Article

Effect of hardpan on the vertical distribution of water stress in a converted paddy field

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Abstract

Upland fields converted from paddy fields often have a hardpan layer that can cause excessive wet and dry conditions. In this study, the water stress affected by this hardpan was evaluated. Data were obtained from a converted maize field that was divided into plots with and without hardpan. A soil-water-atmosphere-plant (SWAP) model was used to estimate the water stress conditions, such as the reduction in the daily transpiration rate and the vertical distribution of reduction in root water uptake for both plots. When hardpan was present, stress due to wet and dry conditions was more intense in the layer above the hardpan, and dry condition persisted after the hardpan dried. Without hardpan, the wet stress decreased more quickly than when the hardpan was present. The vertical distributions of reduction in root water uptake revealed that dry and wet conditions occasionally occurred at the same time in both plots. In this situation, without hardpan, the shallow layer dried and the deeper layer was wet. With hardpan, the same condition occurred, or the hardpan dried and other shallower and deeper layers were wet. It was considered that the latter condition was a unique characteristic of plots with hardpan—converted fields. The vertical distributions of reduction in root water uptake with the SWAP model can be used as an index to examine the relationship between water stress and tillage practices.

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... Research has shown that hardpan effects the vertical distribution of water stress in a converted paddy field. Abiotic stress often intensifies with alternate prolonged wet and dry conditions in the surface and sub-surface layer above the hardpan, dry condition persisted after the hardpan dried (Hamada et al. 2021). Our research found relatively poor seed germination and sprouting because excess water could not leak out from the trays and resulting death of seedlings. ...
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The agro-hydrological Soil–Water–Atmosphere–Plant (SWAP) model was calibrated and validated to simulate water–salt transport based on field experiments in an arid region of China. The simulation results show lower soil water content but higher salt concentration under deficit irrigation. Soil water and salinity below 95 cm at 80% evapotranspiration (ETc) treatments and 65 cm at 60% ETc treatments were hardly affected by irrigation. With deficit irrigation, the maximum water uptake and salt accumulated layer moved upward. The SWAP model was also used to predict long-term deficit irrigation with saline water. The salinization process reached equilibrium after utilization of saline water for a few years. In summary, the numerical model proves to be a useful tool for studying water–salt transport under different scenarios and for evaluating irrigation practices for a long period.
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