the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Oct 2024
Groundwater in fractured granite: implications for tropical dry forest development and water sustainability
Abstract. Seasonality is one of the most important features of Tropical Dry Forests (TDFs), then water scarcity must be overcome by perennial sources dependent of groundwater flows. Groundwater recharge processes in TDFs are controlled by (i) the seasonal dynamics of the components of the water cycle through their interaction with the soil and underlying geological environment, and (ii) the phenological rhythms of the vegetation that simultaneously influence infiltration and evapotranspiration. The daily hydrological dynamics of a TDF that grow atop fractured granite with a thin layer of sandy soil were studied in three basins subject to conservation in the Pacific coast of southern Jalisco, Mexico along the 2019–2020 hydrological year. Automated climatological and streamflow instrumentation was used to obtain data for a detailed analysis of the rain-streamflow response and new instruments were placed to measure interception and soil moisture. Results show that annual precipitation was 1.179 mm (above the average of 832 mm) distributed in 80 highly variable events. The phenological stage of the vegetation and the accumulation of litter strongly influenced interception. Thin sandy soils (~ 0.30 m) controlled the rapid infiltration of 85 to 98 % of the precipitation that reached the ground along seasons, reducing the effect of evapotranspiration by percolation, aided by the fact that most of the precipitation events were nocturnal. The rain-streamflow response showed that groundwater discharge in the streams represented up to 70 % of the percolation volume and the remaining 30 % correspond to groundwater flow and temporary storage in the fractured medium. These two processes may explain the zonation of two subtypes of vegetation, their phenology and survival in the dry months. The deciduous tropical forest (DTF) in the study area developed in groundwater recharge zones, while the sub-deciduous tropical forest (SDTF) emerged in the discharge zones, where evapotranspiration values of up to 0.140 mm d-1 were obtained from the diurnal variations of the base flow. Analyses of daily data highlight the importance of the fractured medium and its temporary saturation, where residence times may made water available in the ecosystem during dry periods. Improving our understanding of these processes will help guide the sustainability of provision of groundwater by the conservation of hydrological ecosystem services in the basins for anthropogenic activities in the region reducing hydrological vulnerability to dry periods and climate change.
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Status: closed
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RC1: 'Comment on egusphere-2024-3117', Anonymous Referee #1, 18 Oct 2024
The paper’s theme is relevant and actual, as the techniques and models used show a new use for the index. Several new critical points about hydrology processes and TDF phenology. It is essential to highlight the novelty of showing the role of fractured rock groundwater in the phenology and TDF distribution.
The paper shows excellent Scientific Significance, Scientific Quality and Presentation Quality. There is only one question that needs to be clarified:
Lines 216-224—These comments are not clear. There is a mix of “Q1” comments with B1 and B3 comments. Is the comment about Q1 related to Figure 2? There is no information about Q1, Q2, or Q3… in figure 4. At the same time, there are comments about the B1 baseflow without details in figure 4.
Citation: https://doi.org/10.5194/egusphere-2024-3117-RC1 -
AC1: 'Reply on RC1', Landy Orozco, 25 Oct 2024
Thank you for your comments. In reply, Figure 4 represents the first streamflow event (Q1) that occurred from July 19 to 31, 2019 in the three study basins (B1, B2 and B3). The streamflow from basin 1 (B1) represented by the black dashed line, the streamflow from basin 2 (B2) by the yellow dotted line and the streamflow from basin 3 (B3) by the solid green line. The paragraph in lines 216-224 explains that the response of the first streamflow event (Q1) in the three basins was different in time and magnitude, particularly with baseflow formation in basins B2 and B3, represented by shaded areas under their lines, with a long duration in the case of basin 3 (B3). In the basins B2 and B3, which presented baseflow, diurnal fluctuations allowed the calculation of evapotranspiration for the sub-deciduous tropical forest.
We will adjust our redaction to better explain this details.
Citation: https://doi.org/10.5194/egusphere-2024-3117-AC1
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AC1: 'Reply on RC1', Landy Orozco, 25 Oct 2024
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CC1: 'Comment on egusphere-2024-3117', Giacomo Medici, 18 Nov 2024
General comments
Good hydrological research on surface / groundwater interaction in fractured rocks. Please, follow my suggestions to improve the manuscript.
Specific comments
Lines 73-75. Sentence not backed up by references on the high hydraulic conductivity and storage properties of fractures. Please, add the two recent and relevant references suggested above on fractured granites:
- Medici , Ling F, Shang F 2023. Review of discrete fracture network characterization for geothermal energy extraction. Frontiers in Earth Science 11, 1328397.
- Agbotui PY, Ewusi A, Seidu J, Brookman-Amissah M, Woode Aforla B 2023. Delineation of preferential flow pathways in a tropical crystalline rock aquifer in Tarkwa, Ghana using integrated hydrogeophysical methods. Hydrology Research 54(5), 722-738.
Line 94. Clearly state the 3 to 4 specific objectives of your research by using numbers (e.g., i, ii, and iii).
Lines 99-185. Consider to insert some basic information on the hydrology and hydrogeology of the study site.
Lines 99-185. Add more information on the tropical climate conditions at your study site.
Lines 390-555. Integrate the relevant and recent literature on fractured rocks suggested above.
Figures and tables
Figure 1. The geographical map on the right is unreadable.
Figure 1. Consider to insert a new and separate figure to fix the issue of Figure 1.
Figure 3. Divide the Figure 3 in three parts A, B and C.
Figures 4, 6 and 7. Make the letters larger on the vertical and horizontal axes. You can also enlarge the entire figures if there is room for it.
Citation: https://doi.org/10.5194/egusphere-2024-3117-CC1 -
AC2: 'Reply on CC1', Landy Orozco, 26 Nov 2024
We are grateful for your valuable feedback, it will indeed help to improve the document, we will take it into account and integrate it into an updated version. We will also review the suggested publications as references.
Citation: https://doi.org/10.5194/egusphere-2024-3117-AC2
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AC2: 'Reply on CC1', Landy Orozco, 26 Nov 2024
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RC2: 'Comment on egusphere-2024-3117', Anonymous Referee #2, 17 Dec 2024
Review of Manuscript: "Groundwater in fractured granite: implications for tropical dry forest development and water sustainability" General Comments
The topic is relevant to understanding the role of vegetation in influencing flows in tropical dry forests (TDFs). However, the manuscript has significant weaknesses in narrative linkages, methodological transparency, and the suitability of datasets used to address the stated objectives. In particular, the methods and data do not directly (in any way) support the paper's main focus, which is groundwater dynamics in fractured granite.
Major Concerns
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Insufficient Data for Groundwater Analysis:
- The paper's title emphasizes groundwater in fractured granite, yet the only subsurface measurements are from two TDR sensors placed at 0–15 cm and 15–30 cm soil depths. This does not provide any information about groundwater behavior in deeper fractured rock layers.
- Given that groundwater is likely quite deep in this system (i would guess on the order of 10 meters at the ridge, as observed in other seasonally dry granitic systems, e.g. the Sierra Nevada in California), it is more plausible that vegetation in the sub-deciduous tropical forest (SDTF) relies on water stored in unsaturated, weathered granite layers, as is typical in similar ecosystems.
-
Methodological Gaps:
- Evapotranspiration (ET) from groundwater: The authors state that ET is calculated from diel oscillations in streamflow, but no method or citation for this calculation is provided. A clear description of the approach and assumptions is necessary.
- Orthophoto Analysis: While results from orthophoto analysis are presented, there is no description of this method in the Methods section.
- Baseflow Separation: The baseflow separation technique is not described. Additionally, baseflow separation algorithms lack a physical basis for distinguishing between groundwater-generated and surface-generated flows. So, I fundamentally disagree with any process interpretations derived from this analysis. Even Hewlett and Hibbert acknowledged this in their original paper that used a baseflow separation method.
-
Ambiguous Conceptual Model of the Subsurface:
- The manuscript frequently refers to the "fractured medium" and assumes water storage below 30 cm is within fractured granite (I think). However, no information is provided about the depth or structure of soils and the transition to fractured rock.
- A figure illustrating the authors' conceptual model of the subsurface structure (soil, weathered granite, and fractured granite) would greatly clarify their assumptions and interpretations.
Specific Comments
-
Abstract:
- The first sentence contains a "then" that does not fit the sentence structure.
- Line 16: "1.179 mm" – Is this a typo? Should it be "1,179 mm" (using a comma for thousands)?
- Line 19: "Evapotranspiration by percolation" is incorrect terminology. Consider rephrasing to "evapotranspiration of infiltrated water."
- Lines 19–21: I’m confused about mass balance here. The authors first say that plants evapotranspire infiltrated water. They then say that 70% of the percolation volume goes to streams, and 30% to groundwater. So, what percent is evapotranspiration then? Does it come from the 30% storage in groundwater? Is that storage really all "groundwater" or could there be significant unsaturated storage below the shallow TDR? Is there any basis for distinguishing groundwater use from unsaturated water use below 30cm?
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Introduction:
- Lines 43–44: The logical connection between sentences is unclear. The "Likewise" transition needs improvement.
- Lines 48–49: Clarify what is meant by "subsoil" – does this refer to weathered bedrock layers beneath the soil?
-
Methods:
- Line 89: Provide the full name of the climate classification system (not just the code).
- Line 100: Typo – "e"stablished.
- Lines 106–109: Clarify how the storages S_prev, \theta, and SS are related. Is SS depth-integrated soil storage? What zones (saturated/unsaturated) are included?
- Line 154: Use appropriate terminology for measurement structures, e.g., v-notch weirs and H-flumes instead of "outlets" and "channels."
- Lines 158–160: ET values from decades-old studies (1980s and 1990s) are used to estimate ET in 2019? This contradicts the authors’ point that ET depends on contemporary water availability. Justify this assumption or provide more recent estimates.
- Line 178: I’m not sure what the authors are describing. The authors state that the total infiltration was compared to “retention”. Do they mean compared to (max_retention - current_retention), so as to compare the infiltration volume to the storage deficit below the maximum? Or are they comparing infiltration to depth-integrated \theta values (which, I assume, would be the definition of retention). If the latter, then I’m not sure I understand what the authors learn by comparing infiltration volumes to soil water storage volumes, and why the threshold comparison of these two values
- Line 183: Provide a description of the subsurface structure, including soil depth, the transition to fractured granite, and conceptual water storage zones.
- Equation 3: Define this equation more clearly, including piecewise conditions for infiltration exceeding the soil storage deficit.
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Results:
- Line 220: The claim that baseflow represents 2.26% of total precipitation appears incorrect. Baseflow appears to contribute much more based on the presented data.
- Lines 222–223: The method for calculating ET from groundwater is not described.
- Lines 251–255: Orthophoto analysis is presented in the results but was not described in the Methods section.
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Discussion:
- The narrative could be improved to connect key findings more logically. For example, the linkage between vegetation zonation and groundwater recharge/discharge zones needs clearer framing.
- Explicitly address the role of unsaturated weathered granite as a potential water source for vegetation.
Conclusion
The manuscript addresses an important topic, but substantial improvements are needed to:
- Provide more appropriate data to support conclusions about groundwater dynamics.
- Clearly describe all methods used, including ET estimation, baseflow separation, and orthophoto analysis.
- Address significant gaps in the conceptual understanding of the subsurface structure and processes.
- Improve the logical flow and clarity of the narrative.
Citation: https://doi.org/10.5194/egusphere-2024-3117-RC2 -
AC3: 'Reply on RC2', Landy Orozco, 06 Jan 2025
The topic is relevant to understanding the role of vegetation in influencing flows in tropical dry forests (TDFs). However, the manuscript has significant weaknesses in narrative linkages, methodological transparency, and the suitability of datasets used to address the stated objectives. In particular, the methods and data do not directly (in any way) support the paper's main focus, which is groundwater dynamics in fractured granite. R: The main focus of the paper is not the groundwater dynamics in fracture granite. Instead, the focus of this study is to quantitatively explore the processes that control the recharge and discharge of groundwater and its relationship with the other seasonal components of the hydrological cycle and the phenological conditions of vegetation and weathered soil in fracture granite. (Lines 70-79).
The paper's title emphasizes groundwater in fractured granite, yet the only subsurface measurements are from two TDR sensors placed at 0–15 cm and 15–30 cm soil depths. This does not provide any information about groundwater behavior in deeper fractured rock layers. R: The measurements of the groundwater behaviour were based on the separation of the direct and base flows from the analysis of the hydrographs obtained in the hydrological year analysed. The TDR sensors allowed us to know the behaviour of moisture in the soil and establish its relationship with the processes of formation of percolation flows. Due to the granitic nature of the study area, the fact that it is located within a Biosphere Reserve and the technical implications required; at this stage, it was not possible to instrument the working basins with piezometers in discrete fractures, so in this work we present an alternative methodology to know the behaviour of groundwater based on environmental indicators (formation of baseflows in runoff, presence of springs and distribution of vegetation).We understand that studying in detail the dynamics of groundwater in the fractured environment requires discrete drilling and instrumentation for the different fractures, which we are considering in future stages of research.
Given that groundwater is likely quite deep in this system (i would guess on the order of 10 meters at the ridge, as observed in other seasonally dry granitic systems, e.g. the Sierra Nevada in California), it is more plausible that vegetation in the sub-deciduous tropical forest (SDTF) relies on water stored in unsaturated, weathered granite layers, as is typical in similar ecosystems. R: Based on the results obtained in our article published in 2023 (Orozco-Uribe et al., 2023) in which we proposed a Conceptual Model for the study basins, the sub deciduous tropical forest is distributed in groundwater discharge zones, being an indicator of relatively near-surface water tables (depth of reach of the roots of the species that represent it), which in the study region have been reported at 5 m in the nearby coastal areas. The groundwater levels within the fractured medium in the study area show seasonal variations, so it is likely that the vegetation of the sub deciduous tropical forest obtains water from the saturated zone at different depths depending on the climatic season.
Evapotranspiration (ET) from groundwater: The authors state that ET is calculated from diel oscillations in streamflow, but no method or citation for this calculation is provided. A clear description of the approach and assumptions is necessary. R: In line 163 of the Methods section, we mention that we follow Cadol et al. (2012) for the calculation of the ET from the daily variations in the flow.
Orthophoto Analysis: While results from orthophoto analysis are presented, there is no description of this method in the Methods section. R: In the Methods section, specifically in lines 109 to 111 we describe the ortophoto analysis. A detailed analysis of the LiDAR flight-generated orthophoto was presented in the article published in 2023 (Orozco-Uribe et al., 2023).
Baseflow Separation: The baseflow separation technique is not described. Additionally, baseflow separation algorithms lack a physical basis for distinguishing between groundwater-generated and surface-generated flows. So, I fundamentally disagree with any process interpretations derived from this analysis. Even Hewlett and Hibbert acknowledged this in their original paper that used a baseflow separation method. R: Indeed, we do not mention the methodology used for baseflow separation, which was carried out using the graphical method following Gonzales et al, 2009. We will make the corresponding correction in the Methods section.
The manuscript frequently refers to the "fractured medium" and assumes water storage below 30 cm is within fractured granite (I think). However, no information is provided about the depth or structure of soils and the transition to fractured rock. R: Agree. Since this study focuses on the application of the Conceptual Model presented in our previous work (Orozco-Uribe et al., 2023), as mentioned from line 76, where a description of the depth and structure of the soils and the characteristics of the fractured environment was made, we will include a brief description of these as was done for other general characteristics of the working basins.
A figure illustrating the authors' conceptual model of the subsurface structure (soil, weathered granite, and fractured granite) would greatly clarify their assumptions and interpretations. R: Agree, we will include a representative figure of the Conceptual Model.
The first sentence contains a "then" that does not fit the sentence structure. R: Agree, we will remove the word "then" from the sentence.
Line 16: "1.179 mm" – Is this a typo? Should it be "1,179 mm" (using a comma for thousands)? R: Indeed, it is a typographical error.
Line 19: "Evapotranspiration by percolation" is incorrect terminology. Consider rephrasing to "evapotranspiration of infiltrated water." R: Infiltration is the process by which water moves through the soil surface into the soil matrix and percolation is the process by which it moves down through the profile and into the underlying weathered rock (Le Maitre et al., 1999; Sophocleous, 2004; Sen, 2015; Gabrielli & McDonell, 2018). In this line of the Abstract, we refer to the fact that the effect of evapotranspiration was reduced due to the rapid movement of water into the fractured medium through the percolation process.
Lines 19–21: I’m confused about mass balance here. The authors first say that plants evapotranspire infiltrated water. They then say that 70% of the percolation volume goes to streams, and 30% to groundwater. So, what percent is evapotranspiration then? Does it come from the 30% storage in groundwater? Is that storage really all "groundwater" or could there be significant unsaturated storage below the shallow TDR? Is there any basis for distinguishing groundwater use from unsaturated water use below 30cm? R: As mentioned in the previous point, plants take water mainly from that which is stored in the soil layers (infiltrated water), between the permanent wilting point and the field capacity; in our study area, it has been shown that 90% of the root biomass is concentrated in the first 40 cm of the soil (Rentería-Rodríguez, 1997 mentioned in Orozco-Uribe et al., 2023). The rest of the water that is nos part of this soil storage, percolates through the soil layers into the fractured medium, forming a new flow that can find an outlet to the surface locally (~70%), forming part of the baseflow. The remaining, can effectively be part of a storage in the unsaturated zone of the fractured medium or incorporated into an intermediate or regional flow based on Tóth, 2016. Certain plant species can access water stored or flowing in the fractured environment through deep roots or, as is the case with the species that make up the sub deciduous tropical forest, access the flowing water in the fractured medium that finds outlet along rivers and streams.
Lines 43–44: The logical connection between sentences is unclear. The "Likewise" transition needs improvement. R: Agree, we will improve the wording.
Lines 48–49: Clarify what is meant by "subsoil" – does this refer to weathered bedrock layers beneath the soil? R: Good observation, we will improve the wording and use a more appropriate term.
Line 89: Provide the full name of the climate classification system (not just the code). R: Agree, we will make the corresponding correction.
Line 100: Typo – "e"stablished. R: Agree, we will make the corresponding correction.
Lines 106–109: Clarify how the storages S_prev, \theta, and SS are related. Is SS depth-integrated soil storage? What zones (saturated/unsaturated) are included? R: Daily storage (S) corresponds to water within the basins (Moore, et al., 2015), i.e., water that was not used by plants or that did not drain during the calculation day, integrating the unsaturated and saturated zones. It is based on equation 3, based on the calculation of the storage of the previous day (Sprev) plus the day's precipitation (P), from which the daily sum of interception (IC + IL), streamflow (Q) and evapotranspirations (ETDTF and ETSDTF) were subtracted. Theta corresponds to the moisture content in the soil from the data obtained from the TDR sensors.
Line 154: Use appropriate terminology for measurement structures, e.g., v-notch weirs and H-flumes instead of "outlets" and "channels." R: Agree, we will make the corresponding correction.
Lines 158–160: ET values from decades-old studies (1980s and 1990s) are used to estimate ET in 2019? This contradicts the authors’ point that ET depends on contemporary water availability. Justify this assumption or provide more recent estimates. R: The evapotranspiration values of the tropical deciduous forest refer to daily rates per season (seasonal) obtained by the authors consulted based on the physiological characteristics of the ecosystem, so they are valid even if they have been carried out in previous decades.
Line 178: I’m not sure what the authors are describing. The authors state that the total infiltration was compared to “retention”. Do they mean compared to (max_retention - current_retention), so as to compare the infiltration volume to the storage deficit below the maximum? Or are they comparing infiltration to depth-integrated \theta values (which, I assume, would be the definition of retention). If the latter, then I’m not sure I understand what the authors learn by comparing infiltration volumes to soil water storage volumes, and why the threshold comparison of these two values R: At this point, we refer to the retention of water in the soil (between the point of permanent wilting and the field capacity), if the water was infiltrated, then there are no Hortonian surface flows, but are part of underground flows or storage that when they exceed the retention capacity of the soil, then they become percolation flows towards the fractured medium.
Line 183: Provide a description of the subsurface structure, including soil depth, the transition to fractured granite, and conceptual water storage zones. R: Agree, we will make the corresponding correction.
Equation 3: Define this equation more clearly, including piecewise conditions for infiltration exceeding the soil storage deficit. R: Agree, we will make the corresponding correction.
Line 220: The claim that baseflow represents 2.26% of total precipitation appears incorrect. Baseflow appears to contribute much more based on the presented data. R: In this case we refer to the first streamflow of the year (Q1) when the soil is completely dry, so the retention will be greater. Likewise, the vegetation will take a large part of the infiltrated water for its greening processes; therefore, percolation will be low. Considering this, and that the baseflow comes from the percolated water to the fractured medium, then the percentage of P that this represents will be very low. We will review our data again to corroborate it and, if necessary, we will expand our discussion in this regard.
Lines 222–223: The method for calculating ET from groundwater is not described. R: The method used was mentioned in lines 160-163 of the Methods section.
Lines 251–255: Orthophoto analysis is presented in the results but was not described in the Methods section. R: Agree, we will make the corresponding correction.
The narrative could be improved to connect key findings more logically. For example, the linkage between vegetation zonation and groundwater recharge/discharge zones needs clearer framing. R: Agree, we will conduct a new and thorough review of our discussion to improve it.
Explicitly address the role of unsaturated weathered granite as a potential water source for vegetation. R: Agree, we will conduct a new and thorough review of our discussion to improve it.
The manuscript addresses an important topic, but substantial improvements are needed to: 1. Provide more appropriate data to support conclusions about groundwater dynamics. 2. Clearly describe all methods used, including ET estimation, baseflow separation, and orthophoto analysis. 3. Address significant gaps in the conceptual understanding of the subsurface structure and processes. 4. Improve the logical flow and clarity of the narrative. R: Agree, we will consider the comments already expressed and our responses to them, we appreciate the interest, which will surely improve our manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3117-AC3
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Status: closed
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RC1: 'Comment on egusphere-2024-3117', Anonymous Referee #1, 18 Oct 2024
The paper’s theme is relevant and actual, as the techniques and models used show a new use for the index. Several new critical points about hydrology processes and TDF phenology. It is essential to highlight the novelty of showing the role of fractured rock groundwater in the phenology and TDF distribution.
The paper shows excellent Scientific Significance, Scientific Quality and Presentation Quality. There is only one question that needs to be clarified:
Lines 216-224—These comments are not clear. There is a mix of “Q1” comments with B1 and B3 comments. Is the comment about Q1 related to Figure 2? There is no information about Q1, Q2, or Q3… in figure 4. At the same time, there are comments about the B1 baseflow without details in figure 4.
Citation: https://doi.org/10.5194/egusphere-2024-3117-RC1 -
AC1: 'Reply on RC1', Landy Orozco, 25 Oct 2024
Thank you for your comments. In reply, Figure 4 represents the first streamflow event (Q1) that occurred from July 19 to 31, 2019 in the three study basins (B1, B2 and B3). The streamflow from basin 1 (B1) represented by the black dashed line, the streamflow from basin 2 (B2) by the yellow dotted line and the streamflow from basin 3 (B3) by the solid green line. The paragraph in lines 216-224 explains that the response of the first streamflow event (Q1) in the three basins was different in time and magnitude, particularly with baseflow formation in basins B2 and B3, represented by shaded areas under their lines, with a long duration in the case of basin 3 (B3). In the basins B2 and B3, which presented baseflow, diurnal fluctuations allowed the calculation of evapotranspiration for the sub-deciduous tropical forest.
We will adjust our redaction to better explain this details.
Citation: https://doi.org/10.5194/egusphere-2024-3117-AC1
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AC1: 'Reply on RC1', Landy Orozco, 25 Oct 2024
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CC1: 'Comment on egusphere-2024-3117', Giacomo Medici, 18 Nov 2024
General comments
Good hydrological research on surface / groundwater interaction in fractured rocks. Please, follow my suggestions to improve the manuscript.
Specific comments
Lines 73-75. Sentence not backed up by references on the high hydraulic conductivity and storage properties of fractures. Please, add the two recent and relevant references suggested above on fractured granites:
- Medici , Ling F, Shang F 2023. Review of discrete fracture network characterization for geothermal energy extraction. Frontiers in Earth Science 11, 1328397.
- Agbotui PY, Ewusi A, Seidu J, Brookman-Amissah M, Woode Aforla B 2023. Delineation of preferential flow pathways in a tropical crystalline rock aquifer in Tarkwa, Ghana using integrated hydrogeophysical methods. Hydrology Research 54(5), 722-738.
Line 94. Clearly state the 3 to 4 specific objectives of your research by using numbers (e.g., i, ii, and iii).
Lines 99-185. Consider to insert some basic information on the hydrology and hydrogeology of the study site.
Lines 99-185. Add more information on the tropical climate conditions at your study site.
Lines 390-555. Integrate the relevant and recent literature on fractured rocks suggested above.
Figures and tables
Figure 1. The geographical map on the right is unreadable.
Figure 1. Consider to insert a new and separate figure to fix the issue of Figure 1.
Figure 3. Divide the Figure 3 in three parts A, B and C.
Figures 4, 6 and 7. Make the letters larger on the vertical and horizontal axes. You can also enlarge the entire figures if there is room for it.
Citation: https://doi.org/10.5194/egusphere-2024-3117-CC1 -
AC2: 'Reply on CC1', Landy Orozco, 26 Nov 2024
We are grateful for your valuable feedback, it will indeed help to improve the document, we will take it into account and integrate it into an updated version. We will also review the suggested publications as references.
Citation: https://doi.org/10.5194/egusphere-2024-3117-AC2
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AC2: 'Reply on CC1', Landy Orozco, 26 Nov 2024
-
RC2: 'Comment on egusphere-2024-3117', Anonymous Referee #2, 17 Dec 2024
Review of Manuscript: "Groundwater in fractured granite: implications for tropical dry forest development and water sustainability" General Comments
The topic is relevant to understanding the role of vegetation in influencing flows in tropical dry forests (TDFs). However, the manuscript has significant weaknesses in narrative linkages, methodological transparency, and the suitability of datasets used to address the stated objectives. In particular, the methods and data do not directly (in any way) support the paper's main focus, which is groundwater dynamics in fractured granite.
Major Concerns
-
Insufficient Data for Groundwater Analysis:
- The paper's title emphasizes groundwater in fractured granite, yet the only subsurface measurements are from two TDR sensors placed at 0–15 cm and 15–30 cm soil depths. This does not provide any information about groundwater behavior in deeper fractured rock layers.
- Given that groundwater is likely quite deep in this system (i would guess on the order of 10 meters at the ridge, as observed in other seasonally dry granitic systems, e.g. the Sierra Nevada in California), it is more plausible that vegetation in the sub-deciduous tropical forest (SDTF) relies on water stored in unsaturated, weathered granite layers, as is typical in similar ecosystems.
-
Methodological Gaps:
- Evapotranspiration (ET) from groundwater: The authors state that ET is calculated from diel oscillations in streamflow, but no method or citation for this calculation is provided. A clear description of the approach and assumptions is necessary.
- Orthophoto Analysis: While results from orthophoto analysis are presented, there is no description of this method in the Methods section.
- Baseflow Separation: The baseflow separation technique is not described. Additionally, baseflow separation algorithms lack a physical basis for distinguishing between groundwater-generated and surface-generated flows. So, I fundamentally disagree with any process interpretations derived from this analysis. Even Hewlett and Hibbert acknowledged this in their original paper that used a baseflow separation method.
-
Ambiguous Conceptual Model of the Subsurface:
- The manuscript frequently refers to the "fractured medium" and assumes water storage below 30 cm is within fractured granite (I think). However, no information is provided about the depth or structure of soils and the transition to fractured rock.
- A figure illustrating the authors' conceptual model of the subsurface structure (soil, weathered granite, and fractured granite) would greatly clarify their assumptions and interpretations.
Specific Comments
-
Abstract:
- The first sentence contains a "then" that does not fit the sentence structure.
- Line 16: "1.179 mm" – Is this a typo? Should it be "1,179 mm" (using a comma for thousands)?
- Line 19: "Evapotranspiration by percolation" is incorrect terminology. Consider rephrasing to "evapotranspiration of infiltrated water."
- Lines 19–21: I’m confused about mass balance here. The authors first say that plants evapotranspire infiltrated water. They then say that 70% of the percolation volume goes to streams, and 30% to groundwater. So, what percent is evapotranspiration then? Does it come from the 30% storage in groundwater? Is that storage really all "groundwater" or could there be significant unsaturated storage below the shallow TDR? Is there any basis for distinguishing groundwater use from unsaturated water use below 30cm?
-
Introduction:
- Lines 43–44: The logical connection between sentences is unclear. The "Likewise" transition needs improvement.
- Lines 48–49: Clarify what is meant by "subsoil" – does this refer to weathered bedrock layers beneath the soil?
-
Methods:
- Line 89: Provide the full name of the climate classification system (not just the code).
- Line 100: Typo – "e"stablished.
- Lines 106–109: Clarify how the storages S_prev, \theta, and SS are related. Is SS depth-integrated soil storage? What zones (saturated/unsaturated) are included?
- Line 154: Use appropriate terminology for measurement structures, e.g., v-notch weirs and H-flumes instead of "outlets" and "channels."
- Lines 158–160: ET values from decades-old studies (1980s and 1990s) are used to estimate ET in 2019? This contradicts the authors’ point that ET depends on contemporary water availability. Justify this assumption or provide more recent estimates.
- Line 178: I’m not sure what the authors are describing. The authors state that the total infiltration was compared to “retention”. Do they mean compared to (max_retention - current_retention), so as to compare the infiltration volume to the storage deficit below the maximum? Or are they comparing infiltration to depth-integrated \theta values (which, I assume, would be the definition of retention). If the latter, then I’m not sure I understand what the authors learn by comparing infiltration volumes to soil water storage volumes, and why the threshold comparison of these two values
- Line 183: Provide a description of the subsurface structure, including soil depth, the transition to fractured granite, and conceptual water storage zones.
- Equation 3: Define this equation more clearly, including piecewise conditions for infiltration exceeding the soil storage deficit.
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Results:
- Line 220: The claim that baseflow represents 2.26% of total precipitation appears incorrect. Baseflow appears to contribute much more based on the presented data.
- Lines 222–223: The method for calculating ET from groundwater is not described.
- Lines 251–255: Orthophoto analysis is presented in the results but was not described in the Methods section.
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Discussion:
- The narrative could be improved to connect key findings more logically. For example, the linkage between vegetation zonation and groundwater recharge/discharge zones needs clearer framing.
- Explicitly address the role of unsaturated weathered granite as a potential water source for vegetation.
Conclusion
The manuscript addresses an important topic, but substantial improvements are needed to:
- Provide more appropriate data to support conclusions about groundwater dynamics.
- Clearly describe all methods used, including ET estimation, baseflow separation, and orthophoto analysis.
- Address significant gaps in the conceptual understanding of the subsurface structure and processes.
- Improve the logical flow and clarity of the narrative.
Citation: https://doi.org/10.5194/egusphere-2024-3117-RC2 -
AC3: 'Reply on RC2', Landy Orozco, 06 Jan 2025
The topic is relevant to understanding the role of vegetation in influencing flows in tropical dry forests (TDFs). However, the manuscript has significant weaknesses in narrative linkages, methodological transparency, and the suitability of datasets used to address the stated objectives. In particular, the methods and data do not directly (in any way) support the paper's main focus, which is groundwater dynamics in fractured granite. R: The main focus of the paper is not the groundwater dynamics in fracture granite. Instead, the focus of this study is to quantitatively explore the processes that control the recharge and discharge of groundwater and its relationship with the other seasonal components of the hydrological cycle and the phenological conditions of vegetation and weathered soil in fracture granite. (Lines 70-79).
The paper's title emphasizes groundwater in fractured granite, yet the only subsurface measurements are from two TDR sensors placed at 0–15 cm and 15–30 cm soil depths. This does not provide any information about groundwater behavior in deeper fractured rock layers. R: The measurements of the groundwater behaviour were based on the separation of the direct and base flows from the analysis of the hydrographs obtained in the hydrological year analysed. The TDR sensors allowed us to know the behaviour of moisture in the soil and establish its relationship with the processes of formation of percolation flows. Due to the granitic nature of the study area, the fact that it is located within a Biosphere Reserve and the technical implications required; at this stage, it was not possible to instrument the working basins with piezometers in discrete fractures, so in this work we present an alternative methodology to know the behaviour of groundwater based on environmental indicators (formation of baseflows in runoff, presence of springs and distribution of vegetation).We understand that studying in detail the dynamics of groundwater in the fractured environment requires discrete drilling and instrumentation for the different fractures, which we are considering in future stages of research.
Given that groundwater is likely quite deep in this system (i would guess on the order of 10 meters at the ridge, as observed in other seasonally dry granitic systems, e.g. the Sierra Nevada in California), it is more plausible that vegetation in the sub-deciduous tropical forest (SDTF) relies on water stored in unsaturated, weathered granite layers, as is typical in similar ecosystems. R: Based on the results obtained in our article published in 2023 (Orozco-Uribe et al., 2023) in which we proposed a Conceptual Model for the study basins, the sub deciduous tropical forest is distributed in groundwater discharge zones, being an indicator of relatively near-surface water tables (depth of reach of the roots of the species that represent it), which in the study region have been reported at 5 m in the nearby coastal areas. The groundwater levels within the fractured medium in the study area show seasonal variations, so it is likely that the vegetation of the sub deciduous tropical forest obtains water from the saturated zone at different depths depending on the climatic season.
Evapotranspiration (ET) from groundwater: The authors state that ET is calculated from diel oscillations in streamflow, but no method or citation for this calculation is provided. A clear description of the approach and assumptions is necessary. R: In line 163 of the Methods section, we mention that we follow Cadol et al. (2012) for the calculation of the ET from the daily variations in the flow.
Orthophoto Analysis: While results from orthophoto analysis are presented, there is no description of this method in the Methods section. R: In the Methods section, specifically in lines 109 to 111 we describe the ortophoto analysis. A detailed analysis of the LiDAR flight-generated orthophoto was presented in the article published in 2023 (Orozco-Uribe et al., 2023).
Baseflow Separation: The baseflow separation technique is not described. Additionally, baseflow separation algorithms lack a physical basis for distinguishing between groundwater-generated and surface-generated flows. So, I fundamentally disagree with any process interpretations derived from this analysis. Even Hewlett and Hibbert acknowledged this in their original paper that used a baseflow separation method. R: Indeed, we do not mention the methodology used for baseflow separation, which was carried out using the graphical method following Gonzales et al, 2009. We will make the corresponding correction in the Methods section.
The manuscript frequently refers to the "fractured medium" and assumes water storage below 30 cm is within fractured granite (I think). However, no information is provided about the depth or structure of soils and the transition to fractured rock. R: Agree. Since this study focuses on the application of the Conceptual Model presented in our previous work (Orozco-Uribe et al., 2023), as mentioned from line 76, where a description of the depth and structure of the soils and the characteristics of the fractured environment was made, we will include a brief description of these as was done for other general characteristics of the working basins.
A figure illustrating the authors' conceptual model of the subsurface structure (soil, weathered granite, and fractured granite) would greatly clarify their assumptions and interpretations. R: Agree, we will include a representative figure of the Conceptual Model.
The first sentence contains a "then" that does not fit the sentence structure. R: Agree, we will remove the word "then" from the sentence.
Line 16: "1.179 mm" – Is this a typo? Should it be "1,179 mm" (using a comma for thousands)? R: Indeed, it is a typographical error.
Line 19: "Evapotranspiration by percolation" is incorrect terminology. Consider rephrasing to "evapotranspiration of infiltrated water." R: Infiltration is the process by which water moves through the soil surface into the soil matrix and percolation is the process by which it moves down through the profile and into the underlying weathered rock (Le Maitre et al., 1999; Sophocleous, 2004; Sen, 2015; Gabrielli & McDonell, 2018). In this line of the Abstract, we refer to the fact that the effect of evapotranspiration was reduced due to the rapid movement of water into the fractured medium through the percolation process.
Lines 19–21: I’m confused about mass balance here. The authors first say that plants evapotranspire infiltrated water. They then say that 70% of the percolation volume goes to streams, and 30% to groundwater. So, what percent is evapotranspiration then? Does it come from the 30% storage in groundwater? Is that storage really all "groundwater" or could there be significant unsaturated storage below the shallow TDR? Is there any basis for distinguishing groundwater use from unsaturated water use below 30cm? R: As mentioned in the previous point, plants take water mainly from that which is stored in the soil layers (infiltrated water), between the permanent wilting point and the field capacity; in our study area, it has been shown that 90% of the root biomass is concentrated in the first 40 cm of the soil (Rentería-Rodríguez, 1997 mentioned in Orozco-Uribe et al., 2023). The rest of the water that is nos part of this soil storage, percolates through the soil layers into the fractured medium, forming a new flow that can find an outlet to the surface locally (~70%), forming part of the baseflow. The remaining, can effectively be part of a storage in the unsaturated zone of the fractured medium or incorporated into an intermediate or regional flow based on Tóth, 2016. Certain plant species can access water stored or flowing in the fractured environment through deep roots or, as is the case with the species that make up the sub deciduous tropical forest, access the flowing water in the fractured medium that finds outlet along rivers and streams.
Lines 43–44: The logical connection between sentences is unclear. The "Likewise" transition needs improvement. R: Agree, we will improve the wording.
Lines 48–49: Clarify what is meant by "subsoil" – does this refer to weathered bedrock layers beneath the soil? R: Good observation, we will improve the wording and use a more appropriate term.
Line 89: Provide the full name of the climate classification system (not just the code). R: Agree, we will make the corresponding correction.
Line 100: Typo – "e"stablished. R: Agree, we will make the corresponding correction.
Lines 106–109: Clarify how the storages S_prev, \theta, and SS are related. Is SS depth-integrated soil storage? What zones (saturated/unsaturated) are included? R: Daily storage (S) corresponds to water within the basins (Moore, et al., 2015), i.e., water that was not used by plants or that did not drain during the calculation day, integrating the unsaturated and saturated zones. It is based on equation 3, based on the calculation of the storage of the previous day (Sprev) plus the day's precipitation (P), from which the daily sum of interception (IC + IL), streamflow (Q) and evapotranspirations (ETDTF and ETSDTF) were subtracted. Theta corresponds to the moisture content in the soil from the data obtained from the TDR sensors.
Line 154: Use appropriate terminology for measurement structures, e.g., v-notch weirs and H-flumes instead of "outlets" and "channels." R: Agree, we will make the corresponding correction.
Lines 158–160: ET values from decades-old studies (1980s and 1990s) are used to estimate ET in 2019? This contradicts the authors’ point that ET depends on contemporary water availability. Justify this assumption or provide more recent estimates. R: The evapotranspiration values of the tropical deciduous forest refer to daily rates per season (seasonal) obtained by the authors consulted based on the physiological characteristics of the ecosystem, so they are valid even if they have been carried out in previous decades.
Line 178: I’m not sure what the authors are describing. The authors state that the total infiltration was compared to “retention”. Do they mean compared to (max_retention - current_retention), so as to compare the infiltration volume to the storage deficit below the maximum? Or are they comparing infiltration to depth-integrated \theta values (which, I assume, would be the definition of retention). If the latter, then I’m not sure I understand what the authors learn by comparing infiltration volumes to soil water storage volumes, and why the threshold comparison of these two values R: At this point, we refer to the retention of water in the soil (between the point of permanent wilting and the field capacity), if the water was infiltrated, then there are no Hortonian surface flows, but are part of underground flows or storage that when they exceed the retention capacity of the soil, then they become percolation flows towards the fractured medium.
Line 183: Provide a description of the subsurface structure, including soil depth, the transition to fractured granite, and conceptual water storage zones. R: Agree, we will make the corresponding correction.
Equation 3: Define this equation more clearly, including piecewise conditions for infiltration exceeding the soil storage deficit. R: Agree, we will make the corresponding correction.
Line 220: The claim that baseflow represents 2.26% of total precipitation appears incorrect. Baseflow appears to contribute much more based on the presented data. R: In this case we refer to the first streamflow of the year (Q1) when the soil is completely dry, so the retention will be greater. Likewise, the vegetation will take a large part of the infiltrated water for its greening processes; therefore, percolation will be low. Considering this, and that the baseflow comes from the percolated water to the fractured medium, then the percentage of P that this represents will be very low. We will review our data again to corroborate it and, if necessary, we will expand our discussion in this regard.
Lines 222–223: The method for calculating ET from groundwater is not described. R: The method used was mentioned in lines 160-163 of the Methods section.
Lines 251–255: Orthophoto analysis is presented in the results but was not described in the Methods section. R: Agree, we will make the corresponding correction.
The narrative could be improved to connect key findings more logically. For example, the linkage between vegetation zonation and groundwater recharge/discharge zones needs clearer framing. R: Agree, we will conduct a new and thorough review of our discussion to improve it.
Explicitly address the role of unsaturated weathered granite as a potential water source for vegetation. R: Agree, we will conduct a new and thorough review of our discussion to improve it.
The manuscript addresses an important topic, but substantial improvements are needed to: 1. Provide more appropriate data to support conclusions about groundwater dynamics. 2. Clearly describe all methods used, including ET estimation, baseflow separation, and orthophoto analysis. 3. Address significant gaps in the conceptual understanding of the subsurface structure and processes. 4. Improve the logical flow and clarity of the narrative. R: Agree, we will consider the comments already expressed and our responses to them, we appreciate the interest, which will surely improve our manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3117-AC3
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