Experiment and numerical simulation for designing layout parameters of subsurface drainage pipes in arid agricultural areas

Subsurface drainage is effective for not only controlling groundwater table depth but also alleviating soil salinization especially in arid and semi-arid agricultural areas. While substantial research efforts have been spent to determine the parameters (e.g., drain depth, drain spacing, and pipe diameter) of subsurface drainage pipe layout for water drainage, less attention has been paid to determine the parameters for salt discharge, which is important in arid and semi-arid regions. This paper presents a comprehensive study that used experimental and numerical methods to determine the pipe layout parameters for both water and salt drainage. A total of nine field experiments were conducted in Yanqi County, an arid area of Xinjiang Province, China, for a combination of different values of drain depth, drain spacing, and pipe diameter. The experimental results indicate that drain depth and spacing are significantly more important than pipe diameter for soil desalinization. Soil water content and salinity measured during the experiments were used to calibrate and validate a numerical model developed for simulating soil water flow and salt transport under drainage conditions. Two proposed indices (critical depth of soil desalinization and ratio of salt discharge and leaching), combined with two existing indices (salt discharge amount and soil desalinization rate), were used to represent the soil desalinization and salt discharge effects under different subsurface pipe layout parameters. The results suggest that drain depth has a significant influence on all indices, while the drain spacing has a significant influence on salt discharge amount and ratio of salt discharge and leaching. The relationships between layout parameters of subsurface pipes and proposed indices were quantified and used to determine appropriate layout parameters of subsurface pipes.