land-use,” whereas Scenario 2 would achieve “high spatial and temporal resolutions.”  More directly connecting these differences to the important questions would be a valuable enhancement to the paper.

Additionally, I found myself wondering about the associated tradeoffs in governance, equity, site access, and data availability between the two scenarios.  Perhaps this is not the place for such discussions; nevertheless, if possible, I think a modest discussion of those issues would be interesting and useful.  What processes could/would be put in place to encourage participation from a broad range of stakeholders from the hydrologic community, particularly for Scenario 2?

Relatedly, while the authors position their two scenarios as end-members, I could imagine a scenario further in the direction of Scenario 1 (perhaps Scenario 0), in which instrumentation is deployed in a purely opportunistic way, taking advantage of construction projects associated with infrastructure upgrades.  For example, one could imagine a policy that stipulates that any time a culvert is rebuilt or rehabilitated (e.g., in response to increasing storm intensity), a suite of monitoring instruments must also be installed.  This would significantly reduce the upfront costs associated with hydrologic observation.  The locations of such added monitoring would be far from planful, but there might be advantages in the sheer number of sites, and ongoing advances in data handling/storage and machine-learning and data-science tools could facilitate new insights.

To make space for those expanded discussions, I think either Figure 2 or Figure 3 could be removed.  Those schematics are nice, but perhaps are not necessary for communicating the central ideas of the paper (vs. the Graphical Abstract, which really presents the Scenarios in a compelling way).

In summary, I appreciate the editorial as a provocation for the hydrologic community to consider and discuss the vision for hydrologic observations.  I recommend expanding the comparison of the two scenarios to help the reader better understand the tradeoffs between them. 

The paper is overall a useful contribution and well written. I hope that my comments below will improve its impact to the larger hydrologic community. I have blended below both higher level comments (approach for have more impact for the international community) and more specific for improving logic. I have no problem with its publication but I am also not impressed with the message it tries to send to the community and what it can accomplish to improve a vision for HOs. Also, its focus on the “UPH” limits its reach and the community that would buy into it, while a more high-level approach of basically understanding complex hydrologic processes to improve modeling and prediction that will allow us to address pressing water related problems… will reach a larger audience and sponsors.

1.

Line 36 -- “Nevertheless we are still a long way from being able to solve the mysteries of hydrologic processes…” – the mysteries of many scientific problems are never completely solved. I would present this differently such as “ Yet, solving important water resources problems requires a deep understanding of the complex hydrologic processes which require long records of observations over diverse environments etc.…”

2.

Line 46 – “help address UPH about the impact of climate and social systems…” –First, is this the only UPH to address from the whole list of UPH? Second, I would present this need here in a more general setting. Recall that one does not even know the long list of UPH and if this is an international effort it has to be presented from an even larger perspective …

3.

Fig 1 is ok but again, cross site synthesis is not the key to many problems but depends on the problem to be addressed… as also articulated later in the paper for ocean missions etc.

4.

Line 82 – I would strongly suggest that the title of this section is changed to something like “The need for HOs to advance scientific understanding of hydrologic processes “ instead of “How to address the UPH” for which probably not everyone agree or might have a different problem not included in that list!

5.

Line 89 – “the extent that anthropogenic stressors influence the hydrologic cycle is not yet fully understood…” – I would argue that if we know the stressor then we can address the forward problem of translating it to an outcome or impact, but the challenge is when we do not know what actions will affect what and how, and we need basic understanding to guide decisions and management for guiding the future of water…

6.

Line 95 – HOS are not always long-term sites

7.

Lines 98-99 – stretching it by much here. If this is to have an international and broad audience, this has to be seen from a higher level. CZOs, NEON etc had nothing to do with the UPH, as an example… -- check their vision when established

8.

Lines 104 – check history papers for some early observatories of Horton (Beven special IAHS volume)

9.

Line 109 – evidence for this exponential growth?

10.

Lines 121-122 – will benefit from some editing

11.

Line 124 – we are beyond this and LiDAR can help with determining surface flow paths etc with a lot of developments over the past decade

12.

Lines 151 on – RS observations are not only to upscale or downscale ground observations but to provide data for larger areas extents and different environments, and the limited ground observations play a fundamental role in that

13.

Line 166 – some discontinuity in arguments and logic here

14.

Fig 1 is ok but not too telling

15.

Line 185 – only SMAP? Precipitation is the most important input to the hydrologic cycle and some reference to GPM, IMERG etc should be given, probably also highlighting the successful international cooperation of NASA, JAXA and ESA…

16.

Fig 2 – UPH is everywhere and distractive. This is not the mission here but process understanding in general. The figure says …“Where the UPH addressed?” No or yes, and depending on the answer we follow a path of “refine approach” or “Hydrological understanding” … First, fundamental questions change and a long-term vision from HOs should not be tied to a limited concept of questions not everyone probably has seen or agrees with…

17.

Line 245 – yes! “Formulate scientifically interesting questions …” not follow “prescribed questions…”

18.

Fig 3 is ok but not impressive

19.

Line 276 – It depends on so many other variables so it is hard to throw this statement as a contradiction …

20.

Line 278 – “observed phenomena” – which phenomena?

21.

Line 312- 316 –Yes, these observatories where designed for specific scientific questions not for “UPH” -- resonates much more with the community at large.

22.

Line 317 – In analogy with the above questions, what would be examples of questions to be addressed by these sites?

23.

Line 324 – Yes I agree with this. This contradicts the whole framing of the paper focused on “the UPH”! Also, the arguments in Lines 335-on defeat the arguments on the starting point of this paper.

24.

Line 374 – Yes but as argued above deep observations in one site can significantly knowledge our knowledge in important problems. Some examples as from the CZOs. So there are some contradicting staments here referring to a “global network” etc. Please check.

25.

Line 393 – 399 -- “We envision a hybrid approach …” Yes, ok but how? This is the question and the end of the paper kind of fails to have a “call to action” and inspire a movement. It is a difficult problem of course but the paper left me at the end with no recommended approach …