the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Apr 2026
Managed Aquifer Recharge in Confined Multi-Layer Aquifers: A Scalable Framework for Drought Resilience in Central Europe
Abstract. Managed Aquifer Recharge (MAR), particularly Aquifer Storage, Transfer and Recovery (ASTR), can enhance groundwater resilience in confined multi-layer aquifers under drought stress. We develop an integrated and scalable framework to assess ASTR feasibility by combining (i) meteorological and groundwater drought analysis using the Standardized Precipitation Evapotranspiration Index (SPEI-12) and Standardized Groundwater Index (SGI), (ii) GIS-based multi-criteria decision analysis (MCDA) for recharge site suitability, and (iii) dynamic assessment of surface-water availability using ecological flow thresholds. Applied to the water-stressed Berlin-Brandenburg region, one of Germany’s driest areas, where water supply relies heavily on induced bank filtration and faces emerging deficits. Results show that groundwater levels closely follow climatic conditions, indicating that climate-based drought indices can guide timely ASTR operations. The MCDA identified 62.5 % of the area (2,154 km²) as viable for ASTR. Flow-threshold analysis at 27 gauges showed that high-potential downstream sites could provide mean annual recharge volumes of 1.6–4.3 Mm³, offsetting 6–79 % of local extractions. At the catchment scale, total mean annual available recharge is 18.2–23.0 Mm³. Literature-based cost estimates (€0.37–0.51 m-³) are substantially lower than regional drinking-water production costs (€1.80 m-³), suggesting potential annual savings of €23–41 million.
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Status: open (until 18 Jun 2026)
- RC1: 'Comment on egusphere-2026-1378', Anonymous Referee #1, 15 May 2026 reply
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RC2: 'Comment on egusphere-2026-1378', Anonymous Referee #2, 08 Jun 2026
reply
Review of manuscript : Abdelrahman et al., submitted to EGUsphere
Abdelrahman Ahmed Ali Abdelrahman, Hagen Koch, Mobarok Hossain, Ronjon Heim, Clara Hauke & Irina Engelhardt, Managed Aquifer Recharge in Confined Multi-Layer Aquifers: A Scalable Framework for Drought Resilience in Central Europe
General:
The authors present a conceptual, comprehensive and quantitative approach for the identification of aquifer sections most suitable for MAR – measures to combat groundwater resources shortages. The approach presented not only considers the actual suitability of aquifers based on the hydraulic characteristics but also the dynamics of boundary conditions, such as the temporal variability of predicted drought patterns and the availability of infiltration water from surface water bodies by considering hydroecological criteria. The authors build their research on a wealth of data on a number of subcatchments in the Lower Spree area, close to Berlin. This allows the consideration of the large variability of the complex structure of a terrain, formed by glacial action. This approach allows, in my opinion, a generalization of the approach because of the number of catchments considered in the study. Furthermore, the authors present cost estimates as decision making criterion for water resources managers.
In sum: the approach is new, quantitative and also considers the temporal dynamics of water resources as well as that of the availability of recharge water and I recommend publication of the manuscript with minor revision, providing the comments below are being taken care of.
There is one main aspect that requires some more attention by the authors: they claim that their technique applies to confined aquifer systems such as Aquifer 2. In Chapter 3.4 they present the different criteria applied for the ASTR suitability assessment (L239 onwards). One criteria is storativity, a critical property within the context of MAR, especially if the aquifer system is confined, implying feasibility or efficiency of MAR measures. I do recommend that the areas with the aquifer 2 being confined to be outlined in Figure 6f and figure 10.
Should artificial recharge be accomplished via the unconfined Aquifer 1 and aquifer 2 be recharged via hydraulic windows, then, different criteria need to be selected on top. In order to clarify the concept for the reader, I recommend to the authors that they present a schematic diagram for different ASTR settings to demonstrate the feasibility of substantial volumes of storage, as well as the expansion of the suitability criteria.
In order to make the results better “digestible” and in order to provide maximum use of the study for the reader, I recommend the authors to provide a table, listing all 24 catchments with individual columns specifying catchment characteristics (material properties, groundwater regime (confined/unconfined), ASTR suitability and in the final column the reason why it was selected suitable /unsuitable). This analysis should then be picked up, grouped into catchment classes and conclusions discussed after L 508.
Specific comments:
In the interest of submitting the review timely and since revision is recommended, just a few comments:
L227: RCP8.5 is highly unlikely; needs a statement, why it is considered here.
L243: Hydraulic conductivity – Kvertical?
- Is there any weighting considered between the different criteria
L239 or L268: A statement is required comparing the method presented here with others already published..
L347: What time intervals are used for the computation? Year/month/day?
L231: Schematic Diagram of ASTR – Process needs to be included
Citation: https://doi.org/10.5194/egusphere-2026-1378-RC2 -
RC3: 'Comment on egusphere-2026-1378', Anonymous Referee #3, 08 Jun 2026
reply
1- The manuscript presents an integrated framework combining drought indices, MCDA, and environmental flow analysis. While the integration itself is useful, it remains unclear which component constitutes the principal scientific innovation relative to existing MAR suitability assessment frameworks.
2-The suitability assessment represents one of the core outputs of the study, yet no uncertainty analysis appears to be conducted.
3-The manuscript should better explain how key hydrogeological controls on ASTR performance are represented. Factors such as aquifer heterogeneity, vertical hydraulic conductivity, leakage through confining units, storage properties, groundwater age, and geochemical compatibility can significantly influence recharge efficiency and recovery rates. The current framework appears to evaluate site suitability largely from a screening perspective and may therefore overestimate practical feasibility. The authors should discuss these limitations more explicitly.
4-The manuscript would benefit from independent validation of the suitability results. For example, the identified high-potential zones could be compared with existing MAR facilities, groundwater management schemes, hydrogeological investigations, or known recharge-favorable areas. Without external validation, it remains difficult to evaluate whether the suitability map reflects actual field conditions or methodological artefacts.
5-ASTR systems are generally most valuable during prolonged drought periods, which are precisely the periods when surface-water availability becomes most constrained. Consequently, mean annual recharge volumes may not accurately reflect operational feasibility. The authors should evaluate recharge availability during critical low-flow years and discuss how climate variability and drought persistence affect the reliability of source-water supplies.
6-The framework identifies potentially available recharge volumes but does not evaluate their hydraulic consequences within the receiving aquifer system. Groundwater flow modelling could provide valuable information regarding water-level recovery, storage efficiency, recovery ratios, residence times, and potential impacts on neighbouring users. The absence of such analysis limits the ability to translate suitability results into operational recommendations.
7-The economic evaluation appears to rely on generalized literature-derived unit costs. However, ASTR economics are strongly site-specific and influenced by infrastructure requirements, treatment processes, monitoring obligations, energy costs, regulatory constraints, and long-term maintenance expenditures. The authors should provide a more detailed discussion of the uncertainties associated with the estimated savings and assess the sensitivity of the results to variations in cost assumptions.
8-Since the proposed framework is intended to enhance drought resilience, its application under future climate conditions should be explored. Incorporating climate projections or at least discussing the potential impacts of changing precipitation patterns, evapotranspiration rates, and streamflow regimes would considerably strengthen the long-term relevance of the study.
9-The future water-demand and climate assessment is based on RCP 2.6, RCP 4.5, and RCP 8.5 scenarios. However, the manuscript does not justify the use of the RCP framework instead of the more recent SSP-RCP scenario framework adopted in CMIP6 and the IPCC AR6. Given that socioeconomic development constitutes a central component of future water demand, the use of standalone RCPs may not adequately represent the interactions between climate forcing and socioeconomic pathways. The authors should clarify:
(i) whether the climate projections originate from CMIP5 or CMIP6 datasets;
(ii) why SSP-based scenarios (e.g., SSP1-2.6, SSP2-4.5, SSP5-8.5) were not considered;
(iii) how socioeconomic assumptions were incorporated into the demand projections; and
(iv) whether the projected climate variables were bias-corrected prior to their use in the hydrological assessment.
10- Since hydrological simulations are highly sensitive to biases in precipitation and temperature, the absence of information regarding bias correction raises concerns about the reliability of the projected water-balance estimates. A detailed description of the downscaling and bias-correction procedures is therefore necessary.
Citation: https://doi.org/10.5194/egusphere-2026-1378-RC3
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- 1
This is an interesting study addressing a real gap in MAR research for confined aquifers in Central Europe. The integration of three methodological components into a single transferable framework is a good contribution. The manuscript is generally well-written and clearly structured. However, several issues require attention before publication.
Major concerns:
1) The authors report a high pearson correlation between SPEI-12 and SGI with "no lag". This is an interesting finding for a confined multi-layer aquifer system, where pressure propagation rather than direct recharge drives head changes. The manuscript does not sufficiently explain the hydrogeological mechanism behind this near-instantaneous coupling. In confined systems, piezometric responses can be rapid, but this is a different mechanism than moisture storage response observed in unconfined systems. This distinction must be clearly discussed and reconciled with the literature.
It would be more correct, rather to state that the absence of lag between SPEI 12 and SGI "indicates that groundwater levels in this confined aquifer respond almost immediately to climatic anomalies" (Discussion section, lines 487-488) to state that the time scale of the response process of groundwater levels to climatic anomalies is not detectable by the aggregated annual scale of SPEI 12 compared with SGI.
Due to the importance of this statement in discussion and conclusion section, to avoid misinterpreting of results and findings, a montly based analisys using SPEI 1 would be recommended to eventualy detect and quantify lags or strongly confirm the "immediate response of GW to climate variability" statements of the whole manusccript.
2) Section 3.2, Line 200-206: Application of SGI to this context should be explained properly and with more details. Some parts of this paragraph are unclear, for example you state that "a common preprocessing step is to remove non-stationarity" but it's not clear if you proceded this way or not.
3) Globally, a flowchart image of the methodology would enrich methodological section.
4) In section 3.4, regarding the MCDA procedure for selecting site suitabilities for ASTR and MAR, you selected six site-selection criteria and classify their suitability scores. MCDA applications in groundwater reletad Issues, such as groundwater potential recharge evaluation and mapping, are very common in scientific literature of recent Years; often exploring and selecting different ranges of criteria due to case specific needs or constrains. In your case, criteria selection seems to be arbitrary or author-knowledge based, as well as the attributed scores in table 1 and their range values, which can be acceptable only if properly justifyied for the characteristics of your case study and strongly linked to scientific literature and to the purpose of your sudy. In any case, you choises have to be properly framed in the context of contemporary scientific literature on the topic. From both aspects, this methodological section of the manuscript is weak and needs improvements.
5) Moreover, regarding MCDA procedure, you apply equal weighting to all six MCDA criteria. It is well-known that weighting choices strongly influence MCDA outcomes in MAR suitability studies. The manuscript acknowledges this only in passing ("based on literature and expert judgment"). A sensitivity analysis testing alternative weighting schemes is strongly recommended.
6) Section 3.6 on Cost Benchmarking from Literature, the cost-analysis is just mentioned as evaluated "from a review of recent european MAR literature". An economic cost analysis would be very interesting and enrich your work, but it hasn't been investigated further since you state it's beyond the scope of your study. At least, you should properly report a summary table of specific informations (if available) for your case study from metioned literature in this paragraph.
It's important to underline that the economic analysis in your study uses a single literature-derived cost range of €0.37–0.51/m³. This range is derived from Ross and Hasnain (2018) and Sprenger et al. (2017), which are based on heterogeneous international datasets. You acknowledge that site-specific analysis was beyond scope, but the manuscript presents potential savings of €23–111 million/year with a level of confidence that is not warranted given the uncertainty in both cost and volume estimates. At minimum, a clear uncertainty paragraph should be introduced, and potential savings claims in the manuscript and in the abstract should be scaled back and framed explicitly only as order-of-magnitude estimates.
7) You note that the study covers only 33% of the total surface-water network. While this limitation is acknowledged in the Discussion, the scale of this gap is substantial and is used to argue that actual regional MAR potential is likely substantially higher. This extrapolation is speculative without further analysis since the unmonitored 67% could include tributaries with lower flow regimes, higher ecological sensitivity, or no overlap with suitable ASTR zones. This claim should be removed or framed as future research direction to investigate and verify.
Following, other minor concerns:
8) Approach 2 relies on hydroecological thresholds that are described generically. The manuscript does not clarify whether these thresholds were derived from local ecological assessments or transferred from literature.
9)The abstract states results clearly but uses the phrase "offsetting 6–79% of local extractions" without specifying which sites this applies to. Readers may misinterpret this as a catchment-wide statement. Clarify that this refers to specific downstream sites.
10) The study focuses on ASTR (Aquifer Storage, Transfer and Recovery), but the framework description and some results sections frequently use ASR and ASTR interchangeably. Since ASTR involves an additional spatial separation between injection and recovery wells compared to ASR, this distinction carries operational significance and should be used consistently throughout.