Soil salinity and sodicity are among the oldest soil and groundwater pollution problems and are widespread across the globe. Where salinity affects crop water uptake and yield, sodicity may additionally cause poorly reversible soil structure degradation and a severely reduced hydraulic conductivity. We use the model HYDRUS‐1D to simulate sodicity development in soils with shallow, Na‐rich groundwater under a normal weather regime with distinct dry seasons. Attention is given to the impact of a sudden fresh water input on the formation of a sodic layer. The complex interplay between soil chemistry, soil physics, soil mechanics (as far as swell–shrink behavior is concerned), and fluctuating atmospheric conditions results in a remarkably regular relation between depth, location, and severity of a sodic layer that forms within the soil as a function of rainfall intensity. A threshold behavior is observed: sodic layer formation is absent at rainfall intensities below this threshold, whereas sodic layer thickness and hydraulic conductivity reduction increase rapidly with intensities exceeding this threshold. This is the case even for different soil types and groundwater depths. Field observations agree with our simulations: the properties of the layer with sodicity‐induced structure degradation are more strongly developed, as this layer is situated at a shallower depth. The implementation of hydraulic conductivity reduction as a function of exchangeable Na percentage and ionic strength in HYDRUS‐1D can be improved towards a smooth reduction function, changing soil physical parameters due to swelling and dispersion of clay and reconsideration of the reversibility of sodicity development.

中文翻译：

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源自边际地下水的土壤碱度

土壤盐分和碱度是最古老的土壤和地下水污染问题之一，在全球范围内普遍存在。在盐度影响作物吸水量和产量的情况下，碱度还可能导致土壤结构的可逆性较差的退化和水力传导率的严重降低。我们使用 HYDRUS-1D 模型来模拟在具有明显旱季的正常天气状况下具有浅层富钠地下水的土壤中的碱度发展。注意突然的淡水输入对钠层形成的影响。土壤化学、土壤物理、土壤力学（就膨胀-收缩行为而言）和波动的大气条件之间复杂的相互作用导致深度、位置、作为降雨强度的函数，在土壤中形成的钠层的严重程度。观察到阈值行为：在低于该阈值的降雨强度时不存在钠层形成，而随着强度超过该阈值，钠层厚度和导水率降低迅速增加。即使对于不同的土壤类型和地下水深度，情况也是如此。现场观察与我们的模拟一致：由于该层位于较浅的深度，因此具有钠度诱导结构退化的层的特性得到了更强烈的发展。作为 HYDRUS-1D 中可交换钠百分比和离子强度函数的水力传导率降低的实现可以改进为平滑的降低函数，