Seepage field simulation of subsurface pipe salt drainage processes in coastal saline-alkali farmland

Underground pipe drainage technology is an effective measure to control saline soil along the coast. To develop a more suitable irrigation and drainage system for high water level conditions in the coastal area, this study took the near coastal underground pipe drainage and salt discharge experimental site of Nandagang Industrial Park in Cangzhou City, Hebei Province, as the simulation study object. By setting different initial water levels, the varying water level conditions of coastal farmland in spring and summer were simulated. COMSOL software was used to conduct numerical simulations, and indoor flume models were designed for comparative verification, simulating the water flow migration process during leaching and desalination and the water flow and salt migration process during groundwater reduction. The research results indicated that: 1) the underground pipe’s drainage capacity and salt discharge capacity increased with the increase in groundwater level, and changes in water level led to changes in hydraulic boundaries, thereby affecting the flow direction and water head distribution in the soil seepage field. The change in hydraulic boundary conditions had a greater impact on the soil seepage field than the change in the water level. When the groundwater level was below the ground, the water level rose by 8 cm, and the seepage velocity increased by 0.21 m³∙d^–1. However, when the groundwater level rose from the ground to the surface, the seepage velocity increased by 1.68 m³∙d^–1. 2) Increasing the vertical distance between the groundwater level and the underground pipe accelerated the water and salt transport rate, reduced the residence time of water carrying salt in the soil, and increased the underground pipe’s drainage and salt discharge rate to prevent salt return from the soil effectively. When the groundwater level rose by 8 cm, the time required for saline water flow from the makeup water tank to the underground pipe was shortened by 720 min. Increasing the burial depth of an underground pipe improved its drainage and salt discharge efficiency effectively. 3) The burial of underground pipes changed the flow path, promoting salt leaching and drainage from the soil. Under different water levels, streamlines were turning around 0.8 meters from the underground pipe, and the water flow changed from flowing underground to flowing towards the underground pipe. The soil near the underground pipe had a high seepage velocity and a short seepage path. The salt in the soil far from the underground pipe was difficult to discharge, and the salt flowed into the deep soil, causing salt accumulation. The use of sand tank physical models and COMSOL software effectively simulated the water and salt transport processes of underground pipe drainage and salt discharge, providing a more convenient experimental method for the optimization of existing leaching strategies, a more scientific basis for the optimization of underground pipe drainage and salt discharge paths, and the design layout of underground pipe construction in coastal saline-alkali land with different textures.