Field and numerical experiments of subsurface drainage systems in saline and low-permeability interlayered fields in arid regions

A reasonable layout of subsurface drainage systems is considered essential for maximizing its drainage and salt control effectiveness. In the saline-alkali farmland of arid regions in Northwest China, low-permeability interlayers (clay, clay-loam or silty-loam) within the soils are common. However, the influence of the low-permeability interlayers on the layout of the subsurface drainage has not been extensively considered in the literature. This study investigated the process of subsurface drainage and salt discharge in salt-affected fields with silty-loam interlayers using field experiments and numerical simulations. Four field experiments were conducted, considering three different relative positions between the drainage pipes and silty-loam interlayers. The results showed that the silty-loam interlayers hindered water infiltration, and the distribution of soil salinity in the soil profile varied with the buried depth of drainage pipes at different positions relative to the silty-loam interlayer. When the buried depth of drainage pipes increased, the amount of water and salt discharged from drainage pipes increased, and the increase rate in salt discharge was greater than water drainage. A numerical model was calibrated and validated using the field experiment data, and 25 orthogonal numerical experiments were conducted to investigate the soil desalination effects of buried depth of drainage pipes, spacing between the pipes, saturated hydraulic conductivity of the interlayer, and position of the low-permeability interlayer. The results showed that the drainage pipe buried depth, spacing, and saturated hydraulic conductivity of the low-permeability interlayer had significant effects on the desalination rate (P < 0.01), while the position of the low-permeability interlayer had no significant effect (P > 0.05). Subsurface drainage pipes should be placed below the low-permeability interlayer. The desalination rate linearly increased with the buried depth of drainage pipe and saturated hydraulic conductivity of the interlayer, and it increased exponentially with decreased spacing. An empirical formula for soil desalination rate considering interlayer and subsurface drainage pipe layout parameters was fabricated and used to determine the appropriate layout parameters.