Ju et al. (2025) Impact of groundwater depth on crop coefficient: An improved evapotranspiration model

Identification

• Journal: Agricultural Water Management
• Year: 2025
• Date: 2025-12-06
• Authors: Qin Ju, Yongjian Ding, Tongqing Shen, Huiyi Cai, Yining Wang, Junliang Jin, Huibin Gao, Shiqin Xu, Yanli Liu, Guoqing Wang
• DOI: 10.1016/j.agwat.2025.110060

Research Groups

• State Key Laboratory of Water Disaster Prevention, Hohai University, Nanjing, China
• Department of Earth, Environmental and Geographic Sciences, University of British Columbia (Okanagan Campus), Kelowna, Canada
• State Key Laboratory of Water Disaster Prevention, Nanjing Hydraulic Research Institute, Nanjing, China
• Fuzhou Research Institute of Sustainable Development in Cities Ltd., Fuzhou, China
• Wantian Township People’s Government, Kecheng District, Quzhou, China
• Hydrology, Agriculture and Land Observation (HALO) Laboratory, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia

Short Summary

This study investigated the exponential relationship between groundwater depth and actual crop evapotranspiration (ETc act) for winter wheat, developing a new Groundwater–Meteorology-Based Actual Crop Evapotranspiration Model (GW–M model) that significantly improves ETc act estimation accuracy in shallow groundwater regions.

Objective

• To investigate the influence of groundwater depth and meteorological factors on actual crop evapotranspiration (ETc act) across different growth stages of winter wheat.
• To develop an ETc act model that integrates groundwater depth with key meteorological variables based on experimental observations.

Study Configuration

• Spatial Scale: Field experiment conducted at the Wudaogou Hydrological Experimental Station in the Huaibei Plain, Anhui Province, China. The experiment utilized 62 large-scale in-ground lysimeters, each with a surface opening area of 0.3 square meters and a net soil depth of 5.5 meters. Winter wheat (Triticum aestivum L., cultivar Zhengmai 136) was grown under controlled groundwater depths ranging from 0.2 meters to 5.0 meters in lime concretion black soil and fluvo-aquic soil.
• Temporal Scale: A four-year field experiment from 31 October 2019 to 31 May 2023. ETc act measurements were taken daily for shallow groundwater levels (0.2–1.0 meters) and every five days for deeper levels (1.0–5.0 meters). Meteorological variables were continuously recorded at 10-minute intervals, and soil temperatures were manually measured three times daily.

Methodology and Data

• Models used:
  • Groundwater–Meteorology-Based Actual Crop Evapotranspiration Model (GW–M model) (developed in this study).
  • FAO-56 Penman–Monteith equation (for reference evapotranspiration, ETo).
  • FAO-56 Kc–ETo method (for comparative analysis).
  • Temperature effect method (for comparative analysis).
  • Cumulative crop coefficient method (for comparative analysis).
  • Radiation-temperature method (for comparative analysis).
  • Geographical detector analysis (to identify influential meteorological factors).
  • Sobol sensitivity analysis (to quantify model output sensitivity to input variables).
• Data sources:
  • Field experiment observations: Actual crop evapotranspiration (ETc act) measured using non-weighing lysimeters.
  • Automatic weather station: Air temperature (K), relative humidity (dimensionless), sunshine duration (s), precipitation (m), wind speed (m s⁻¹), net radiation (W m⁻²), and soil heat flux (W m⁻²).
  • Manual measurements: Soil temperatures (K) at depths of 0, 0.05, 0.10, 0.15, 0.20, 0.40, 0.80, 1.60, and 3.20 meters.
  • Soil physical properties: Lime concretion black soil and fluvo-aquic soil characteristics (texture, clay content (%), silt content (%), bulk density (kg m⁻³), field capacity (%), permanent wilting coefficient (%)).

Main Results

• Both actual crop evapotranspiration (ETc act) and actual crop coefficients (Kc act) of winter wheat exhibited a clear exponential relationship with groundwater depth, showing a significant decreasing trend as the depth increased.
• Mean temperature (K), net radiation (W m⁻²), and sunshine duration (s) were identified as the primary meteorological drivers of ETc act across different growth stages of winter wheat, with Q-values exceeding 0.3.
• Interactions between meteorological factors and groundwater depth significantly enhanced their explanatory power for ETc act; for example, the interaction between relative humidity and daily minimum temperature yielded a q-value of 0.908 during the tillering–overwintering stage in lime concretion black soil.
• The developed Groundwater–Meteorology-Based Actual Crop Evapotranspiration Model (GW–M model) demonstrated high accuracy in reproducing ETc act, with correlation coefficient (R) values exceeding 0.7 during the fitting stage and Nash–Sutcliffe efficiency (NSE) values generally exceeding 0.7 during the validation period across various groundwater depths and soil types.
• The GW–M model achieved superior overall performance compared to the classical FAO-56 Kc–ETo method and three other empirical models (temperature-effect, cumulative crop coefficient, and radiation–temperature methods).
• Sobol sensitivity analysis indicated that groundwater depth was the dominant factor influencing actual evapotranspiration, and interactions between meteorological variables and groundwater depth were non-negligible, exceeding their corresponding main effects.

Contributions

• Quantified the exponential relationship between groundwater depth and actual crop evapotranspiration (ETc act) and actual crop coefficients (Kc act) for winter wheat based on a four-year field experiment.
• Developed an improved Groundwater–Meteorology-Based Actual Crop Evapotranspiration Model (GW–M model) that explicitly integrates groundwater depth and key meteorological factors into the crop coefficient parameterization.
• Demonstrated the superior accuracy and adaptability of the GW–M model compared to existing methods for simulating ETc act in shallow groundwater regions.
• Provided a flexible and scalable modeling framework for simulating ETc act under variable groundwater conditions, offering theoretical support for regional-scale agricultural water management.
• Enhanced the understanding of the pivotal role of groundwater depth and its interactions with meteorological factors in regulating ETc act dynamics.

Funding

• National Natural Science Foundation of China (Grant No. 52179013, 52525902, 52325902)
• Open Research Fund Program of National Key Laboratory of Water Disaster Prevention (Grant No. 2024490611)
• Open Research Fund of National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety (Grant No. 1524020003)
• China Scholarship Council (Grant No. 202406710130)

Citation

@article{Ju2025Impact,
  author = {Ju, Qin and Ding, Yongjian and Shen, Tongqing and Cai, Huiyi and Wang, Yining and Jin, Junliang and Gao, Huibin and Xu, Shiqin and Liu, Yanli and Wang, Guoqing},
  title = {Impact of groundwater depth on crop coefficient: An improved evapotranspiration model},
  journal = {Agricultural Water Management},
  year = {2025},
  doi = {10.1016/j.agwat.2025.110060},
  url = {https://doi.org/10.1016/j.agwat.2025.110060}
}