STEMMUS-MODFLOW v1.0.0: Integrated Understanding of Soil Water and Groundwater Flow Processes: Case Study of the Maqu Catchment, north-eastern Tibetan Plateau
Abstract. How to efficiently and physically integrate the soil water dynamics with groundwater flow processes has drawn much attention. We present a coupled soil water-groundwater model, considering the two-way feedback coupling scheme, and verified its performance using two synthetic cases (using the fully 3D variably saturated flow (VSF) model simulations as the ‘reference’) and one real catchment case (using the groundwater table depth and soil moisture profile measurements). By the cross-validation between the observations and various model simulations, the two-way coupling approach is proven physically accurate and is applicable for large-scale groundwater flow problems. Compared to the simulation by groundwater model alone (i.e., only MODFLOW), the coupling of MODFLOW with one soil column reduced the overestimation of groundwater table simulation (taking the VSF model simulations as the reference). The results were further improved as more soil columns were used to represent the heterogeneous soil water-groundwater interactions. Compared to the HYDRUS-MODFLOW, the two-way coupling approach produces a similar spatial distribution of hydraulic heads while better performs in mimicking the temporal dynamics of groundwater table depth and soil moisture profiles. We attribute the better performance to the different coupling strategies across the soil-water and groundwater interface. It is thus suggested to adopt the two-way feedback coupling scheme, together with the moving phreatic boundary and multi-scale water balance analysis, to maintain physical consistency and reduce coupling errors. The realistic implementation of the vadose zone processes (with STEMMUS), coupling approach, and spatiotemporal heterogeneity of soil water-groundwater interactions were demonstrated critical to accurately represent an integrated soil water-groundwater system. The developed STEMMUS-MODFLOW model can be further equipped with different complexities of soil physics (e.g., coupled soil water and heat transfer, freeze-thaw, airflow processes), surface hydrology (snowfall, runoff), soil and plant biogeochemical processes, towards an integrated "from bedrock to atmosphere" modeling framework.
Lianyu Yu et al.
Status: open (until 29 Jun 2023)
- RC1: 'Comment on gmd-2022-221', Anonymous Referee #1, 05 Jun 2023 reply
Lianyu Yu et al.
2018–2019 dataset in Maqu, the Tibetan Plateau https://doi.org/10.17026/dans-z6t-zpn7
Multi dimensional observation data set of single watershed in Maqu County, Qinghai Tibet Plateau (2018–2019) https://doi.org/10.11888/Hydro.tpdc.271221
Model code and software
Simultaneous Transfer of Energy, Momentum and Mass in Unsaturated Soils (STEMMUS) https://github.com/yijianzeng/STEMMUS
Lianyu Yu et al.
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The presentation of the aims and of the methods are not clear enough to allow a proper review. The english level should also be improved. Thus I recommend to reject this manuscript, and to encourage the authors to resubmit after a thorough rewritting step.
General comments :
- The abstract is unclear, it should allow to understand what in this work is compared : coupling approaches or simulation softwares, or both. If comparisons of both approaches and softwares are made, they should be articulated with each other (different coupling approaches may be implemented using either of the considered softwares).
- To the knowledge of the reviewer, an important example of hydrological model that couples dimensionnaly heterogeneous descriptions of flow in the saturated zone and in the unsaturated zone is MIKE-SHE (e.g.: Graham and Butts, 2005), which is for instance included in recent international benchmarking efforts for physically based hydrological modeling (e.g.: Kollet et al., 2016). The fact that works related to MIKE-SHE do not appear in the references of the manuscript make me think that the bibliographical survey on which the presentation of the background of the study is done should be consolidated.
- Since I did not understand the interest that the authors see for the proposed approach (I am not saying that there is no interest ; it is just not stated), and since I did not understand the approach itself, I stop my review at the section 2.2.4. Obviously in such circumstances I will not be able to review the application cases and their discussions.
l 22 « and physically » clumsy expression (also l 27 « proven physically accurate »)
l 23-24 : « We present … and verified » : time concordance problem
l 32 : « HYDRUS-MODFLOW » : do you mean the variably saturated flow reference soultion, or something else ? If this is, as I think, something else, it should be explicited.
l 38-39 : « spatiotemporal heterogeneity of soil water-groundwater interactions » sounds strange.
L 43 : “from bedrock to atmosphere” Why is there quotation marks here ? Are you citing some one ?
l 115 : « In this study, we coupled the soil water model (STEMMUS) with the groundwater model (MODFLOW) in a two-way feedback manner. ». The authors should explicitely state here why did they choose these softwares and this coupling method. What is the added value compared to the existing literature ?
L 120 : section 2 should be splitted, with a dedicated section for the governing equations and the resolution methodologies, and another section dedicated to the set up of the test cases.
L 125-126 : « MODFLOW assumes that the ground water is with constant density and the porous medium is noncompressible » : the scientific english here should be improved. « assumes that the density of water is constant and that the porous medium is incompressible » would sounds better I think. Overall I think that the level of english langage should be improved. From now on I stop pointing examples of this kind.
L 161 : « After a certain number of iterations, STEMMUS model will converge ». What is the involved iterative process ? Is STEMMUS run until equilibrium, is this a fixed point method, is this something else ? This key point should be explained in detail.
L 156 Figure 1 : The figure is too rich with both informatic (e.g. : allocate array storage) and algorithmic (e.g. : Solve equation, converge yes/no …) for the two models, and unsufficiently explained. For instance what is the ‘stress periods’ in MODFLOW ? What is the loop of soil profil in STEMMUS ? Of course this very important part of the paper should be as complete and informative as possible, and this requires more explanations.
Section 2.2.2 : Hard to follow. The explanation are supposed to be given in the appendix A, which is not understandable (for instance in Figure 2a where are Zs and Zt(t) mentionned in l 728 ?).
l 180-181 : « Within the given time step, STEMMUS model will adapt its own time step (variable between 10 0 -10 3 s) to the converged simulation results. » Clumsy. I think here the authors are making an assumption of separation of time scales, and this should be clarified and argumented.
L 189-191 : « By scale matching of the water budget components at the interface of respective scales, water balance is conserved and the upper boundary flux for the groundwater flow model can be achieved. » I don’t understand what scale matching means here. In this section once again the authors point the reader to the appendix A, but the term scale matching does not appear in this appendix.