Reconstructing 273 Years of Potential Groundwater Recharge Dynamics in a Near-Humid Monsoon Loess Unsaturated Zone Using Chloride Profiling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is interesting, but there is significant potential to improve it. There might also be some problems with the data, see below

 

-          What is the exact definition of groundwater recharge employed in the study? There are numerous ways to define it, and it would be good if the authors explicitly mention it. See a recent paper by Gong where the numerous definitions are discussed: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2023WR034920

-          I think there is an incoherence in the data. The tritium peak is in 1963, so about 61 years from now. The distance of peak-displacement is 14.1 meters. The average moisture content in the profile is about 0.25 . Based on this we can calculate recharge (see e.g. Environmental soil physics by Hillel, chapter on movement of solutes, sample problem 1) : v= q/water content

The average velocity v is 14.1m/61a=0.24m/a

Considering the water content this gives a flux q =0.05m/a . this seems to be in contradiction with the data in figure 3, where the average recharge for the period 1960 to 2010 is clearly higher. Please explain this difference.

-          The use of the word paleo is unclear. Typically paleao refers for much larger timescales

-          The conclusions are mainly a summary.

-          I am not sure how important the temperature effect is. Consider deleting it. The main driver is precipitation, landuse and depth to groundwater

-          There is  no relation between sun activity and recharge. R=0.27 is too low.

-          Only one profile is shown. Dont you have more data?

-          There is no independent comparison with groundwater recharge. There will be certainly some published indicated on recharge in this region and it would be good to discuss the robustness on this basis Was there any change in landuse over the period? This would have a much more important and direct influence on recharge, compared to sun activity, see e.g. Salix psammophila afforestations can cause a decline of the water table, prevent groundwater recharge and reduce effective infiltration https://doi.org/10.1016/j.scitotenv.2021.146336

-          Note that employed chloride balance is a steady state formulation. I am not sure how this equation was applied to the non.steady profiles, please explain.

-          It is unclear why e.g. magnetic susceptibility was used.

-          Is there any indication of the variations in the groundwater level? This would be very important to discuss and consider

Comments on the Quality of English Language

The paper is interesting, but there is significant potential to improve it. There might also be some problems with the data, see below

 

-          What is the exact definition of groundwater recharge employed in the study? There are numerous ways to define it, and it would be good if the authors explicitly mention it. See a recent paper by Gong where the numerous definitions are discussed: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2023WR034920

-          I think there is an incoherence in the data. The tritium peak is in 1963, so about 61 years from now. The distance of peak-displacement is 14.1 meters. The average moisture content in the profile is about 0.25 . Based on this we can calculate recharge (see e.g. Environmental soil physics by Hillel, chapter on movement of solutes, sample problem 1) : v= q/water content

The average velocity v is 14.1m/61a=0.24m/a

Considering the water content this gives a flux q =0.05m/a . this seems to be in contradiction with the data in figure 3, where the average recharge for the period 1960 to 2010 is clearly higher. Please explain this difference.

-          The use of the word paleo is unclear. Typically paleao refers for much larger timescales

-          The conclusions are mainly a summary.

-          I am not sure how important the temperature effect is. Consider deleting it. The main driver is precipitation, landuse and depth to groundwater

-          There is  no relation between sun activity and recharge. R=0.27 is too low.

-          Only one profile is shown. Dont you have more data?

-          There is no independent comparison with groundwater recharge. There will be certainly some published indication on recharge in this region and it would be good to discuss the robustness on this basis Was there any change in landuse over the period? This would have a much more important and direct influence on recharge, compared to sun activity , see e.g. Salix psammophila afforestations can cause a decline of the water table, prevent groundwater recharge and reduce effective infiltration https://doi.org/10.1016/j.scitotenv.2021.146336

-          Note that employed chloride balance is a steady state formulation. I am not sure how this equation was applied to the non.steady profiles, please explain.

-          It is unclear why e.g. magnetic susceptibility was used.

-          Is there any indication of the variations in the groundwater level? This would be very important to discuss and consider

Author Response

Comments from Reviewer: #1

1. What is the exact definition of groundwater recharge employed in the study? There are numerous ways to define it, and it would be good if the authors explicitly mention it. See a recent paper by Gong where the numerous definitions are discussed: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2023WR034920.

Response: Thanks for your valuable advice. To clarify the concept of groundwater recharge in this study, we have revised the sentence as: “Potential groundwater recharge (PGR), defined as net-infiltration below the extinction depth, in the unsaturated zone (USZ) serves as a critical link be-tween surface climate elements (e.g., precipitation, evapotranspiration, runoff) and groundwater.”, and cite the paper of Gong et al. (2006) in line 34-37.

2. I think there is an incoherence in the data. The tritium peak is in 1963, so about 61 years from now. The distance of peak-displacement is 14.1 meters. The average moisture content in the profile is about 0.25. Based on this we can calculate recharge (see e.g. Environmental soil physics by Hillel, chapter on movement of solutes, sample problem 1) : v= q/water content. The average velocity v is 14.1m/61a=0.24m/a. Considering the water content this gives a flux q =0.05m/a . this seems to be in contradiction with the data in figure 3, where the average recharge for the period 1960 to 2010 is clearly higher. Please explain this difference.

Response: Thanks. Both the tritium peak method and the chloride mass balance method can be utilized to calculate groundwater recharge rates. However, the tritium peak method computes the average recharge rate over nearly 60 years from 1961 to 2010, whereas the chloride mass balance method determines groundwater recharge at specific time points rather than averaging over time. Comparing the arithmetic mean of recharge rates at several time points within this period to the former is not reasonable. In fact, the average moisture content in the profile is approximately 0.255 cm³ cm⁻³, and we sampled the deep soil core in 2019, causing the ³H peak to descend over 56 years. Based on these data, the average recharge flux would be 64.2 mm/yr (0.255 × 14.1 × 10³ / 56), which is essentially consistent with the average recharge amount of 66.7 mm/yr calculated based on Cl, as shown in Figure 3a.

Sorry for not specifying the exact sampling time, which may have led to inaccuracies in your estimation. We have added the sampling time to the text:“A soil core was drilled on the Changshou tableland in 2019, reaching the ground-water table at a depth of 64 m.” in line 107.

3. The use of the word paleo is unclear. Typically paleao refers for much larger timescales.

Response: Thanks for your suggestion. We have made revisions to the manuscript and replaced the term "paleo" with either "potential recharge history" or "PGR".

4. The conclusions are mainly a summary. I am not sure how important the temperature effect is. Consider deleting it. The main driver is precipitation, landuse and depth to groundwater. There is no relation between sun activity and recharge. R=0.27 is too low.

Response: Thanks. Correlation analysis revealed a significant relationship between potential recharge rates and both temperature and solar activity. The correlation coefficient between recharge rates and sunspot numbers was 0.27 without considering lag effects, but it increased to 0.57 when lag effects were accounted for. The influence of these factors on the long-term history of groundwater recharge appears to be indirect and characterized by a distinct lag effect. Furthermore, our results indicate that accounting for lag effects amplifies the observed impact of temperature and solar activity on groundwater recharge. Consequently, we assert that our data supports the existing conclusions.

5. Only one profile is shown. Dont you have more data??

Response: Sampling from the deep vadose zone is time-consuming and labor-intensive, rendering samples from deep profiles valuable. More importantly, in the Chinese loess plateau region, soil layers exhibit horizontal homogeneity, enhancing the representativeness of a single profile. To more clearly reflect the characteristics of the loess plateau soil, we have provided explanations in lines 101-102: “Additionally, the soil layers exhibit horizontal homogeneity on the loess-tableland.” and cited another paper ([23] Lu, Y., et al., Elucidating controls of the variability of deep soil bulk density. Geoderma, 2019. 348: p. 146-157.).

6. There is no independent comparison with groundwater recharge. There will be certainly some published indicated on recharge in this region and it would be good to discuss the robustness on this basis. Was there any change in landuse over the period? This would have a much more important and direct influence on recharge, compared to sun activity, see e.g. Salix psammophila afforestations can cause a decline of the water table, prevent groundwater recharge and reduce effective infiltration https://doi.org/10.1016/j.scitotenv.2021.14633.

Response: Thanks for your questions. In the study region of Weinan, there is a paucity of long-term historical data on groundwater recharge. Nevertheless, we compared the potential groundwater recharge history from a sub-humid region, Changwu, as detailed in lines 284-292. The comparative analysis revealed a high degree of consistency in the historical patterns between the two regions.

Indeed, land use change is a significant factor affecting groundwater recharge. Nonetheless, its impact is predominantly confined to short time scales (or the non-deep vadose zone), as per the study you mentioned. Conversely, this study examined the potential groundwater recharge history over three centuries.

7. Note that employed chloride balance is a steady state formulation. I am not sure how this equation was applied to the non.steady profiles, please explain.

Response: One of the fundamental assumptions of the chloride mass balance method is that soil water and chloride movement in the unsaturated zone occurs under steady-state conditions. Several studies have demonstrated that soil water movement in the deep vadose zone of the Chinese Loess Plateau is primarily governed by steady-state piston flow. Therefore, employing the chloride mass balance method to compute the long-term groundwater recharge history in this region is justified. Furthermore, the presence of a peak in the ³H signal within the soil profile indicates that piston flow predominates as the mode of soil water transport in our study area.

8. It is unclear why e.g. magnetic susceptibility was used.

Response: Magnetic susceptibility is a commonly used indicator for analyzing deep loess profiles. The distribution of soil magnetic susceptibility (MS) exhibits distinct peaks and valleys corresponding to interglacial and glacial sediments. Here, we utilize it to characterize the entire soil profile's stratigraphic structure. Moreover, high magnetic susceptibility signifies advanced soil weathering and a heightened presence of clay particles, enhancing water retention capacity. Consequently, magnetic susceptibility shows synchronous variations with the morphological profile of moisture content. We elucidated this relationship in the results section (Lines 189-190).

9. Is there any indication of the variations in the groundwater level? This would be very important to discuss and consider

Response: In this study, the long-term potential groundwater recharge history was reconstructed in this study based on soil water chemical properties, rather than the actual groundwater recharge reaching the water table. Fluctuations in actual groundwater levels may affect the calculation of historical recharge rates using the chloride mass balance method under steady-state conditions. However, we excluded the 5-meter capillary fringe influenced by groundwater levels (lines 209-214), ensuring that our inferred recharge history is reliable.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Please refer to the pdf file for feedback and comments for improvement.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Author Response

Comments from Reviewer: #2

1. Page 2, Line 51: Revise “…method. [a,Edmunds[12] delineated..”

Response: Thanks. Done as suggested.

2. What makes understanding PGR and its controlling factors essential for predicting hydrological cycle processes under future climate change? Is the location of study selected is prone to climate change problem?

Response: Thank you for your question and valuable advice. Groundwater recharge is a crucial hydrological component. Therefore, studying the history and patterns of potential groundwater recharge in the vadose zone can provide significant insights for predicting future trends in groundwater recharge. Our statement may have been inaccurate and led to misunderstandings. To address your concern, we have revised the relevant content as follows: “Therefore, studying the history of PGR and its controlling factors is crucial for understanding the evolution of PGR and predicting groundwater recharge under future climate change.” in lines 37-40.

3. Can the author indicate and emphasize the novelty of the current project, beyond the study location? Have any changes been made from the commonly used method (e.g., CMB method).

Response: Thanks. The Chloride Mass Balance method was previously primarily used to reconstruct potential groundwater recharge history in arid regions. However, this study attempts to apply the method in near-humid areas to reflect the historical groundwater recharge with higher resolution. In addition to this innovation in the study region, we also employed spectral analysis to examine the dominant factors of groundwater recharge history at different time scales, providing a deeper insight into the mechanisms of groundwater recharge through deep vadose zones. To better highlight the innovation of the article, we add more content in the abstract and conclusion: “Spectral analysis was employed to identify the principal influencing factors on PGR across various time scales.” in lines 18-19, and “We examined the characteristic cycles of recharge and other climate indicators to explore their interconnections using spectral analysis.” in lines 370-372.

4. How does the integration of meteorological data (e.g., precipitation, temperature) from 1961 to 2018 with soil measurements (bulk density, water content, magnetic susceptibility) contribute to understanding groundwater recharge patterns on the Changshou tableland?

Response: Thanks for your question. Precipitation and soil moisture content, as well as bulk density, are essential parameters for reconstructing the history of groundwater recharge using the chloride mass balance method. Additionally, temperature and precipitation are important influencing factors on groundwater recharge. Therefore, the role of these data is detailed in the Materials and Methods section, where precipitation and chloride concentration in precipitation are used to calculate the atmospheric chloride input flux, and soil moisture content and bulk density are used to calculate the soil storage capacity in the deep profile.

5. Can the authors explain how studying chloride concentration in the soil profile serves as a proxy for climatic and environmental dynamics? Additionally, how do these dynamics influence groundwater recharge processes over time?

Response: Thanks. Chloride ions serve as an excellent natural tracer in the environment due to their high solubility and stability. Assuming a constant atmospheric chlorine input, the chloride concentration within the soil profile is inversely proportional to water content. Consequently, the chloride concentration can indirectly indicate the water flux under piston flow conditions, as depicted in Equation (1). When precipitation infiltrates the soil surface, it progressively percolates over time, preserving the historical atmospheric chloride signal in the soil pore water along depth. Thus, variations in chloride concentration across the spatial extent of the soil profile can be translated into soil water recharge over past timescales, representing potential groundwater recharge.

6. Considering factors beyond precipitation, how does soil magnetic susceptibility correlate with fluctuations in volumetric water content?

Response: High magnetic susceptibility is indicative of a high degree of soil weathering and a high clay content, which enhances the soil's water-holding capacity. Consequently, magnetic susceptibility shows synchronous variations with water content along the deep soil profile. This has been proved by our (Figure 2a&b, lines188-190) and several published studies (e.g., Lu et al., 2020).

7. Briefly describe the impact of chemical fertilizers on chloride concentrations in upper soil layers. Are there seasonal or long-term trends in chloride deposition resulting from these practices?

Response: Thanks. The extensive use of agricultural fertilizers in China began around 1980, and thus the impact of fertilizers on the chloride concentration in soil pore water is primarily concentrated in the shallow soil layers. To minimize the influence of fertilizers on the study results, the research site was selected in farmland that was cultivated in 2017, and we used chloride concentration data from soil water at depths of 2-59 m to calculate potential recharge rate. To address your concern, we added more content about the sample site in lines 106-107: “The sample site was selected in a newly cultivated farmland to minimize the effect of fertilizer on chloride input.”

Furthermore, as noted by Lu et al. (2023), the influence of fertilizers is primarily observed on short time scales and does not substantially impact the long-term average chloride deposition over multiple years. In the Weinan region, Lu's reconstructed chloride deposition rate using tritium tracer was 1064.3 mg m^-2 yr^-1.And our observed precipitation chloride deposition rate was 1027.2±87.9 mg m^-2 yr^-1, which is consistent with Lu’s result. This suggests that the chloride deposition flux data we employed possess a certain level of reliability. Therefore, we added the following content in lines 208-209: “The average Cl concentration of precipitation and annual precipitation amount were 1.77 mg L^-1 and 581 mm respectively. Therefore, annual Cl deposition was 1027.2±87.9 mg m^-2 year ^-1 in the study area, which is consistent with Lu's results.”

8. What are the main factors driving the bell-shaped distribution of tritium isotopes in the soil profile? How does this distribution pattern relate to local precipitation dynamics and groundwater recharge processes?

Response: Thank you for your questions. Tritium (³H) within the soil profile derives from precipitation tritium. As is known, the atmospheric tritium concentration peak occurred around 1963 from nuclear tests. Once precipitation infiltrates into soil by piston flow, the tritium would exhibit a bell-shaped distribution along the vertical soil profile. Conversely, the bell-shaped tritium distribution in the vadose zone suggests that piston flow is the predominant mechanism for groundwater recharge. Naturally, the depth and peak value of the tritium peak are correlated with precipitation, with greater precipitation typically leading to a deeper tritium peak position.

9. What are the main sources of uncertainty in evaluating long-term groundwater recharge rates? How do these uncertainties vary across different time scales and affect the interpretation of historical recharge patterns?

Response: Thanks. Both the chloride deposition flux and the soil chloride ion concentration affect the accuracy of estimating the long-term history of groundwater recharge. We have conducted uncertainty analysis on the recharge rate estimation at different time scales in line 240-246.

10. Can the authors briefly explain how solar activity impacts variations in groundwater recharge, including the observed positive correlation over an 11-year cycle and the implications of lag effects?

Response: Solar energy is the driving force behind the global water cycle, primarily altering the intensity of evapotranspiration, which subsequently influences atmospheric circulation and precipitation. Groundwater recharge, however, is predominantly sourced from precipitation, especially in regions with deep vadose zones like our study site. The time it takes for precipitation to infiltrate the soil and reach groundwater varies, depending on the thickness of the vadose zone. Therefore, there is a lag effect between solar activity and the dynamics of groundwater recharge. The influence of solar activity is of great significance to predict the future change of groundwater recharge, and the lag of its influence will provide abundant time for human utilization and regulation of groundwater resources. To illustrate the significance of the lag effect, we added more content in lines 363-365: “These effects and their lags on ground water recharge will be of great significance to predict the future change of groundwater recharge, allowing ample time for human utilization and regulation of groundwater resources.”

11. From authors’ perspective, how do multi-cycle precipitation patterns impact long-term groundwater recharge trends? What implications does this have for managing water resources and adapting to climate change in selected study region?

Response: Our study reveals that the history of potential groundwater recharge shows a significant correlation with precipitation over specific time scales (e.g., 2-7 years), indicating that precipitation could be a key factor influencing potential groundwater recharge. Gaining insights into the evolutionary patterns of potential groundwater recharge and climatic variables (like precipitation) is essential for enhancing our simulations and forecasts of groundwater recharge time and flux in this region or other similar areas with deep vadose zones under future climate conditions. To address your concern, we added more content in lines 363-365: “These effects and their lags on ground water recharge will be of great significance to predict the future change of groundwater recharge, allowing ample time for human utilization and regulation of groundwater resources.”.

12. How do the results from reconstructing a 273-year hydroclimate record using USZ Cl proxies in a near-humid location contribute to our understanding of global hydrological and climatic processes?

Response: Thanks. We may not have accurately expressed the significance of this study in our conclusions. In order to elaborate more rigorously in the conclusion, we revised the content as“These findings underscore the potential of using USZ Cl proxies to reconstruct historical groundwater recharge and improve our understanding of the groundwater recharge evolution in near-humid regions.”in lines 394-397.

13. does the authors believe that the current research could influence strategies for future water resource management?

Response: Thanks for your valuable comment. Our study reconstructed potential groundwater recharge history from 1745 to 2007 AD, providing valuable insights into its underlying patterns and dynamics. This knowledge is essential for accurately predicting future changes in groundwater recharge, thereby facilitating effective water resource management. To address your concern, we added more content in lines 363-365: “These effects and their lags on ground water recharge will be of great significance to predict the future change of groundwater recharge, allowing ample time for human utilization and regulation of groundwater resources.”.

14. Apart from precipitation, temperature, and solar activity, what other aspects of climatic variability warrant further study to increase our understanding of long-term groundwater recharge dynamics? In what ways could including more climate proxies or advanced modeling techniques improve our understanding of the susceptibility of recharge patterns to climate change?

Response: Beyond the precipitation and temperature examined in this study, hydrological or meteorological extreme events—including heavy rainfall, abrupt drought, and evapotranspiration—which emerge within the context of climate change, can significantly influence groundwater recharge. In addition to chloride signals, other hydrochemical indicators, such as stable isotopes within the vadose zone, may also function as proxies for documenting past climate element. We made some prospects at in lines 394-397: “Besides chloride in USZ, other hydrochemical indicators, such as water-stable isotopes sensitive to climate change, may also be valuable tools for reconstructing climate history and need further study.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

thanks for the revision