the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Apr 2024
Changes in mean evapotranspiration dominate groundwater recharge in semi-arid regions
Abstract. Groundwater is one of the most essential natural resources and is affected by climate variability. However, our understanding of the effects of climate on groundwater recharge (GR), particularly in dry regions, is limited. Future climate projections suggest changes in many statistical characteristics of the potential evapotranspiration (ETref) and the rainfall that dictates the GR. To better understand the relationship between climate statistics and GR, we separately considered changes to the mean, STD, and extreme statistics of the ETref and the rainfall. We simulated the GR under different climate conditions in multiple semi-arid locations worldwide. We find that changes in the average ETref have the most significant impact on GR. Interestingly, we find that changes in the extreme ETref statistics have much weaker effects on the GR than changes in extreme rain statistics. Contradictory results of previous GR studies may be explained by the differences in the projected climate statistics.
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CC1: 'Comment on egusphere-2024-433', Giacomo Medici, 11 Apr 2024
General comments
Good research in the field of groundwater hydrology that has been approached with a worldwide angle. However, important detail is missing. Please, take into account my minor points to fix the issues.
Specific comments
Line 18. “In recent years, much effort has been devoted to the analysis of the sensitivity of groundwater systems to climate change”. Add recent literature on the effects of climate change in mountain ranges, the aquifer recharge from the snow is very sensitive to the climate:
- Lorenzi, V., Banzato, F., Barberio, M. D., Goeppert, N., Goldscheider, N., Gori, F., Lacchini A., Manetta M, Medici G, Rusi S, Petitta, M. (2024). Tracking flowpaths in a complex karst system through tracer test and hydrogeochemical monitoring: Implications for groundwater protection (Gran Sasso, Italy). Heliyon, 10(2).
- Langman, J. B., Martin, J., Gaddy, E., Boll, J., & Behrens, D. (2022). Snowpack aging, water isotope evolution, and runoff isotope signals, Palouse Range, Idaho, USA. Hydrology, 9(6), 94.
Line 48. Cleary mention the 3 to 4 specific objectives of your research by using numbers (e.g., i, ii and iii).
Line 78. “l is the pore connectivity”. Your research appears to focus on porous aquifers of siliciclastic nature (plio-quaternary age?). This point is not clear by reading the manuscript.
Lines 80-81.“Sand, silt, and clay contents”. The geological nature of your aquifers have not been disclosed, see also my comment above.
Line 184. “Under some future climate predictions, the frequency of extreme events is expected to double”. Please, be more specific. Are you talking about semi-arid / arid regions?
Line 184. “Under some future climate predictions, the frequency of extreme events is expected to double”. This sentence should be expanded and moved to the discussion section.
Lines 231-240. The conclusion is too short, it needs more detail.
Lines 232-234. “Our results suggest...rainfall statistics”. The sentence is unclear and too long. Please, split it in two parts.
Line 237. “Focused processes”. Which processes? Please, be more specific.
Figures and tables
Figure 1a. You also have study sites and aquifers in highly arid settings, this is not clear in the text. You don’t have only semi-aridity.
Figure 1c. You can also report Mean Error, Mean Absolute Error and RMS in the graph.
Citation: https://doi.org/10.5194/egusphere-2024-433-CC1 -
AC3: 'Reply on CC1', Tuvia Turkeltaub, 21 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-433/egusphere-2024-433-AC3-supplement.pdf
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AC3: 'Reply on CC1', Tuvia Turkeltaub, 21 Jun 2024
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RC1: 'Comment on egusphere-2024-433', Anonymous Referee #1, 20 Apr 2024
This manuscript investigates the expected effects of climate change on groundwater recharge. The manuscript presents a model (based on 1D Darcy-Richardson that simulates diffuse recharge) that is modeled with current climate conditions (which show generally good agreement with the data in the selected semi-arid regions) and synthetic climate (which is essentially the historical climate but with its statistical properties changed). The model also assumes that ET is limited to evaporation from the topsoil, soil retention curves and hydraulic conductivity are according to Van Genuchten-Mualem (see Eq. 2-3). From the model simulations the paper concludes that
- changes in the average ETref have the most significant impact on GR.
- Changes in the extreme ETref statistics have much weaker effects on the GR than changes in extreme rain statistics.
- In addition, it is stated, that contradictory results of previous GR studies may be explained by the differences in the projected climate statistics.
While the paper has several useful elements, I have a few concerns that inhibit me from recommending publication in its current format. My main concerns are:
- A major conclusion seems to misrepresent actual recharge changes. Namely, it is concluded that mean ET has a bigger influence on recharge than P, but the latter is only true when the ratio of recharge to precipitation is considered, but absolute (in mm/y) or relatively (in %) changes in recharge are very likely much bigger due to precipitation changes. Such an amplifying effect of precipitation on recharge (versus PET on recharge) is because changes in precipitation both the ratio of this precipitation becoming recharge, and the total amount of precipitation that can become recharge. In contrast, changes in PET only affect the ratio of precipitation becoming recharge. The latter is also highlighted in cited most recent work on the climate sensitivity of recharge
- The model claims to be accurate within 5% of recharge but given that recharge/rainfall is typically very low in these arid places so being within 5% can still mean the recharge is off by a lot (for example several 100%). These uncertainties are not reflected in the projections. In addition, several data points appear to exceed the 5% error? More generally it is unclear why the climate projections can be considered accurate?
- The mathematical derivation (Eq. 6-9) is applicable for ET but not for ETref. The argument that is made (that ET is proportional to the ETref) to circumvent this problem is not valid in semi-arid systems. If we follow the Budyko framework as a reference, one can see that in extremely energy-limited systems indeed ET is expected to (almost exactly) linearly (and one-on-one) scale with ETref. However, in more arid places changes in ETref will not be associated will similar ET changes, nor will be their relationship be linear.The expectation of how this behaves can be exemplified using simplified version of the Budyko framework: E/P= 1-exp(-φ) = E/P= 1-exp(-Eref/P). Thus: E= P*(1-exp(-ETref /P)). Thus, dE/d(ETref )= P/(P - exp(ETref/P) P + ETref), which is a nonlinear function for ETref>P. Therefore, the physical relevance of the derivation provided in the manuscript remains unclear to me.
- The model assumes that all evaporation is soil evaporation and no overland flow. It is unclear why this is realistic even with somewhat sparse vegetation. Most of these regions will still have vegetation that evaporates relevant parts of total ET
- The manuscript states surprise that the extreme ETref statistics have much weaker effects on the GR than changes in extreme rain statistics. Isn’t this a result in line with obvious expectation?
- The use of symbols is highly confusing with GR standing for “recharge” and R for “rain”. However, to readers it would make interpretation a lot easier if a single letter was used for a process, and maybe a subscript is needed for further specifications. This avoids that GR reads as G times R. In addition, the use of P for precipitation, and R for recharge seems slightly more conventional. Such formulation would also come in handy when the symbols are used to derive new equations (Eq. 6-9).
Citation: https://doi.org/10.5194/egusphere-2024-433-RC1 -
AC1: 'Reply on RC1', Tuvia Turkeltaub, 04 Jun 2024
Dear Reviewer
We thank you for the careful assessment of our manuscript and the constructive criticism. In the attached PDF file, we address your comments one by one. For clarity, we use a black font for the quoted comments and a blue italic font for our responses. Note that the comments in green font refer to changes we made in the text.
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RC2: 'Comment on egusphere-2024-433', Anonymous Referee #2, 12 May 2024
Dear Editor, I have carefully read the submitted manuscript, which aims to evaluate the impact of the variation in mean and standard deviation of the statistical distributions of rainfall (R) and potential evapotranspiration (ETref) on groundwater recharge (GR). I am in trouble, because the objective of the study seemed quite interesting and totally in line with the HESS journal topics, but I note that behind the desire to internationalize the study with global datasets (a condition rightly much appreciated by the journal) there are major uncertainties on the definition of the parameters (GR), on the relevance of the simulations to the reality of the physical data (ETref and R variability) and on the initial hypotheses (infiltration and soil heterogeneity considered only for the first cm of topsoil). I don't want to sound too critical, but I think there are some gaps in this study, which need to be fixed at least by defining the limits more and also recalibrating the text and parameter definition. For example, regarding the GR parameter there is a big divergence between what I mean (and also other authors) and what I read here. I suggest the authors to keep this reference in mind: https://doi.org/10.1017/CBO9780511780745.
The last analytical part which considers the derivative of the groundwater recharge (GR) and Rainfall (R) ratio (GR/R) with respect to ET is not correct, in my opinion. Actual evapotranspiration depends both on rainfall and potential evapotranspiration and cannot be considered as independent. Based on that part, conclusion section could be misleading too.
All specific comments are available in the attached pdf file.Furthermore, I would like the authors to explain more in depth the differences between this new study and the previous one (also published on HESS, https://doi.org/10.5194/hess-27-289-2023).
Finally, it would be desirable for the authors to make the raw datasets and part of the simulations available and public as supplementary material. Surely, it would also be helpful for us reviewers to confirm or resolve some doubts.
In conclusion, my opinion is that the manuscript is not acceptable in the present form for publishing in HESS and should be carefuly revised, considering some key concepts differently, better presenting both the methodology and the application limits. All the raised issues should be discussed and fixed before considering the publication.-
AC2: 'Reply on RC2', Tuvia Turkeltaub, 21 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-433/egusphere-2024-433-AC2-supplement.pdf
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AC5: 'Reply on RC2', Tuvia Turkeltaub, 21 Jun 2024
Dear Reviewer
We thank you for the careful assessment of our manuscript and the constructive criticism. In the attached PDF file, we address your comments one by one. For clarity, we use a black font for the quoted comments and a blue italic font for our responses. Note that the comments in green font refer to changes we made in the text.
Citation: https://doi.org/10.5194/egusphere-2024-433-AC5
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AC2: 'Reply on RC2', Tuvia Turkeltaub, 21 Jun 2024
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CC2: 'Comment on egusphere-2024-433', Christian Leduc, 29 May 2024
The present paper addresses the impact of changes in rainfall distribution and potential evapotranspiration on groundwater recharge (GR) in semi-arid areas. In their introduction based on the scientific literature, the authors note that "no conclusive generic outcomes can be drawn regarding the relationship between changes in climate conditions and the resulting changes in GR rates", "it is unclear whether the climate variability is amplified or smoothed in the GR response" and "even the trend of the GR response is uncertain". These assessments are reasonable and I fully agree with them.
Several factors may explain this large range of GR responses to present climatic changes, which leads to apparent contradictions in their recent evolution. The first explanation is the variability of environmental conditions in the natural state. The second explanation is the multiplicity of scientific approaches (various methods using various types of data sets monitored at different scales in space and over time), which logically lead to heterogenous results. Moreover, depending on methods, the calculated GR represents an integration over a very variable time. The third explanation is that in the last decades GR may have changed a lot as a consequence of the climate change and the multifaceted human modifications of semi-arid landscapes (e.g. changes in land use and land cover; water conservation works). The GR estimates found in the literature aggregate values from various stages of this evolution between areas still in a mostly natural state and others deeply modified. Depending on areas, direct human intervention may be a much stronger factor than climate change. For instance, the increase in GR by one order of magnitude in southwestern Niger (e.g. Favreau et al., 2009) and by two orders of magnitude in eastern Australia (e.g. Allison et al., 1990) was explained by a change in the vegetation cover only. Therefore raw data from the literature should be used with a cautious reference to their specific contexts, which is not really the case in the submitted text.
An important concern is the geographical extent of this work. The authors used GR estimates from 200 semi-arid locations in different continents, in a wide range of soils and climate conditions. The annual rainfall in the sites considered ranges from 180 to 1044 mm, with more than one half between 400 and 600 mm. In fact, 60 % of the sites are in Australia, 20 % in Africa, 10 % in North America and 10 % in the other continents, which differs significantly from the distribution of semi-arid areas in the world. Does this selection bias the final conclusions? The Figure 1-a will probably surprise many readers who usually see a much larger extent of semi-arid areas in global maps; this singularly restricted coverage should be justified.
Another important concern is the very restrictive assumptions for the calculation: (i) GR occurs only through diffuse recharge (i.e. without any focused recharge); (ii) transpiration is negligible vs. evaporation; (iii) surface runoff is negligible; (iv) there is no preferential flow in the unsaturated zone. The fist assumption contradicts the observation that focused and diffuse recharge often coexist in the same area. Their respective proportions depend on local geomorphological conditions (e.g. Cuthbert at al., 2019). At the global scale, it is generally accepted that the proportion of focused recharge increases with aridity and as a consequence the driest semi-arid regions would be excluded from this calculation. The second assumption neglects transpiration uptake while the vegetation cycle in semi-arid areas is closely linked with the rain distribution, which is also the driver for GR. The third assumption requires to limit the application of the calculation to very flat areas and/or very low rainfall. The fourth assumption requires a very poor biological activity (roots and fauna).
These four assumptions together are so constraining that the geographical extent of the concerned semi-arid areas is probably very small. The practical relevance of this text appears therefore limited and the added value for researchers working on groundwater in semi-arid areas may be seriously questioned. The authors are conscious of these weaknesses and in their conclusion they mention the possibility of extending their work, but this last precaution is not enough to give the text a convincing strength.
Allison et al., 1990. doi.org/10.1016/0022-1694(90)90030-2
Cuthbert et al., 2019. doi.org/10.1038/s41586-019-1441-7
Favreau et al., 2009. doi.org/10.1029/2007WR006785
Citation: https://doi.org/10.5194/egusphere-2024-433-CC2 -
AC4: 'Reply on CC2', Tuvia Turkeltaub, 21 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-433/egusphere-2024-433-AC4-supplement.pdf
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AC4: 'Reply on CC2', Tuvia Turkeltaub, 21 Jun 2024
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