the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jun 2024
Assessing the cumulative impact of on-farm reservoirs on modeled surface hydrology
Abstract. On-farm reservoirs (OFRs) are essential water bodies to meet global irrigation needs. Farmers use OFRs to store water from precipitation and runoff during the rainy season to irrigate their crops during the dry season. Despite their importance to crop irrigation, OFRs can have a cumulative impact on surface hydrology by decreasing flow and peak flow. Nonetheless, there is limited knowledge on the spatial and temporal variability of the OFRs' impacts. Therefore, to gain novel understanding on the cumulative impact of OFRs on surface hydrology, here we propose a novel framework that integrates a top-down data driven remote sensing-based algorithm with physically-based models by leveraging the latest developments in the Soil Water Assessment Tool+ (SWAT+). We assessed the impact of OFRs in a watershed located in eastern Arkansas, the third most irrigated state in the USA. Our results show that the presence of OFRs in the watershed decreased annual flow on average between 14 and 24 %, and the mean reduction in peak flow varied between 43 and 60 %. In addition, the cumulative impact of the OFRs was not equally distributed across the watershed, and it varied according to the OFR spatial distribution, and their storage capacity. The results of this study and the proposed framework can support water agencies with information on the cumulative impact of OFRs, aiming to support surface water resources management. This is relevant as the number of OFRs is expected to increase globally as an adaptation to climate change under severe drought conditions.
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RC1: 'Comment on hess-2024-148', Anonymous Referee #1, 24 Jul 2024
General comments
The article deals on an important issue, the impact of on-farm reservoirs on the hydrology.
The article propose an interesting application, with an innovation that consist in imposing surface and hence volume of the reservoirs in the model as derived from satellite data.
The application is in Arkansas, USA, with about 300 OFRs on a basin of 530km2, known for the importance of irrigation.
One issue with such reservoirs is the lack of data on their management.
From what I understood, the study tries to retrieve some parts of the management of the OFRs by imposing the extension of surface water of the OFRs which is very interesting
However, some elements of the methods are not clear, and I couldn’t understand how the model really works, and how the OFRs are managed.
My main comments is that the way OFRs are models and their management should be clarifiedDetails questions
Introduction : Peak flow is mentioned in the introduction and analysed in the study, but never defined… In the study, it seems to be maximal annual monthly flow, which is quite far to what can expect as peak flow, ie, flood… Can you define ?
Section 2.1: It is stated that there are with about 330 OFRs. The size of the basin, given later in the text is 7107 km2. Thus, the density of the OFRs is lower than 0.05 OFRs/km². This corresponds to a small density, especially considering large annual precipitation (1300mm/year) when compared to previous studies review by Habets et al., 2018 refered in the article. It is stated that 95 % of the OFR are smallest than 50ha, and Fig 4, it is shown than only about 10 aggragated OFRs are smaller than 10ha. According to the hypothesis used in the study of an average depth of 10m, 10ha corresponds to a capacity of 1 million cubic meter, which is quite large.
So is there only 330 OFRs because there are actually rather large OFRs. Or Is it possible that smaller OFRs are missing ?Line 274 « For each of the aggregated OFR, the water volume was calculated using SWAT+ default rule, which is a simple multiplication of the OFR surface area by a factor of 10 » : Do you mean that the maximum volume capacity of the OFRs is the maximum surface area multiplied by 10m ? Or do you mean that the water level within the OFR is constant and fixed to 10 ? This is unclear...
Line 282 : I find it weird to have details on how the river channel are divided in 4 classes, while, no details on the way the OFRs impacts the water balance are given…
You need to provide the water balance of the OFRs:- what are the inputs and ouptuts of the OFRs ?
- Is there water abstraction from the OFRs ? How much ? How is it computed ? What are the temporal variations ?
- Is there evaporation from the OFRs ?
- Does the water level vary ? Is the water level affect the outflow ?, due to the spillway
- The surface of the OFRs is changing. But is it the only way to have a change on the volume of the OFRs ?
- What’s happening if the surface of the OFRs is decreasing ? Does it lead to an outflow of water downstream ? Does the associated volume is expected to be abstracted for irrigation ? How this volume is estimated ?
- What happens when the surface area increase ? Does the inflow feels the reservoir ? Again how the corresponding volume is estimated ? What if the inflow is not large enough to fill the OFRs ?
I’d like to have a detail explanation of how it works.
Section 2.4 Scenario Analysis
line 297 ; "The daily surface area time series of each OFR was used to simulate three scenarios (i.e., lower, mean, and upper) representing the OFRs’ capacity in terms of surface area." Sorry, but, again, this part is not clear. Figure 2 is not very helpfull to understand what is done…
It is stated that the « daily OFRs’ surface area change between 2017 and 2020 » is derived,, and then that (line 300) « The mean scenario represents the regular condition of the OFRs, and it is the mean of the daily surface area time series derived from the Kalman filter , The lower and upper scenarios represent the lowest and highest capacities of the OFRs, and they are based on the surface area 95% confidence interval limits, calculated using the daily time series. »
So there is 4 value for each day of the year, how do you compute a 95 % confidence ?
Line 304 : » For each scenario, the OFRs were simulated at full capacity (i.e., maximum storage at the lower, mean and upper scenarios), and this capacity was kept constant during the simulation period »
⇒ this seems to be in contradiction with the previous sentence… Do you mean that the maximum capacity is set constant ? That the volume is set constant… ? Please, make it clearer...
Moreover, this part is the innovative part of the article. And only the distribution of the area of the aggregated OFRs is given Figure 4, and no details are given on the annual cycle of the OFRs … I strongly suggest that the daily evolution of the surface area of the OFRs be presented, for the 3 scenarios.
And again, please explain how the evolution of the surface impact the evolution of the volume….Does this surface change gives indication on the volume of water used for irrigation ? If yes, please, provide the estimated values…
Line 360 : The impact of the OFRs on monthly flow varied throughout the year…. Ok, but here, the reason of this impact is not clear : there is no information on the management of the OFRs, so, no idea of when they are filled, when the water is used/ abstracted, the condition for the water to spill out… So, again, please provide an description of the water balance and describe the hypotheses… the dynamic of the storage is clearly missing….
All this part is difficult to follow since key informations are missing…
Section 3.3 : impact on peak flow… You have to define what you consider as peak flow...
Section 3.4 Again, difficult to understand as the hypotheses on the functioning of the OFRs are not clear. You should present the evolution of the OFRs volume and surface in this section, as well as the evolution of the abstracted water….
Section 4 : Discussion
line 512 : « our validation and calibration was done using the flow measurements, and the OFRs surface area scenarios were based on an algorithm that was validated with an independent higher spatial resolution dataset (Perin et al., 2022). »
==> This sentence is not clear : OFRs surface area is an input data, not an output to be validated….
line 515 : « Furthermore, differently from previous research, our results showed that the OFRs may have a positive (< 9%) impact on flow (Fig. 5, classes 3 and 4) »
⇒ This is true only for some months…But you don’t explain why. This can occur only if OFRs release some water. But the management of the OFRs is not presented at all…line 527 : « This could be explained by the level of details in our analyses. »
⇒Well to achieve this, it is necessary to know how much of the water in the OFRs is used, directly in the OFRs, but also, if the OFRs releases water in the river, to have a clear idea of the pumping directly in the river.Moreover, it is not clear that this increase of riverflow with OFRs improves the model results, thus, correct a bias of the model without OFRs.
line 542 « For instance, during January and May the mean monthly percent change ranged between -35.8 ± 6% and -32.0 ± 7%, and during June and December it varied between -8.8 ± 5% and -5.4 ± 6% for the three surface area scenarios »
⇒ Do these values refer to the evolution of the surface of the OFRs ? Such info is needed earlier to understand the method and the resultsline 559 : « Our findings highlight that the impacts of the OFRs on flow and peak flow have a significant intra- and inter-annual variability (Figs. 5, 6, and 7) »
⇒ Fig 5 only present monthly flow. So, again, it depends on what you call peak flow...line 593 « Our results indicate that OFRs do not have an equally distributed impact on mean and peak flow across the watershed. Hence, assessing the OFRs location as well as their numbers across the watershed is important when aiming to manage the construction of new OFRs. »
⇒ I don’t understand how you can reach such statement… 1st, because no indication is given on the OFRs management, 2nd, the density of the OFRs varies in space….line 620 : « there is no bathymetrie » Does it means that the water level in the OFR is constant !
Reference
Yaeger et al : Trends in the construction of on-farm irrigation reservoirs in response to aquifer decline in eastern is not well referenced
Citation: https://doi.org/10.5194/hess-2024-148-RC1 - AC2: 'Reply on RC1', Vinicius Perin, 31 Oct 2024
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RC2: 'Comment on hess-2024-148', Anonymous Referee #2, 30 Sep 2024
Dear Authors,
The study innovatively assessed the spatial and temporal variability of the cumulative impact of OFRs at the watershed and subwatersheds levels, quantifying the annual impacts of the OFRs on flow and peak flow at the channel scale. Although the manuscript presents relevant novelty, there are some issues, which must be addressed or clarified by the Authors, prior to the study's publication, in my point of view. Please, see below my comments and suggestions.
Major comments:
- The influence of OFRs on surface hydrology and their mathematical representation could not be adequately evaluated, because there is no streamflow monitoring in the areas with the presence of the OFRs (see Fig. 1).
- Very simple considerations were assumed for the water volume and water releases of OFRs (see Lines 273-278). This is a critical issue of the study, which should be addressed by fieldwork or at least by a sensitivity analysis.
- Please, explain in detail how hydrologically the OFRs could positively impact streamflow, as you found.
- I did not fully understand what kind of results you would like to explore in section 3.5 Overall impact of OFRs. Please, improve this section.
- In the beginning of the manuscript, you mentioned that one of your focuses is the analysis of the interannual flow variability. However, I could not find such an analysis. An example of simulated time series (section 3.4) is not enough. For example, what are the impacts of OFRs on low-flow and high-low years?
Minor comments:
- Lines 385-386: “In general, the OFRs contributed to decreased monthly flow. However, the OFRs' impact on flow had a significant intra- and inter-annual variability…” Figure 5 is only on intra-annual variability or, also called, seasonal variability.
Citation: https://doi.org/10.5194/hess-2024-148-RC2 - AC1: 'Reply on RC2', Vinicius Perin, 31 Oct 2024
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