Dear Hongzhang Xu , thanks for sharing and your call for opinions. From my experience in urbanized deltas and managing urban flood risk, river regulation (turning these into narrow strips) and land use (urbanization - more paved areas - loss of natural land e.g. forests, wetlands) are of major influence to our increased vulnerability to floods. Climate change is amplifying these effects and makes the impact of flood worse. (image: Henk Ovink, Water as Leverage, 2019)

Dear Nanco Dolman

Thanks a lot for your kind response. Very helpful!

That's exactly what I thought based on my many years of research experiences on dams and basin management. From my perspectives, land-use change and river regulation are always the major drivers of flood change for most basins.

Also, as a response to this article, I know only the Yangtze River basin has over 52,000 dams and this paper only mapped thousands in China and then based on unconvincing methods to get such results.

I just could not believe such a big claim could be accepted by a 'respectful' international academic journal, and it seems just too entertaining for me.

Read more on Yangtze River dams here: Article Policy changes in dam construction and biodiversity conserva...

I agree with you Hongzhang Xu The conclusion is not evidence-based.

Floods are generally related to the presence of the ENSO phenomenon, and this phenomenon may be related to climate change

Dear Emmanuel V Murray

Thanks. I felt much better while finding I am not the only person who questions that.

Dear Edgard Gonzales

Thanks for raising up a very important point, the ENSO phenomenon. It's very important but hard to claim it plays more important roles than river regulation and land-use change.

Also, ENSO is a continuous loop for quite a long time but large-scale and distinctive flood changes happened more recently, with more frequent human activities, especially accompanied by a much higher speed of dam construction and urbanization.

Actually, that's quite true. For example, many farmers here in Australia don't believe in climate change and regarded the change of climate as a part of the loop of the ENSO. See: doi:https://doi.org/10.1016/j.gloenvcha.2021.102274

I have to say that that claim is false under a logic perspective because there is a relationship between variables that cannot be treated as separable ones. So after a profound analysis, outcomes have to do with the interaction of those features within a dynamic scenario; a micro-climatic event fosters land use, and the presence or not of a river can even be the dominant factor within a determined phenomenon. That is why there is a very fertile valley around the Nilo river, for instance. Another example of this kind of behavior can be found around massive continental water bodies; there is an interaction between water reservoirs and local climate, resilience weather systems, and several phenomena as floods. So I am afraid that it is a huge mistake to set out this kind of conclusion.

Hey Henry Arellano Peña

Thanks. You raised a very important concept, micro-climate. I know that the construction of many big reservoirs, such as the Nuozhadu Dam on the upper Mekong and the Three Gorges Dam on the Yangtze, have changed the local climate significantly. I have been there before and after constructing some of these giant dams and can feel the distinctive change of micro-climate.

Indeed, this kind of conclusion is quite misleading. I am not sure whether the conclusion shaping has been impacted by politics. Some countries are just too eager to embrace climate change, and let climate change to 'carry all issues' so that they can show how 'far-sighted' they are in setting up ambitious climate goals.

Dear Eugene A. Simonov

Thank you very much for opening another discussion on defining the ‘flood’. That’s quite an important point. I quite agree with you that flood is not always undesirable and causing problems. In contrast, flood help to generate lives, such as larvae fishes and other aquatic animals. In Australia, flood sustains a big area of wetlands and a range of different plans.

However, if we are talking about flood changes as the paper does, we are talking about hydrological changes, such as the changes in the flood regime. I agree climate plays an important role but it's not safe enough to say it is the biggest driver, especially through the way of this paper.

This paper fails to convince me because of its weak argument. If someone aims to argue that A plays more important roles than B and C on hydrological changes, she/he should clearly state the effects of A, B and C on the same level, and then compare. Unfortunately, it is not the way adopted by this paper. Also, the division of watersheds into near-natural and human-modified is quite blurring and thereby questionable. Moreover, as stated by Henry Arellano Peña , the three things compared are not independent.

Regarding your point on the 2020 Yangtze flood, it is very hard to find the causes. Is it because of the poorly operation of the Three Gorges to protecting the upstream or the weather change? What if there is no such big dam and other cascade dams?

Cheers,

Hong

I definitely agree with you, Hong. River management, urban land-use management and stormwater management have direct impacts and consequences on urban flood control. Climate change, for sure, has secondary and indirect impacts on urban flood hazards. For example, this paper (Article Urbanization and air quality as major drivers of altered spa...

Good on you, Prabhasri Herath

Thanks a lot for confirming that and add your thoughts. Do appreciate your contribution!

Cheers,

Hong

Hi Hongzhang Xu could you perhaps give more detail on why you disagree?

I read through the paper and could not disagree with the conclusion.

The key points for me was that

1) flood trends are similar across both regulated and unregulated systems

2) flood trends correlate with trends in climate-related drivers (rainfall, soil moisture, snow depth)

Note that here in my response I write "flood", but really I am referring to "peak flood discharge" as the authors do.

Note also that they are talking about *trends*, not magnitude of event. Governance/management can be regulating the magnitude of events (e.g., controlling/preventing flooding), but it cannot be controlling the trend which are governed by climate factors.

Hey Takuya Iwanaga

Please find my responses to our folks above and their responses as well. I believe there are enough details

Responses to your key points can be found in answers from Henry Arellano Peña

In summary, climate-related drivers are greatly controlled by the local microenvironment due to large-scale reservoir construction. Climate is not independent enough to be compared with land use and river regulation. They are all closely related. For example, 'peak flood discharge' has already regulated by the operation of numerous (upstream) dams.

In addition, the title is very misleading. It is not talking out trends but broadly "flood changes". The mapping of the near-natural watershed and human-modified watersheds is not transparent at all, and I can find a lot of overlapping regions and missing big dams.

Anyway, this paper makes a too big claim.

Hi Hongzhang Xu

Yes I read the other responses but I felt they didn't fully relate to what I had in mind.

> climate-related drivers are greatly controlled by the local microenvironment due to large-scale reservoir construction

I think this is a big claim in itself. Yes, of course there are localised effects but climate itself is a global system. Further, this point is addressed in the paper - namely that previous studies that study the influence of large-scale infrastructure development constrain their investigations of the impact to the local area (whether this is true or not is a different matter). But here they are interested in larger multi-basin scale effects.

> For example, 'peak flood discharge' has already regulated by the operation of numerous (upstream) dams.

The authors point out (correctly in my view) that construction of those dams then make the influence of hydro-climatic events downstream more important in understanding river behavior. As you say, events upstream are regulated and so their influence is reduced, at least partially, but dams upstream cannot regulate events downstream.

> Climate is not independent enough to be compared with land use and river regulation

In my view they adequately controlled for this to allow this comparison. This statement also seems to suggest that policies are the only relevant factor and that climatic influences are negligible which I disagree with.

I will return to this point further below, but briefly, local water management policies cannot influence global climate factors, but climate factors can influence local water management (both implementation and their development).

> It is not talking out trends but broadly "flood changes"

"peak flood discharge" however is a concept independent of regulation (i.e., floods will have a peak regardless of whether there is a dam upstream or not). I believe it is a common indicator for changes in flood so I do not have an issue with this conceptual framing.

> The mapping of the near-natural watershed and human-modified watersheds is not transparent at all, and I can find a lot of overlapping regions and missing big dams.

Thinking on this point (which you made in a response elsewhere, hence why I've had some time to think on it) I've come to the conclusion that cleanly separating regulated and unregulated basins actually doesn't matter to come to the same conclusion the authors present.

(To your point about transparency, they've made code and data for their analysis publicly available, although you'd have to understand MATLAB it's there if you care to take a look.)

For one, speaking to the data presented in the paper, we see correlation in flood trends with climate factors across all basins included in the study.

Let's assume the basins they investigated are indeed mixed together as you say.

1) If the basin includes more regulated areas, then the policies may be contributing to decreasing peak flood discharge (as these events become more regulated) and increases in other places, say for agricultural and environmental management purposes.

Water releases for agricultural and environmental management typically happen during times of need, however. Farmers/environmental users need water during dry times (why order more water when your fields are flooded?). Yet the paper shows a clear correlation with climate conditions instead and this correlation occurs for all basins investigated.

A big number of unregulated systems cannot explain the correlations shown in my view, and based on your statement that "I can find a lot of overlapping regions and missing big dams", it seems that they may actually be under-representing the number of regulated systems.

If policies were the only governing factor, I would then expect no or negative correlation with climatic factors as these policies would (presumeably) aim to mitigate the influence of climatic inputs.

2) A further point as a thought experiment is to go in the opposite direction - droughts instead of floods.

We have the Basin Plan in place for the Murray-Darling here in Australia aiming to provide more environmental water, but we see drying trends in line with projected with climate change regardless of local/state and federal policies.

Now you could argue, as some have done, that the Plan is not fit for purpose - partly as it does not fully account for the level of change we're seeing with the climate system. In other words, the influence of climate is having more of an effect on the hydrology because our policies are not well suited to the conditions.

The above two points go to the authors' conclusion - that "[f]lood management policies need to go beyond local measures and adapt to climate change effects" (p 7).

Thanks for the interesting discussion

Hi Takuya Iwanaga

Thank you very much for your very detailed responses. Do appreciate!

I will respond to each of your points a bit later.

Here I think it's necessary and urgent to highlight the argument of this paper before going into details. For me, the argument itself is a 'straw man' (https://en.wikipedia.org/wiki/Straw_man).

"Straw man" e.g. You cannot say apple is a fruit; Peter hates eating apples; so, Peter hates eating fruit. (Peter may like all other fruit except apple).

In the abstract, it says: "Existing studies suggest that flood changes in China are mainly a result of land use change and the construction of reservoirs."

This paper only targets a small part of the flood changes, that is, the 'annual flood peak discharge'. However, it goes to the conclusion, "Our data-driven analyses echo recent modelling studies that show climate plays a more important role in flood changes than land and water management", directly.

The analysis itself cannot support such a big claim. I agree with you that "[T]he need for flood management policies to go beyond local measures and adapt to the effects of climate change" is an acceptable conclusion. However, the authors are not satisfied by only making this claim but a much bigger one as shown by the title of this article.

Cheers,

Hong

I think then that we've hit the central point of disagreement.

Changes to peak discharge is still a change to flood regime. I am not sure what other indicator you would have preferred (flood duration?) but peak discharge is a reasonable and common indicator of changes to flood regime. I imagine there would be some disagreement on whether it is the most appropriate indicator across all basins but generally I am hard pressed to argue against it (as I wrote before).

Because of my interpretation, I don't see this as a strawman, and I personally don't see a disconnect with the title and its conclusion - as I said above, changes to peak discharge is still a change to flood behavior.