Manuscript number: hess-2023-172

Title: Wetting and drying trends in the Land-Atmosphere Reservoir of large basins around the world

Salazar et al. introduces the concept of the Land-Atmosphere Reservoir (LAR), which explicitly includes atmospheric processes as a critical component of the basin water budget. This is in contrast to traditional approaches that treat atmospheric processes as an external forcing to the basin. They have undertaken a rigorous analysis of obtained time-series data of monthly river discharge from the HYdro-geochemistry of the AMazonian Basin (HYBAM) and the Global Runoff Data Centre (GRDC). They selected specific gauging stations for each of the six large basins studied and used data from the ERA5 reanalysis for the years 1979–2020 to estimate moisture convergence for each basin. The authors find considerable latitudinal trends in water storage within the studied basins' LAR, with tropical basins getting wetter and temperate basins getting drier.

While the findings are intriguing and somewhat intuitive given the state-of-knowledge in climate science, the paper could go deeper into the mechanisms (atmospheric and hydrologic processes) driving these latitudinal trends. I will elaborate on this below.

Challenges and questions in applying the LAR Framework to catchment hydrology

The state-of-knowledge in catchment hydrology suggests that hydrologic processes (infiltration, flowpaths, runoff generation, etc.) are not too different in ‘tropical’ (low-latitude, to be more precise) and ‘temperate’ (mid- to high-latitude) catchments. Admittedly, most of these catchment hydrology studies are at spatial scales much smaller than the large basins considered by Salazar et al. And that is because of the obvious reason that understanding hillslope- and catchment-scale surface and subsurface processes is logistically limiting (if not impossible to perform) in large basins. That said, from the perspective of catchment hydrology (what the authors here refer to as ‘Land Reservoir’), the hydrologic processes in low and mid-latitude catchments are not too different. This then, in my view, begs the question: what explains the latitudinal trends, from a process-based, mechanistic perspective, in both Land and Atmosphere domains?

The artificial reservoir analogy that the authors used to explain their results is simple, but too simplistic. It falls short in providing and explaining the underlying mechanisms. For the LAR framework to gain traction (as it should), it would have to be clear to catchment hydrologists how it could potentially change their worldview, i.e., how they perform their work both from experimental and modeling standpoints. I suggest that the authors address this comment by discussing the challenges and questions in applying LAR to catchment hydrology. Here’s an outline or guide that they might find useful in the revision:

1. Scale Mismatch: Catchment hydrology studies are often conducted at much smaller spatial scales than the large basins considered in the Land-Atmosphere Reservoir (LAR) framework. How can the LAR framework be adapted or scaled to be relevant for catchment hydrologists? Note that LMR is not exclusive to ‘large basins’. LMR has also be found in small catchments, for example, in the low latitudes.
2. Complexity vs. Simplicity: The LAR framework is simple, but it may be too simplistic to capture the nuances of hydrologic processes at the catchment level. How can the framework be refined to include more complex processes?
3. Interdisciplinary Integration: The LAR framework needs to be clear and compelling to catchment hydrologists to change (or refine!) their worldview. How can the framework be presented in a way that is relevant to multiple disciplines, including catchment hydrology?

I would discourage the authors from resorting to the usual ‘beyond the scope of this study’ excuse that comes with [most] revisions.

Other comment:

L294-295. In the previous page (L278), you suggested that TWS estimates from GRACE are contradictory. But here, GRACE suddenly figures prominently and is used to support the claim that wetting and drying trends are ‘underway worldwide’? You cannot question GRACE’s TWS estimates only to use the same to support your claim. That is a self-contradiction.

The authors introduce the concept of the Land-Atmosphere Reservoir (LAR), which explicitly considers land-atmosphere interactions such as moisture recycling when computing a basin water budget. The LAR is in contrast to traditional approaches that assume atmospheric processes as external effects. Based on the LAR concept, the authors study long-term storage trends of the six largest river basins using river discharge data from HYBAM and GRDC and meteorological data from ERA5 reanalysis, and find a contrasting latitudinal trend, with tropical basins getting wetter and temperate basins getting drier. The study is interesting, and the topic is suitable for publication in the Hydrology and Earth System Sciences. However, I have some comments that should be addressed before publication. 

• Is there any reason why the authors apply the LAR only to the largest basins? Given that GRDC and ERA5 data are available globally, a similar analysis could be conducted for other basins (with different sizes and climatic conditions) with relatively little effort.
• The authors applied the LAR concept to show the long-term trends in the large basins (e.g. Figs. 3 and 4). Is there something we can learn here that we didn't know from previous studies using the traditional approaches? I would like to see more detailed analysis and discussion in this aspect. 
• The paper has no in-depth explanation about physical mechanisms behind the revealed long-term trends of the basins. For instance, why do we see the contrasting wetting and drying trends between the tropical and temperate basins? Why has the trend in the Congo basin changed since 2000?
• Just as a minor suggestion, Fig. A9 to 14 and Fig. A15 to 20 can be combined into a figure, respectively, to avoid too many figures.