Timescale of Groundwater Recharge in High Percolation Coastal Plain Soils

Round 1

Reviewer 1 Report

General Comments

1. The paper deals with the actual problem of recharge formation by analyzing the moisture movement in the unsaturated zone. This approach is of interest to a wide range of specialists dealing with shallow groundwater and the vadose zone.

2. The chosen research methodology, combined with numerical experiments using Hydrus 1D, is generally satisfactory to me, and the results obtained are of a certain novelty.

3. the main drawback of this paper is the practical impossibility of using the obtained approximations (9-16) by other researchers, if the set of van Genuchten parameters differs from the parameters characteristic of the Myakka soil.

4. Instead of scaling (7-9), the authors should use scaling that includes the whole set of van Genuchten parameters, not only hydraulic conductivity and Zcz. Examples where capillary and gravity lengths and dimensionless time are functions of the van Genuchten parameter set can be found in Lehmann et al. 2008, Pozdniakov et al. 2019, and other publications.

5. I believe that in connection with the above-mentioned, the authors should explicitly state in the conclusion section their opinion whether the approximations obtained by them are universal.

specific remark

in equation 8, the parameter Zcz is introduced for the first time. It is necessary to explain explicitly what it is and to show this length in Fig. 1. This figure is labeled capillary zone. And we need to explicitly state that it is Zcz

Literature

Peter Lehmann, Shmuel Assouline, and Dani Or Characteristic lengths affecting evaporative drying of porous media  Phys. Rev. E 77, 056309 - Published May 16, 2008

Pozdniakov, S. P., Wang, P., & Lekhov, V. A. (2019). An approximate model for predicting the specific yield under periodic water table oscillations. Water Resources Research, 55. https://doi.org/ 10.1029/2019WR025053

Author Response

Dear Reviewer, 

We truly appreciate the comments and literature suggestion. Please see the attachment of our response.

Author Response File: Author Response.pdf

Reviewer 2 Report

water-2966993

Timescale of Groundwater Recharge in Coastal Plain Soils

Authors: Qing Du, Mark Ross

Dear Authors,

Thank you for this contribution. The paper is interesting and scientifically sound. I propose some further investigations to improve its impact and deepen the understanding and the mutual connection among physical phenomena.

Many natural phenomena can be described by employing exponential functions, also called Gibbs’ Distributions.

1)     Please provide exponential interpolations of your results. In particular:

-        normalize water table depths in Fig. 3a-d and evaluate the exponent of Gibbs’ functions for each panel.

-        Repeat the estimation for the arrival time, hence for Figures 4, 5, and 6; for the latter ones, an exponential fitting seems more appropriate than linear or quadratic ones.

-        Repeat the estimation for Figures 8 and 9 (water table rise), 10 and 11 (completion time vs depth), and 13 (recharge).

The power of Gibbs’ distribution stems from the fact that they link different phenomena. This seems to me enabling to empower your research, since you come up with an estimation of exponential exponents upon normalized variables, even if they are different. Please adequately comment on the range of your estimated exponents and how much they vary for each considered variable. Note that these operations are easy (e.g. in Excel), but I suggest improving the quality and the cleanliness of the plots by doing them in a MATLAB or R environment rather than in Excel.

 2)     Introduce appropriately Gibbs’ Distributions in the Introduction, employing the references to support their usage: Troutman and Karlingen (1992); Schiavo (2023).

 3)     Can your analysis be employed within a stochastic framework using HYDRUS, as well as done e.g. for the MODFLOW environment by the following Authors? Please introduce this aspect in the Introduction and as a final (future) research direction, upon reference: Janetti et al. (2019).

 After these operations are done, the paper can be indubitably published. Good work, and best regards.

Further references:

Janetti, E. B., Guadagnini L., Riva, M., and Guadagnini, A. (2019). Global sensitivity analyses of multiple conceptual models with uncertain parameters driving groundwater flow in a regional-scale sedimentary aquifer. Journal of Hydrology 574 (544-556). https://doi.org/10.1016/j.jhydrol.2019.04.035

Schiavo, M. (2023). Entropy, fractality, and thermodynamics of groundwater pathways. J. Hydrol. 617 (4), 128930. DOI: 10.1016/j.jhydrol.2022.128930

Troutman, B.M., Karlinger, M.R. (1992). Gibbs’ Distribution on drainage networks. Water Resour. Res. 28 (2), 563–577. https://doi.org/10.1029/91WR02648

Author Response

Dear reviewer, we appreciate your time taken into giving different insights into our paper, please see the attachment of our response.

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript entitled ‘Timescale of groundwater Recharge in Coastal Plain Soils’ focuses on influence of intensive rainfalls on water table level and water content in unsaturated zone of sandy soils in Florida. The parametric dependencies between time and depth of wetting front arrival for different water table depth and hydraulic conductivity were proposed. The authors present a deep knowledge on vadose zone processes and groundwater recharge in shallow aquifer, however I have some concerns according to the HYDRUS-1D model. There is no information about vegetation cover or root water uptake set up in the model description (chap. 2), while Fig. 6b shows some data according to root zone (?). If I understood correctly, the authors intention was to represent a real soil profile thus assuming no evapotranspiration and zero-flux boundary condition at the bottom seems not appropriate in this case (L:179-181). More insightful explanation would be necessary here. More information about soil parameters’ source is also required because L:185-187 are confusing, if the soil data was obtained by laboratory tests or calibration of numerical simulation. The model discretization set to 101 nodes is reasonable for shallow profiles, but for 10 m deep profile it results in 10 cm intervals between the nodes, which is rather outstanding – usually the interval ought to be no larger than 2 cm. My second general remark is according to figures – please improve the quality, it is difficult to read, especially the legends. The dependencies and the parameters presented in the manuscript as a novelty need to have units defined. Below you can find the minor comments:

L:199-208 – I assumed the model discretization for the main simulations is identical with the preliminary simulations, however it should be specified in the text;

Figure 1 – the legend is lacking;

Table 1 – value of 224 is impossible for the L parameter, I guess it is a mistake;

L:252-253 – I do not quite understand this statement, it needs to be rephrased;

Figure 2 – there is a problem with axis;

L:346-347 – I am not sure what variable was extended – is it the final time of the simulation performance or set up according to time step length? 5 years is not the extreme time scale, please provide an explanation.

L:418, Eq. 15 – adding the units is necessary here. For 1D profile it does not matter if the P’, P, Scz are volume or length (depth), but for 2D/3D domain it could. Why the P’ is described as P divided by Scz instead of difference between this volumes (P-Scz)?

Tables 6-7 – the tables’ descriptions need to be improved, it is not clear that ta was calculated using proposed formulas (based on HYDRUS simulations) and then compared to simulated results. The 100% relative error for 30 cm depth ought to be commented in the text.

The manuscript is easy to follow, however the authors could use more scientific language and avoid laconic explanations in the text.

Author Response

Dear reviewer, we are thankful for your review and comments, please see the attachment of our response.

Author Response File: Author Response.pdf