the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2026
Antecedent groundwater depletion amplifies hydrological drought severity: aquifer-river connectivity and cascading recovery in the 2022 Po Plain extreme event
Abstract. The 2022 Po River drought produced a 78 % reduction in annual discharge at Pontelagoscuro despite a precipitation deficit of only 20–25 %, a discrepancy not attributable to meteorological anomalies alone. Using GRACE-FO groundwater storage anomalies (GWSA), ERA5-Land soil moisture fields, and a 25-year daily discharge record, we show that antecedent aquifer depletion altered aquife-river connectivity, amplifying the hydrological response beyond what meteorological forcing alone would predict.
GWSA had reached −149 mm by August 2021, more than a year before the drought peak. During the 2022 irrigation season, groundwater abstraction substantially exceeded recharge once Alpine-sourced surface water became unavailable, producing the largest single-season storage loss in the 22-year satellite record (−246 mm, Z = −2.32). The recession coefficient k increased by 90.7 % between 2021 and 2022, exceeding the storage-dependent trend by 90.5 % and consistent with a threshold transition in connectivity near GWSA = −150 mm, a level corresponding to roughly the lowest quintile of the 2002–2024 August GWSA record. Direct runoff collapsed by 72 %, leaving baseflow as the dominant hydrograph component.
Recovery proceeded in three tiers: meteorological indices within 8 months, river discharge within 12 months, and soil moisture within 14 months; GWSA had not returned to historically normal levels within the observational record, leaving a period of persistent hydrological drought of at least 7 months undetected by standard precipitation-based indices. Antecedent groundwater depletion thus alters river discharge sensitivity to meteorological forcing, with recovery in groundwater-dependent irrigated basins extending well beyond the meteorological signal. We propose a diagnostic framework based on GWSA, k, and BFI as an early-warning tool for data-sparse irrigated basins.
- Preprint
(1999 KB) - Metadata XML
-
Supplement
(724 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2026-4113', Matthew Currell, 04 Sep 2026
-
AC1: 'Reply on RC1', shuhan yang, 10 Oct 2026
Response：We sincerely thank Reviewer 1 for the positive assessment of our manuscript and for the constructive comments, which have helped us strengthen the evidence supporting our main findings. In response, we have updated the hydrological records through September 2026 and re-examined the daily streamflow and baseflow series using the existing hydrograph-separation outputs. We have conducted additional analyses of historical low-flow thresholds, low-flow duration, baseflow recession behaviour, and the timing of changes in the 2022 hydrograph. We have also revisited the comparison between groundwater-storage anomalies and streamflow responses, reassessed the evidence supporting the proposed groundwater-storage threshold, and conducted a preliminary comparison between the 2022 drought and the available 2026 hydrological records. These analyses have helped us clarify which findings are directly supported by the observations and where interpretations require greater caution.
In the further revision, we will continue to strengthen the manuscript by refining the discussion of baseflow recession and groundwater–river connectivity in light of relevant literature, clarifying the limitations of satellite-derived groundwater-storage estimates and the applicability of the proposed diagnostic framework, and incorporating the additional analyses and updated figures where appropriate. We will also ensure that the conclusions distinguish clearly between observed hydrological patterns and inferred mechanisms, particularly regarding the proposed threshold behaviour and its potential transferability to other basins. We believe these revisions will improve the robustness, transparency, and broader relevance of the study.
Comment 1: Line 29: Typo – correct to aquifer.
Response 1: We thank the reviewer for pointing this out. The typo has been corrected to “aquifer”.
Comment 2: Line 81: Wada et al 2010 reference is missing from references list. Famiglietti et al 2014 did not specifically look at the Mediterranean region. You might re-phrase this to indicate that groundwater depletion is a growing global issue, but that there has been limited study (using GRACE or other tools) in the Mediterranean region.
Response 2: We thank the reviewer for this helpful comment. We have added the missing reference to Wada et al. (2010) and revised the statement regarding the Mediterranean region. We now frame groundwater depletion as a growing global concern, while noting that its spatial characteristics and impacts remain less well documented in the Mediterranean region, particularly using large-scale satellite-based observations.
Comment 3: Line 93: This last point may be controversial – does GRACE really remove the need for in-situ observations? I think there is a strong case to say the two must be used in conjunction wherever possible.
Response 3: We thank the reviewer for raising this important point. We agree that GRACE observations cannot replace in-situ groundwater observations. GRACE provides valuable spatially integrated information on large-scale terrestrial water-storage variations, but its coarse spatial resolution and the indirect nature of groundwater-storage estimation make in-situ observations important for local validation and interpretation. We have therefore revised the statement to emphasize that GRACE and in-situ observations are complementary rather than interchangeable..
Comment 4: Line 113-114: Is this meant to read 2 to 10? Seems like a small typo.
Response 4: We thank the reviewer for pointing out this typo. The value has been corrected to “2 to 10”.
Comment 5: Line 117: If characteristic of the groundwater, it would be good to include one or more hydrographs showing groundwater levels through time (either individual wells, or an aggregated average level). The same applies for the surface water gauging site – would be great to present a typical hydrograph early in the paper for background.
Response 5: We thank the reviewer for this helpful suggestion. We agree that representative hydrographs would provide useful background for interpreting the groundwater–river interaction. For the surface-water component, we will add a representative discharge hydrograph for the Pontelagoscuro gauging station to illustrate the river-flow regime and the pronounced low-flow conditions during the 2022 drought. For groundwater, we agree that a representative groundwater-level hydrograph would be valuable. We are currently making further efforts to identify suitable long-term groundwater-level observations or monitoring-well datasets for the Po Plain that could be used to complement the GRACE-based groundwater-storage analysis. If a sufficiently long and spatially representative dataset can be identified, we will incorporate it into the revised manuscript.
Changes in manuscript: We will add a representative discharge hydrograph and further evaluate the availability of suitable groundwater-level observations for comparison with the GRACE-based groundwater-storage signal.
Comment 6: Line 179: Were other possible recession filters applied and assessed for the level of fit? There are many available filters – some discussion of how you chose the Lyne & Hollick filter as being the most appropriate for the data would be helpful.
Response 6: We thank the reviewer for this important suggestion. We performed a sensitivity analysis using alternative baseflow separation approaches and parameters, including Lyne–Hollick with α values of 0.900, 0.925, 0.950, 0.975 and 0.990, as well as the Eckhardt, Chapman and Boughton–Chapman approaches. The sensitivity analysis indicates that the magnitude of the estimated baseflow and, particularly, the BFI are sensitive to the choice of separation method and parameterization. However, the pronounced reduction in absolute baseflow during the 2022 drought is consistently identified across the tested approaches. The results obtained using the Eckhardt method are also broadly consistent with those from the primary Lyne–Hollick configuration for 2022. We therefore retained the Lyne–Hollick filter with α = 0.925 as the primary method, while treating the BFI as a method-dependent diagnostic rather than a direct measure of the fraction of groundwater-derived streamflow. The sensitivity analysis has been added to demonstrate the robustness of the main conclusion regarding the 2022 baseflow suppression.
Changes in manuscript: The baseflow-separation methodology and its sensitivity to alternative filters and parameterizations have been clarified, and the sensitivity results have been added to the revised analysis.
Comment 7: Line 249: Clarify whether the low-flow deficits are relative to 2021 mean discharge, or another metric (e.g. baseflow during 2021).
Response 7: We thank the reviewer for pointing this out. The summer low-flow deficit of approximately 78% was calculated relative to the climatological mean discharge for the same summer period (June–August) over 2000-2021, rather than relative to the 2021 discharge or baseflow. Specifically, the mean June-August discharge was 233.7 m3 s-1 in 2022, compared with a 2000–2021 climatological mean of 1050.3 m3 s-1, corresponding to a deficit of approximately 78%. We have clarified this definition in the manuscript to avoid ambiguity.
Changes in manuscript: The reference period and calculation of the summer low-flow deficit have been explicitly stated in the revised manuscript..
Comment 8: Figure 3: Label size is far too small on the figure. Please enlarge the figure label and axis text by at least 50%. Same applies to Figure 4, including axis labels and all other text.
Response 8: We thank the reviewer for pointing this out. We have enlarged the figure labels, axis text and other annotations in Figures 3 and 4 to improve readability.
Changes in manuscript: Figures 3 and 4 have been reformatted with larger labels and axis text.
Comment 9: Line 350: It appears the mean baseflow is almost 100% of the mean streamflow (when comparing the mean baseflow values reported here, and the mean 2021 and 2022 total streamflow at line 248). This seems unlikely to be realistic, or at least requires some further discussion and evidence in support. This point underscores the need to carefully assess the most appropriate baseflow/hydrograph separation method and check the realism of the output. Adjusting either the filter method (exploring other options like Boughton, Chapman, Eckhardt), and/or the adopted coefficient(s), and checking whether this significantly affects the mean baseflow estimates, is an important step to ensure estimates are robust and the method appropriate. Is there literature supporting the Po River as being a dominantly baseflow hydrograph?
Response 9: We thank the reviewer for raising this important concern. We agree that the realism and robustness of the estimated baseflow should be carefully assessed, particularly given the relatively high baseflow index obtained for the Po River. In response, we updated the baseflow analysis in two ways. First, we extended the discharge record and repeated the Lyne–Hollick separation using different recession parameters (α = 0.900, 0.925, 0.950, 0.975 and 0.990). Second, we compared the results with alternative baseflow separation approaches, including the Eckhardt, Chapman and Boughton–Chapman methods. This sensitivity analysis allowed us to assess whether the main conclusions were dependent on the selected separation method and parameterization. The updated analysis shows that the absolute magnitude of estimated baseflow and the BFI are indeed sensitive to the separation method and parameter values. Using our primary Lyne–Hollick configuration (α = 0.925), the long-term mean BFI is 0.742. For 2021, the mean discharge and baseflow were 1108.6 and 901.5 m3 s-1, respectively (BFI = 0.813), whereas in 2022 they decreased to 535.3 and 438.7 m3 s-1, respectively (BFI = 0.820). Thus, although the relative BFI remained high in 2022, the absolute baseflow magnitude decreased by approximately 51% compared with 2021. Importantly, the pronounced reduction in absolute baseflow during the 2022 drought is also identified across the alternative separation methods and parameterizations. The sensitivity analysis therefore indicates that the absolute value of BFI is method-dependent, but the substantial suppression of absolute baseflow during the 2022 drought is a robust feature of the analysis. We have therefore revised our interpretation of the baseflow results. We no longer interpret the relatively high BFI as direct evidence that a specific proportion of the Po River discharge is groundwater-derived, nor do we use it alone to characterize groundwater dominance. Instead, we focus on the robust reduction in absolute baseflow magnitude during the drought and explicitly acknowledge the methodological uncertainty associated with hydrograph separation.
Changes in manuscript: The baseflow-separation methodology and its sensitivity to alternative filters and parameterizations have been described in greater detail. The interpretation of BFI has also been revised to distinguish the method-dependent relative contribution from the more robust result of absolute baseflow suppression during the 2022 drought.
Comment 10: Line 372 to 388: I’d like to see some more reference to literature on baseflow recession and streamflow hydrograph responses to drought here. You have made some observations and claims about a possible ‘threshold’ change in aquifer-river connection, reflected in the baseflow coefficients. It is important to support this with some relevant literature on the topic – e.g. where else has this been observed and what is the theoretical underpinning? The work of Dr Timothy Peterson and colleagues comes to mind (e.g., Peterson et al. Watersheds may not recover from drought, Science 372, 745-749).
Response 10: We thank the reviewer for this helpful suggestion. We will expand the Discussion to provide a clearer theoretical context for interpreting baseflow recession and streamflow responses to drought. Specifically, we will review relevant literature on the relationship between groundwater storage, baseflow recession behaviour, and hydrological drought, and discuss the physical processes that may explain changes in river flow under prolonged low-storage conditions. We will also examine relevant studies on post-drought streamflow recovery, including the work suggested by the reviewer (Peterson et al., 2021), to place the Po Plain case study in a broader hydrological context. This review will help us clarify which aspects of the observed response are consistent with established hydrological understanding and which remain specific to our study or require further investigation. In addition, we will revise the interpretation of the 2022 event in light of the updated recession analysis, distinguishing the pronounced reduction in absolute baseflow from conventional recession events. We will also clarify the uncertainty associated with the proposed low-storage transition and avoid presenting it as a definitive threshold unless supported by sufficient evidence.
Changes in manuscript: We will strengthen the theoretical background and literature review in the Discussion, incorporate relevant research on drought-related streamflow responses and post-drought recovery, and refine the interpretation of the 2022 baseflow response and the potential low-storage transition.
Comment 11: Line 416-417: Can you give some further supporting evidence about the ‘suppressed’ baseflow during this period? Are you basing this conclusion just on the elevated baseflow recession constant, or is there other evidence you can point to in the hydrograph - e.g. comparing flow percentiles during low-flow periods across different years?
Response 11: We thank the reviewer for this suggestion. We agree that the evidence for suppressed baseflow should not rely solely on recession characteristics. We therefore examined the low-flow behavior directly from the discharge record. During July 2022, the mean daily discharge was approximately 159 m³ s⁻¹, compared with approximately 829 m³ s⁻¹ for July in the preceding reference years, corresponding to only about 19% of the reference July mean. The updated baseflow analysis also shows a marked reduction in absolute baseflow during 2022, with mean baseflow decreasing from 901.48 m³ s⁻¹ in 2021 to 438.68 m³ s⁻¹ in 2022. Importantly, our revised analysis does not rely on treating 2022 as a conventional recession event. Instead, we interpret 2022 as a period of pronounced low-flow and baseflow suppression.
Changes in manuscript: We have added the low-flow comparison and revised the discussion to distinguish the 2022 baseflow suppression from conventional recession events..
Comment 12: Line 426: Your earlier text and Figure 8 suggest that the lag is longer than 7 months – please check for consistency throughout.
Response 12: We thank the reviewer for raising this important point. We have recalculated the lagged relationship between groundwater storage anomalies (GWSA) and baseflow using the updated, temporally matched datasets. The preliminary results show weak contemporaneous correlation, while negative correlations emerge at several subsequent lags. However, these statistical associations do not, on their own, establish a physically meaningful groundwater–river response time. We will further examine the sensitivity of the lagged relationship to seasonal variability and clarify the direction and definition of the lag used in the analysis. We will also reassess whether the available evidence supports a specific response timescale. If the inferred delay is not robust to these checks, we will revise the relevant interpretation accordingly rather than assign a definite hydrological response time based solely on lagged correlation.
Changes in manuscript: We will clarify the lag-analysis methodology, evaluate the influence of seasonal variability, and revise the interpretation of the inferred delay in accordance with the updated results.
Comment 13: Line 440: Is there strong evidence that the antecedent aquifer condition (deficit) was significantly greater in the lead-up to the 2022 drought than previous droughts? This is implied, but no strong evidence is cited here. The GWSA data are an obvious place to look.
Response 13: We thank the reviewer for this important observation. We agree that the claim of unusually severe antecedent groundwater depletion before the 2022 drought requires direct evidence from the groundwater-storage record. We will examine the updated GWSA time series to characterise groundwater-storage conditions in the months preceding the 2022 drought and compare them with those preceding previous drought periods. To ensure a meaningful comparison, we will use consistent reference periods and examine both the absolute storage anomalies and their evolution before the onset of each drought. We will then revise the relevant interpretation according to the results. If the updated data do not demonstrate that antecedent groundwater depletion before 2022 was significantly greater than that preceding previous droughts, we will avoid making this comparative claim and instead focus on the observed groundwater-storage decline during 2022 and the persistence of the subsequent storage deficit.
Changes in manuscript: We will add a comparison of antecedent GWSA conditions across selected drought periods and revise the corresponding statements in the Results and Discussion to ensure that the strength of the conclusions is consistent with the evidence.
Comment 14: Line 444-447: This should be testable, by examining hydrographs from previous years with similar precipitation deficit, but less of a negative GWSA. It is important to attempt this to ensure the conclusion is supported.
Response 14: We thank the reviewer for this helpful suggestion. We extended the analysis by comparing the 2022 event with historical years that had broadly comparable May–September precipitation totals but less negative antecedent groundwater storage anomalies (GWSA). The years 2005 and 2006 were selected as preliminary comparison years. Their May–September precipitation totals were 539.3 and 530.9 mm, respectively, compared with 494.5 mm in 2022. However, their January–April mean GWSA values were substantially higher than in 2022. We compared the corresponding hydrographs and quantified low-flow persistence using fixed thresholds derived from the May–September daily-flow distribution over 2000–2021. The comparison shows that low-flow conditions also occurred in 2005 and 2006, but the 2022 event exhibited a more persistent low-flow regime. In 2022, daily discharge remained below the historical Q10 threshold for 121 of the 153 days from May to September, with a longest continuous episode of 92 days. The corresponding values were 65 days and 21 days in 2005, and 74 days and 74 days in 2006. Baseflow-based diagnostics also indicate prolonged low-flow conditions in 2022. These comparisons provide additional evidence that the 2022 event was distinguished not only by low discharge but also by the persistence of low-flow conditions in the context of strongly depleted groundwater storage. We nevertheless acknowledge that similar seasonal precipitation totals do not ensure identical within-season rainfall patterns or antecedent hydrological conditions. We therefore interpret these results as supporting a hydrological association, rather than as establishing a causal effect of groundwater depletion independently of other drivers.
Changes in manuscript: We will add a historical comparison of the 2022 event with selected years (2005 and 2006) that had broadly comparable May–September precipitation totals but less negative antecedent GWSA. We will incorporate the comparison of hydrographs and fixed historical low-flow thresholds into the Results, and revise the Discussion to clarify the association between antecedent groundwater storage conditions and low-flow persistence while acknowledging the limitations of the historical comparison.
Comment 15: Line 478: The California Central Valley comes to mind here – please have a look at the relevant literature and data from this region to see if the same holds true.
Response 15: We thank the reviewer for this valuable suggestion. Previous studies in California’s Central Valley provide useful context for interpreting groundwater–streamflow relationships under drought conditions. Integrated hydrological modelling of the San Joaquin River basin has demonstrated that groundwater contributions to streamflow vary across temporal scales and exhibit hysteresis, highlighting the dynamic nature of groundwater–surface water interactions (Gilbert and Maxwell, 2017). Regional assessments further show that groundwater–stream exchange varies spatially, with groundwater depletion and pumping potentially reducing summer streamflow in some reaches (Faunt, 2009). Studies of the Central Valley also demonstrate that drought-related groundwater depletion and subsequent recovery depend on multiple factors, including climatic conditions, surface-water availability, and groundwater management (Alam et al., 2021; Liu et al., 2022). These findings provide a broader context for interpreting the threshold-like change in hydrological behaviour identified in the Po River Basin. However, the threshold identified here should be regarded as an empirical feature of the analysed storage and streamflow records, rather than a universal threshold for groundwater–river connectivity. Its physical interpretation and transferability to other basins require further investigation, particularly where hydrogeological conditions and human water management differ.
Changes in manuscript: We will expand the Discussion to incorporate relevant studies from California’s Central Valley on groundwater–surface water interactions, groundwater depletion, and post-drought recovery. These studies will be used to place our findings in a broader hydrological context, clarify the potential influence of hydrogeological heterogeneity and human water management, and distinguish the empirically identified threshold-like change from a directly verified physical transition in groundwater–river connectivity.
Comment 16: Line 496: Is there a particular point on the streamflow hydrograph where you can identify this change specifically, and show the reader?
Response 16: We thank the reviewer for this helpful suggestion. We re-examined the daily streamflow and estimated baseflow hydrographs for May–September 2022 and compared their temporal patterns using 7-, 14-, and 21-day moving averages. The analysis identifies a clear change in the streamflow trend around late June to early July, whereas the change in the smoothed baseflow trend occurs later, around 18–24 July for the 14- and 21-day windows. Daily streamflow and estimated baseflow both reach their minimum on 25 July 2022. However, the timing of the baseflow trend change depends on the smoothing window, and the monthly groundwater-storage observations do not allow us to identify an exact daily threshold-crossing date. We will therefore annotate the observed streamflow minimum and the relevant periods of changing hydrograph behaviour in the revised figure. We will distinguish these observable hydrological changes from the inferred threshold-like response discussed in the manuscript and avoid interpreting them as direct evidence of a precisely dated physical transition in groundwater–river connectivity.
Changes in manuscript: We will revise the relevant hydrograph figure and its caption to identify the late-June/early-July change in streamflow trend and the subsequent baseflow behaviour, while explicitly acknowledging the temporal resolution of the groundwater-storage data and the sensitivity of the inferred baseflow transition to the smoothing window.
Comment 17: Line 523-531: Indeed, these are important findings for river management.
Response 17: We thank the reviewer for this encouraging and constructive comment. We agree that these findings have important implications for river management, particularly for understanding how groundwater-storage deficits and their delayed recovery may affect streamflow and hydrological drought persistence. We will further emphasize the practical implications of these findings for drought monitoring and integrated surface-water and groundwater management in the revised Discussion.
Comment 18: Line 537: Is such monitoring sparse within the study area, specifically?
Response 18: We thank the reviewer for this important question. Our intention was to highlight the broader challenge of obtaining consistent, accessible, and spatially representative in-situ groundwater observations across different irrigated regions, rather than to claim that groundwater monitoring is necessarily sparse within the Po River Basin. Groundwater observations may be limited by uneven monitoring coverage, discontinuous records, and restricted data accessibility, which can complicate cross-regional assessments (Li et al., 2019). We acknowledge that the original wording did not clearly distinguish this broader data-availability challenge from the specific monitoring conditions within our study area. We will revise the text to clarify that the proposed framework does not require in-situ groundwater observations as direct inputs, while recognizing that the availability and quality of satellite-derived groundwater-storage estimates and discharge records remain essential. We will also qualify the potential applicability of the framework to other irrigated basins according to their data availability and observational limitations.
Changes in manuscript: We will revise Section 5.5 to describe the motivation in terms of the limited accessibility, continuity, and spatial representativeness of in-situ groundwater observations in many regions, rather than implying that groundwater monitoring is absent or poorly distributed throughout the Po River Basin. We will also clarify the data requirements and limitations of the proposed framework.
Comment 19: Section 5.5: This section does not really convince the reader that you have identified a specific threshold where baseflow recession behavior has qualitatively shifted to a new state. This would require some more in-depth analysis of the hydrograph to demonstrate the key change point (e.g. change in recession slope or shape that corresponds to a threshold GWSA). You have done this semi-quantitatively by means of looking at recession constants but the work is not very precise. This part doesn’t necessarily have to be part of the scope of the paper – your findings are significant enough regardless, but I think if you are going to make this claim, you need to substantiate it better.
Response 19: We thank the reviewer for this important suggestion. We agree that the evidence for a sharp groundwater-storage threshold should be evaluated more rigorously. We therefore performed a breakpoint analysis of the relationship between monthly GWSA and baseflow. The analysis identified a potential transition around approximately −165 mm GWSA. However, the improvement over a single linear relationship was modest, with a low overall explanatory power, and the evidence was not sufficiently strong to support a sharply defined threshold. We have therefore revised our interpretation. Rather than presenting −150 mm as a statistically established threshold, we now describe approximately −150 to −165 mm as a potential low-storage transition zone, within which the river–groundwater response may change. We explicitly acknowledge that this transition should not be interpreted as a universal or sharply defined threshold.
Changes in manuscript: The threshold-related statements have been substantially revised, and the breakpoint analysis and its limitations are now discussed..
Comment 20: Looking at recent (July-August 2026) SPEI data for the European alpine region, there has been a very significant meteorological drought this summer. Somewhere in your introduction or conclusion, I would suggest mentioning that your research has very important implications for subsequent drought periods, including the current severe drought being experienced in central Europe.
Response 20: We thank the reviewer for this valuable suggestion. We agree that the implications of our findings extend beyond the 2022 Po Plain drought, particularly in the context of the severe drought affecting central Europe and the Alpine region during summer 2026. In response, we extended our analysis of the available hydrological records through 19 September 2026 and conducted a preliminary comparison between the 2022 and 2026 conditions. The updated records show sustained low-flow and baseflow conditions during summer 2026. In addition, the available monthly groundwater-storage anomalies became progressively more negative from May to July 2026, reaching −191.6 mm in July. This indicates that the 2026 drought provides a relevant context in which to further examine the relationship between groundwater-storage deficits and streamflow behaviour. We emphasize, however, that the 2026 record is incomplete, with groundwater-storage anomalies currently available only through July. We therefore do not interpret these preliminary observations as evidence that the groundwater–river connectivity changes identified in 2022 have necessarily recurred. Instead, they illustrate the potential value of integrating groundwater-storage information with streamflow-based indicators when assessing ongoing droughts. We have revised the manuscript to highlight the relevance of our findings to subsequent drought events and to acknowledge the need for continued monitoring and further assessment as additional data become available.
Changes in manuscript:We have added a preliminary comparison based on the updated hydrological records through September 2026 and expanded the Conclusions to discuss the relevance of the proposed diagnostic perspective to ongoing and future drought assessment, while explicitly acknowledging the incomplete coverage of the 2026 groundwater-storage record.
Citation: https://doi.org/10.5194/egusphere-2026-4113-AC1
-
AC1: 'Reply on RC1', shuhan yang, 10 Oct 2026
-
RC2: 'Comment on egusphere-2026-4113', Anonymous Referee #2, 15 Sep 2026
The authors investigated the role of groundwater depletion in amplifying the 2022 Po plain drought event. They found that the aquifer depletion in 2021 amplified the drought event and groundwater storage remained low level after meteorological conditions had recovered. This study also indicates that meteorological drought indices may underestimate hydrological drought risk. The paper is well written and the structure is well organized. It is worthy of being published after minor revision. Some minor comments are shown as below:
1, I understand the paper was prepared several years ago and the groundwater level has not returned to its normal level at that time. But it is submitted two years later. Is it possible to update the data and give the time when the groundwater has recovered in Po plain?
2, In Figure2, there is an overlap between the catchment boundary and legend in the lower-left. Maybe the figure can be updated.
3, in section 3.5, SPEI-6 is the first time to appear, there should be an explanation about it and how it is calculated.
4, Line 219, delete ‘and’
5, about the baseline, in line 239, it is 2015-2021, but it becomes to be 2009-2009 in line 242. Why do you choose it as the baseline? Why the two baselines are different?
6, Line 376-380, it is too long and difficult to understand this sentence. Please rephrase it.
Citation: https://doi.org/10.5194/egusphere-2026-4113-RC2 -
AC2: 'Reply on RC2', shuhan yang, 10 Oct 2026
Response: We sincerely thank the reviewer for the positive assessment of our manuscript and for the careful and constructive comments. We appreciate the time and effort devoted to evaluating our work. The reviewer’s suggestions have helped us further improve the clarity, consistency, and presentation of the manuscript. In response, we have updated the groundwater storage anomaly record and refined the discussion of groundwater storage recovery, clarified the definition and calculation of SPEI-6, and addressed the inconsistencies in the reference periods used in different analyses. We have also revised the relevant figure layout and corrected typographical and textual issues to improve readability. We have carefully considered each comment and provide our point-by-point responses below. We believe that these revisions have strengthened the manuscript and improved the clarity of our methods and interpretations.
Comment 1: I understand the paper was prepared several years ago and the groundwater level has not returned to its normal level at that time. But it is submitted two years later. Is it possible to update the data and give the time when the groundwater has recovered in Po plain?
Response 1: We thank the reviewer for this important suggestion. In response, we have updated the groundwater storage anomaly (GWSA) record through July 2026 and extended the associated hydrological analysis using the available data. The updated record shows that groundwater storage remained substantially depleted during the most recent available observation period, with GWSA reaching −191.6 mm in July 2026. Based on the currently available data, we cannot identify a sustained recovery date at which groundwater storage returned to its historically normal seasonal range. We have therefore revised the manuscript to clarify the updated data coverage and to distinguish between short-term fluctuations in groundwater storage and sustained recovery to normal conditions. We have also clarified that the recovery assessment is based on groundwater storage anomalies derived from satellite data, rather than direct measurements of groundwater levels. Because the available GWSA record extends only through July 2026, the timing of any subsequent recovery cannot yet be determined. We have revised the relevant text and recovery discussion accordingly to avoid implying that groundwater storage remained continuously depleted throughout the entire extended period without assessing the intervening observations.
Changes in manuscript: We have updated the GWSA record through July 2026, revised the groundwater recovery discussion and clarified the observational limitation on determining the timing of sustained recovery.
Comment 2: In Figure2, there is an overlap between the catchment boundary and legend in the lower-left. Maybe the figure can be updated.
Response 2: We thank the reviewer for pointing this out. We have updated Figure 2 by adjusting the legend placement and figure layout to eliminate the overlap with the catchment boundary and improve the overall readability of the map.
Changes in manuscript: Figure 2 has been revised to improve the placement of the legend and the visibility of the catchment boundary.
Comment 3: in section 3.5, SPEI-6 is the first time to appear, there should be an explanation about it and how it is calculated.
Response 3: We thank the reviewer for this helpful suggestion. We have revised Section 3.5 to define the Standardized Precipitation–Evapotranspiration Index (SPEI) and explain the meaning of the 6-month timescale. We have also added a brief description of its calculation, including the use of accumulated climatic water balance (precipitation minus potential evapotranspiration) over a six-month period and its standardization against the reference climatology.
Changes in manuscript: The definition and calculation of SPEI-6 have been added at its first occurrence in Section 3.5.
Comment 4: Line 219, delete ‘and’
Response 4: We thank the reviewer for identifying this typographical issue. We have removed the unnecessary word “and” from Line 219.
Changes in manuscript:The redundant word “and” has been deleted.
Comment 5: about the baseline, in line 239, it is 2015-2021, but it becomes to be 2009-2009 in line 242. Why do you choose it as the baseline? Why the two baselines are different?
Response 5: We thank the reviewer for raising this point. The two reference periods serve different purposes. The 2015–2021 period is used to compare annual precipitation in 2022 with the recent pre-drought mean, whereas 2004–2009 is retained as the reference period for the satellite-derived terrestrial water storage anomalies and the associated water-storage analysis. We have clarified these distinct purposes in the manuscript to avoid confusion and ensure consistency between the text, tables, and figure captions.
Changes in manuscript: We have clarified the rationale for using the two reference periods and checked the corresponding descriptions in the text, tables, and figure captions for consistency.
Comment 6: Line 376-380, it is too long and difficult to understand this sentence. Please rephrase it.
Response 6: We thank the reviewer for this helpful suggestion. We have rewritten the sentence in Lines 376–380 to improve readability and clarify the logical connection between the findings and their interpretation. The revised text separates the main result from its explanation, making the argument easier to follow.
Changes in manuscript: The original long sentence has been divided and rephrased for clarity.
Citation: https://doi.org/10.5194/egusphere-2026-4113-AC2
-
AC2: 'Reply on RC2', shuhan yang, 10 Oct 2026
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 210 | 135 | 50 | 395 | 47 | 41 | 39 |
- HTML: 210
- PDF: 135
- XML: 50
- Total: 395
- Supplement: 47
- BibTeX: 41
- EndNote: 39
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
This study looks at a very important topic – the response of major, groundwater-dependent rivers to drought, in cases where there is pre-existing groundwater depletion within the catchment. The authors have (I believe) demonstrated their case that antecedent groundwater depletion results in far more significant and long-lasting reductions in streamflow than could be predicted using meteological data and indices. I think this paper is worthy of publication and an important contribution to knowledge. However, there is some further work required to strengthen the evidence base for the main findings, and integrate the results with relevant literature and data that has examined similar questions elsewhere.
Specific comments:
Line 29: Typo – correct to aquifer.
Line 81: Wada et al 2010 reference is missing from references list. Famiglietti et al 2014 did not specifically look at the Mediterranean region. You might re-phrase this to indicate that groundwater depletion is a growing global issue, but that there has been limited study (using GRACE or other tools) in the Mediterranean region.
Line 93: This last point may be controversial – does GRACE really remove the need for in-situ observations? I think there is a strong case to say the two must be used in conjunction wherever possible.
Line 113-114: Is this meant to read 2 to 10? Seems like a small typo.
Line 117: If characteristic of the groundwater, it would be good to include one or more hydrographs showing groundwater levels through time (either individual wells, or an aggregated average level). The same applies for the surface water gauging site – would be great to present a typical hydrograph early in the paper for background.
Line 179: Were other possible recession filters applied and assessed for the level of fit? There are many available filters – some discussion of how you chose the Lyne & Hollick filter as being the most appropriate for the data would be helpful.
Line 249: Clarify whether the low-flow deficits are relative to 2021 mean discharge, or another metric (e.g. baseflow during 2021).
Figure 3: Label size is far too small on the figure. Please enlarge the figure label and axis text by at least 50%. Same applies to Figure 4, including axis labels and all other text.
Line 350: It appears the mean baseflow is almost 100% of the mean streamflow (when comparing the mean baseflow values reported here, and the mean 2021 and 2022 total streamflow at line 248). This seems unlikely to be realistic, or at least requires some further discussion and evidence in support. This point underscores the need to carefully assess the most appropriate baseflow/hydrograph separation method and check the realism of the output. Adjusting either the filter method (exploring other options like Boughton, Chapman, Eckhardt), and/or the adopted coefficient(s), and checking whether this significantly affects the mean baseflow estimates, is an important step to ensure estimates are robust and the method appropriate. Is there literature supporting the Po River as being a dominantly baseflow hydrograph?
Line 372 to 388: I’d like to see some more reference to literature on baseflow recession and streamflow hydrograph responses to drought here. You have made some observations and claims about a possible ‘threshold’ change in aquifer-river connection, reflected in the baseflow coefficients. It is important to support this with some relevant literature on the topic – e.g. where else has this been observed and what is the theoretical underpinning? The work of Dr Timothy Peterson and colleagues comes to mind (e.g., Peterson et al. Watersheds may not recover from drought, Science 372, 745-749).
Line 416-417: Can you give some further supporting evidence about the ‘suppressed’ baseflow during this period? Are you basing this conclusion just on the elevated baseflow recession constant, or is there other evidence you can point to in the hydrograph - e.g. comparing flow percentiles during low-flow periods across different years?
Line 426: Your earlier text and Figure 8 suggest that the lag is longer than 7 months – please check for consistency throughout.
Line 440: Is there strong evidence that the antecedent aquifer condition (deficit) was significantly greater in the lead-up to the 2022 drought than previous droughts? This is implied, but no strong evidence is cited here. The GWSA data are an obvious place to look.
Line 444-447: This should be testable, by examining hydrographs from previous years with similar precipitation deficit, but less of a negative GWSA. It is important to attempt this to ensure the conclusion is supported.
Line 478: The California Central Valley comes to mind here – please have a look at the relevant literature and data from this region to see if the same holds true.
Line 496: Is there a particular point on the streamflow hydrograph where you can identify this change specifically, and show the reader?
Line 523-531: Indeed, these are important findings for river management.
Line 537: Is such monitoring sparse within the study area, specifically?
Section 5.5: This section does not really convince the reader that you have identified a specific threshold where baseflow recession behavior has qualitatively shifted to a new state. This would require some more in-depth analysis of the hydrograph to demonstrate the key change point (e.g. change in recession slope or shape that corresponds to a threshold GWSA). You have done this semi-quantitatively by means of looking at recession constants but the work is not very precise. This part doesn’t necessarily have to be part of the scope of the paper – your findings are significant enough regardless, but I think if you are going to make this claim, you need to substantiate it better.
Looking at recent (July-August 2026) SPEI data for the European alpine region, there has been a very significant meteorological drought this summer. Somewhere in your introduction or conclusion, I would suggest mentioning that your research has very important implications for subsequent drought periods, including the current severe drought being experienced in central Europe.