the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Oct 2025
The influence of small farm reservoir network characteristics on their cumulative hydrological impacts
Abstract. In many regions of the world, the use of infrastructure to store runoff and stream water, such as small farm reservoirs, is the only way to enable irrigation and thereby secure and increase food production. The presence of multiple reservoirs in one catchment has cumulative impacts that are not necessarily the sum of the individual impacts. However, we still have little knowledge of the spatial factors that drive these cumulative impacts. In this work, the effects of the distribution of small reservoirs in a catchment on their hydrological impacts are investigated with a modeling approach. Our numerical experiment consists of randomly generating multiple small reservoir networks in the same catchment with realistic reservoir numbers, capacities, and spatial distributions and then comparing their hydrological impacts over a 20-year period. We focused on two variables, namely, the outlet discharge and the mean proportion of the network in low flow, which we computed annually and seasonally. We used the distributed agrohydrological model MHYDAS-small-reservoir, which represents small reservoirs and their links with the hydrological network and the irrigated plots. In our context and with current reservoir management rules, we found that the impacts of reservoirs are more important in summer, with discharges reduced by more than 20 % and up to 60 % compared with the reference situation without reservoirs. Moreover, low flow proportions are always higher than those in the reference situation. For these two indicators, the main explanatory factors are the number and distribution of reservoirs, with a limited effect of the storage capacity. The effects of the study factors on the seasonal and annual indicators were thoroughly interpreted with respect to the hydrological functioning of the catchment and the timing and amount of irrigation. This work contributes to a better understanding of the drivers of the cumulative hydrological impacts of small reservoirs. Although many questions remain, our results can help scientists and water managers choose the best representation of small reservoirs in their models to address their needs.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-4737', Anonymous Referee #1, 21 Nov 2025
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AC1: 'Reply on RC1', Henri Lechevallier, 08 Dec 2025
Dear Referee,
Thank you for the thorough review of our manuscript and your valuable feedback, which will undoubtedly help improve the clarity of our paper.
Please find attached our answers to each of your comments. Your comments are written in black, our replies in blue, and our proposed edits to the manuscript in red.
Best regards,
Henri Lechevallier, on behalf of co-authors.
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AC1: 'Reply on RC1', Henri Lechevallier, 08 Dec 2025
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RC2: 'Comment on egusphere-2025-4737', Anonymous Referee #2, 25 Nov 2025
The article presents an analysis on impacts of the variability of small reservoirs networks configuration in terms of their number, storage capacity and distribution in space. As the authors state, this type of investigation is a new addition to the study of small reservoirs impacts on the water cycle. The article is well written, and the authors described with clarity and detail the configuration tested and the outcomes they achieved. Below I’ve listed some comments to the authors, mainly clarification of some parts, then some edits to the text.
I suggest the article to be published after minor revision addressing the comments below.
Comments/revisions
Line 94: only agricultural and natural surfaces are referenced, is the model not able to represent urban surfaces, or was this a deliberate choice? This could be addressed in the text
Line 109-110: I think a small justification for not considering percolation from the bed and walls could be provided here. The authors for example state in another part that the soil is mostly impermeable, this could be an explanation but it is not explicit
Line 117: is this decision backed by some prior knowledge e.g. based on small reservoirs functionality?
Line 179-180: the authors could provide more details explaining what are the "specific locations" of the reservoirs
Line 180: at Line 96, the hydrological network is defined as the RSs. In Figure 1, REs are not all located on RSs. This is in conflict with the sentence at line 180-1.
Line 221: the authors could state here at what time steps the simulations are performed
Line 298: the authors introduce here "irrigation return flows", which are later addressed in a dedicated section, but it is not clear to me what these flows are or why they happen, are they a component of the model? How and why does it return to the hydrological network? The reservoirs are emptied at the end of the irrigation season? Is this happening in reality?
Line 391-392: withdrawals are based on the decision model. Can observation (i) be proposed as an "absolute" observation as it is proposed here?
Edits
Line 13: here, “low flow proportions” is not too clear, I would change to “the proportion of low flow”
Line 14: would change to "For the two indicators", since they are presented a few sentences before
Line 43-44: reference to Colombo et al., 2024 can be done as a more recent example of small reservoirs cumulative impacts study through modeling (https://doi.org/10.1016/j.jhydrol.2024.130640)
Line 78: streamflow
Line 276: i would suggest to put (no reservoirs) after "reference situation"
Line 418: I would remove "figure not shown"
Table 2 caption: where the upward and downward arrows are indicated I would add text stating something like "increase" and "decrease", as it could be misunderstood as "positive" and "negative"
Citation: https://doi.org/10.5194/egusphere-2025-4737-RC2 -
AC2: 'Reply on RC2', Henri Lechevallier, 08 Dec 2025
Dear Referee,
Thank you for the thorough review of our manuscript and your many suggestions.
Please find attached our answers to each of your comments. Our replies are written in blue, and our proposed edits to the manuscript in red.
Best regards,
Henri Lechevallier, on behalf of co-authors.
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AC2: 'Reply on RC2', Henri Lechevallier, 08 Dec 2025
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RC3: 'Comment on egusphere-2025-4737', Anonymous Referee #3, 30 Nov 2025
This manuscript examines the impact of small dams on the flow regime in a small catchment in south western France. The aim of the study appears to be to systematically assess how the spatial arrangements and physical characteristics of dams affect their hydrological impact.
The study uses a series of randomly generated arrangements of dams placed on the stream network with randomly generated physical characteristics. While many studies tend to focus on annual flows, this study assesses additional flow metrics including seasonal flows and the proportion of the stream network experiencing low flows. Notably, it models the effect of irrigation return flows which is novel. This appears to have a significant effect on results.
The results highlight some interesting relationships between the hydrological impacts and the number, volume, and arrangement of dams. However, there are a small number of assumptions which limit the applicability, including the use of water for irrigation with return flows, passing environmental flows, and limiting withdrawals to the top 75% of dam capacity. Modifying these parameters (perhaps in future studies) could provide results which are relevant to many other contexts and jurisdictions.
The article is well written, easy to understand, and clearly structured. The discussion and results are relevant for the fields of water resources management and hydrology. I am keen to see this paper published, however as noted by other reviewers, it could be improved with some minor revisions.
Specific comments:
L109 – Evaporation from the surface of each dam is assumed to be equal to 60% of ET. This may be appropriate, but it would be good to see some justification for this. Other evaporation variables may be more suitable for this purpose, such as Morton’s shallow lake evaporation (refer to doi 10.5194/hess-17-1331-2013).
L114 – In the results section, many references are made to return flows from irrigation. This feature of the model and assumptions around it are not described anywhere in the method (eg. percent of flow returned, which reach does irrigation water return to). I would expect to find this information in Section 2.1.3. I realise this information may be present in one of the references given, but it appears to be important for this study so it needs to be provided here.
L117 – As others have noted, assuming no withdrawals if the dams is below 25% capacity seems arbitrary. Is this a regulated limit? Or standard operating procedure? In most parts of the world, farmers will extract water from their dams until it is just a small pool of mud. A 10 word explanation is fine.
L155 – I do not quite understand the scenarios. In particular, the phrase ‘each combination of modalities is repeated 5 times’ is not clear. Exactly what is repeated 5 times? And what is different about each of these 5 scenarios?
Table 2 – Typically, I would expect dams to have effects throughout summer and into autumn while they are filling. This extends the summer low flow season further into autumn. Some results here seem to show slightly different autumn results where irrigation return flows are significant. An interesting follow up study could be to repeat the study with water withdrawals for stock or domestic use with no irrigation returns, or without the 10% environmental flow requirement which is not required in many other jurisdictions. The results may appear quite different. I recommend the paper briefly recognises that these results are dependant on some assumptions which may not always be applicable in other locations.
Citation: https://doi.org/10.5194/egusphere-2025-4737-RC3 -
AC3: 'Reply on RC3', Henri Lechevallier, 08 Dec 2025
Dear Referee,
Thank you for reviewing our manuscript and your constructive comments. Many of your remarks were also raised by the other referees, which confirms the need to clarify these points in the revised version.
Please find attached our answers to each of your comments. Our replies are written in blue, and our proposed edits to the manuscript in red.
Best regards,
Henri Lechevallier, on behalf of co-authors.
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AC3: 'Reply on RC3', Henri Lechevallier, 08 Dec 2025
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- 1
This manuscript describes the cumulative impact of small reservoirs on the hydrology of a small catchment area in south-western France. These reservoirs are generated randomly and are connected to the river network. Their characteristics include their water storage capacity, their spatial distribution across the catchment area and their number. The metrics used to assess the impact of these different components are annual flows, summer flows, and the proportion of the hydrographic network at low flow each year. The study is based on 20-year numerical simulations. It shows that the impact of reservoirs is cumulative along the river network and assesses the effects of the distribution, number and capacity of reservoirs on flows, and studies the associated processes. This work is very interesting and also sensitive because it attempts to provide answers to societal questions, in particular the storage of water for irrigation in dedicated reservoirs.
I find this article very well structured and well written. The conclusions are based on objective evidence and clear figures, and the limitations of the study are well presented in the discussion. I recommend publication of this article after minor revisions.
Comments:
- L109: evaporation from the reservoir is considered to be 60% of the reference evaporation, which I assume corresponds to potential evapotranspiration. How valid is this approximation and what is the associated uncertainty? It is important to give an order of magnitude for annual evaporation, as well as annual withdrawals, as these contribute to direct mass loss.
- L117: withdrawals are possible if the volume of water is greater than 1/4 of the reservoir's capacity: what average water depth does this correspond to, given that the shape is an inverted pyramid? Is this compatible with the characteristics of the withdrawal pumps?
- L114: on the map of France in Figure 1, there are white pixels in the Rhone valley that should not be there. What do they correspond to?
- L127: average annual rainfall is 675 mm: how was this calculated? Using SAFRAN data or measurements from rain gauges located in the catchment area?
- L128: The total volume capacity of the reservoirs in the basin is estimated at 205,000 m³. Is this estimate based on the pyramidal shape of the reservoirs or does it come from data describing the various structures?
- L140: What do you mean by “better match expectations”?
- L147: An example of one or two reservoir distributions would be helpful. For example, showing one distribution of the 7, 14 or 21 downstream reservoirs with the two capacities (two colours) as in Figure 1 and the main irrigable crop would help to clarify the ideas.
- L180: what is the proportion of reservoirs not connected to the hydrographic network compared to those that are connected? They also contribute to water storage for irrigation.
- L225: a warm-up period of 5 years is considered: does this mean that the simulated data used start in September 2000? Or is the warm-up from 1990 to 1995? Please clarify.
- L233: references to Vidal et al. (2010) and Le Moigne et al. (2020) can be added for SAFRAN. Can you clarify if the reference evapotranspiration is computed from SAFRAN data and at what frequency? Same question for min and max temperatures, do they come from the SAFRAN reanalysis?
- L236: You state that meteorological data variability is taken into account: I would temper this statement, as the climate in such a small area, covered by four contiguous points, probably does not vary greatly, at least you have not demonstrated that it does. I suggest removing this idea of variability. However, it is interesting to note that Gélon covers only four cells of the SAFRAN grid. Further on, you only take one cell (8558) into account in your comparison. You could at least show that the annual precipitation for these four grid points is very similar, which would allow you to take only one into account.
- L252: Figure 2, Medians are extending before 2001 and after 2019: do they use data covering 2000-2020? If not lines must be croped to adjust to 2001-2019.
- L300: the sentence 'highlights the role of weather' is not very adapted and accurate: please rephrase
- L321: (i) and (v) are the same, (v) is the autumn proportion of the framework in low flow
- L327: Fig 5 (a) and (d) respectively
- L329: 'the boxes are more separated' is not very adapted, perhaps change it into 'each year, the departure to the median is systematically more pronounced in (a) as compared to (d)'
- L403: equation should be y=-(3/4)x ; in Figure 7 equations should contain brakets y=-(a/b)x
Edits:
- L222: parameterization
- L166: reservoir
- L176: 1.06 km⁻²
- L250: reference simulation