the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Dec 2025
Quantification of Delayed Recharge by Soil Surface and Riverbed Infiltration in a Deep Groundwater Depression Zone in the North China Plain
Abstract. Agriculture on the North China Plain (NCP), home to over 300 million people, heavily relies on groundwater extraction to feed its irrigation systems. This has created a large groundwater depression zone up to 80 m deep, severely limiting sustainable groundwater extraction and crop-production. Effective recharge is essential to restore this depleted zone and secure future sustainability. Few studies, however, have quantified recharge delays and efficiencies in deep vadose zones with complex lithology. Here we simulated infiltration times and percolation velocities in the Ningbailong groundwater depression zone, a typical overexploited site in the NCP using HYDRUS-1D with measured borehole lithology and hydro-meteorological data. Two infiltration modes were considered: precipitation-fed and riverbed infiltration. Spatial distributions of infiltration times and percolation velocities were obtained, and recharge efficiencies were compared between these two infiltrations. Results showed that times for precipitation-fed recharge averaged 446 days and varied with lithology and thickness, from 10 days in western Taihang foothills (dominated by coarse sands) to 1,395 days in central/eastern plains (finer clays and loams). Riverbed recharge was markedly faster, averaging 91 days, indicating higher efficiency than precipitation under equivalent lithological conditions. Regression equations were derived to predict percolation velocities from vadose zone thickness and soil particle fractions. These findings elucidate how vadose zone thickness and lithology amplify recharge lags and control recharge efficiency. They also highlight the potential for managed aquifer recharge strategies, such as constructing infiltration basins for flood capture, offering strategies to reduce groundwater over-exploitation in similar depression zones.
Competing interests: The authors have the following competing interests: Yonggen Zhang is a member of the editorial board of Hydrology and Earth System Sciences. The authors declare that they have no other competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-5651', Nima Zafarmomen, 03 Dec 2025
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RC1: 'Comment on egusphere-2025-5651', Anonymous Referee #1, 22 Dec 2025
In this paper, the authors present a solid and well-structured study addressing an important issue in groundwater sustainability in the North China Plain. The use of HYDRUS-1D to analyze recharge delays in thick vadose zones is appropriate and well justified. In addition, the comparison of recharge efficiency and time lag under identical vadose zone conditions for two distinct recharge modes is novel and provides clear practical relevance for managed aquifer recharge (MAR) planning in groundwater depression zones.The results, as currently presented, also seem reasonable. I encourage the authors to further extend their analysis to address several concerns and questions that other readers may also raise regarding the flexibility of the approach and the range of conditions under which the method can be reliably applied.
My main concerns are summarized as follows:
1. The authors clearly highlight heterogeneity in horizontal boundary conditions across the North China Plain (Table 1), where some locations are characterized by precipitation–evaporation–infiltration processes, while others are governed by constant-head riverbed infiltration. At the same time, strong vertical heterogeneity in soil types is emphasized (Figure 2). Given these heterogeneities, it is not fully clear whether a purely one-dimensional modeling framework can adequately resolve the dominant flow processes. For example, when infiltrating water encounters low-permeability layers at depth, lateral flow along stratigraphic interfaces may occur. Such lateral redistribution could potentially influence infiltration times and recharge efficiency. Under these heterogeneous horizontal boundary conditions, lateral soil water flow may not be negligible, unlike in traditional large-scale studies where lateral flow is often assumed to be insignificant.The manuscript would therefore benefit from a discussion of the potential magnitude of lateral flow and its implications, as neglecting horizontal flow may limit the applicability of a one-dimensional approach in settings with heterogeneous boundary conditions.
2. Equations (2)–(6) describe the van Genuchten–Mualem constitutive relationships. However, the formulation appears inconsistent in places, and some parameters (e.g., θₛ and θᵣ) are not clearly defined when first introduced. Clarifying the parameter definitions and ensuring consistency with standard van Genuchten notation would improve transparency and reproducibility.
3. The terms “infiltration time” or “recharge time” are used throughout the manuscript. I recommend explicitly defining these quantities early in the Methods section, preferably in mathematical form, and using the terminology consistently thereafter to avoid ambiguity.Citation: https://doi.org/10.5194/egusphere-2025-5651-RC1 -
RC2: 'Comment on egusphere-2025-5651', Anonymous Referee #2, 13 Jan 2026
This manuscript investigates the mechanisms of soil water movement and groundwater recharge in the North China Plain, a region facing significant water scarcity issues. The authors employ numerical simulations combined with multiple regression analysis to quantify the influence of vadose zone thickness, soil texture (clay and sand fractions), and lithology on percolation velocities and recharge rates. The study compares different infiltration modes (such as precipitation vs. managed aquifer recharge/riverbed infiltration) and aims to provide theoretical support for sustainable groundwater extraction and crop production. The topic is highly relevant to regional water resources management and addresses a pressing global concern regarding aquifer depletion. But there are significant methodological concerns that must be addressed to ensure the validity of the results.
General comments:
- Since measured initial soil water content was unavailable, the reliability of the simulation results depends heavily on the sufficiency of the spin-up period. It is unclear from the current text whether the spin-up period has made the initial state reached a dynamic equilibrium state, especially for deep soil layers. The authors should provide justification or graphical evidence (e.g., time series of soil water content or pressure head deeper than a certain depth at the profile) to demonstrate that the model achieved a robust equilibrium prior to the main simulation period.
- The multiple regression models employ clay, sand fractions, and depth as independent predictors. Since soil textural components are compositional data (summing to 100% with silt, clay and sand), there is an inherent negative correlation between these variables. To ensure the robustness of the regression coefficients presented in Tables A.1 and A.2, please check the independence of the input variables or calculate the Variance Inflation Factors for the predictors. If high multicollinearity is detected, the authors should discuss how this affects the physical interpretation of the coefficients.
- The Conclusions suggest the construction of recharge basins; however, the implications for groundwater management could be further strengthened. Given the limited land resources in the North China Plain, does this recommendation adequately consider land-use constraints?
Specific comments:
- Line 18 “…sustainable groundwater extraction and crop-production,” “Crop production” is typically not hyphenated unless used as a compound modifier before a noun.
- Line 19, The word “however” is redundant following “few studies”.
- Line 23 “…compared between these two infiltrations,” the word “infiltrations” is a non-standard pluralization in this context. Please revise “compared between these two infiltrations” to “compared between these two infiltration modes”.
- Line 33, there is a subject-verb agreement error. “…the depletion of aquifers have become…” should be corrected, as the subject “depletion” is singular.
- Line 33, in the same sentence, “pressing global concerns” should be changed to “a pressing global concern” to match the singular subject.
- Line 43, In soil science, the standard term is “matric potential.” Please change “matrix potential gradient” to “matric potential gradients”.
- Line 118, A preposition is missing in the phrase “holistic understanding soil water movement.” Please change it to “holistic understanding of soil water movement”.
- Line 198, it should be corrected to “and t represents time”.
- Line 202, “set up” should be written as two words when used as a verb.
- Line 244, in the sentence “…and then apply the same average water level data…” the verb “apply” should be in the past tense to match the preceding “were obtained”.
- Line 328, for more formal academic tone, please change “interplay between these factors in controlling water movement” to “interaction between these factors in governing water movement.”
- Line 335, to ensure grammatical parallelism with “thickness,” please change the adjective “lithological” to the noun “lithology”.
- The fonts in Figures 5 and 8 are inconsistent with those in the manuscript. If there is no special meaning, it is recommended to make them consistent with the fonts of the other figures.
- In the titles of Figures 5 and 8, the unit of “Groundwater recharge time” should be marked as (d).
Citation: https://doi.org/10.5194/egusphere-2025-5651-RC2
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- 1
The paper addresses an important and very topical problem: delayed recharge in deep vadose zones within a major groundwater depression cone in the North China Plain, comparing precipitation-fed vs riverbed recharge using HYDRUS-1D plus borehole lithology. The regional perspective and explicit focus on lag times and percolation velocities are valuable and fit well within hydrology / groundwater journals. I recommend it for publication after considering below comments:
Abstract
The phrase “two infiltration modes were considered: precipitation-fed and riverbed infiltration” could be tightened to “precipitation-fed soil infiltration and riverbed infiltration”.
When mentioning the regression equations, briefly state the key predictors (vadose zone thickness and particle fractions) to give the reader more context.
Introduction
Some paragraphs are quite long and dense (e.g., lines 41–64, 85–110). Consider splitting into shorter paragraphs to improve readability.
When you review past work (HYDRUS applications, global lag studies), explicitly state the remaining gap you are addressing (combined effect of deep vadose zones, complex lithology, and comparison of two recharge sources under identical profiles). You do this, but it could be more sharply framed at the end of the Introduction.
Study area
It might be helpful to explicitly mention average annual precipitation and reference ET, if available, to characterize the climate quantitatively.
The description of boundaries (Taihang Mountains, Shijiazhuang, Hengshui) is good, but consider adding one sentence stating dominant land use (e.g., double cropping, wheat–maize rotation) to connect with the root uptake assumptions.
Data section (Table 1)
“Depth (cm)” is given for boreholes, but values like 8,080 cm (= 80.8 m) etc. Make clear that these are vadose zone thicknesses down to shallow groundwater table or borehole depth; the phrase “Depth (cm)” is ambiguous.
You might add a column indicating vadose zone thickness vs. total borehole depth if they differ.