Determination of Optimal Surface Drainage Depth for Sunflower Cultivation on Saline Clay Soils in Southern Bangladesh

In low‐lying coastal areas of Bangladesh, excess soil water after the monsoon delays the timely establishment of dry season crops. Late sowing exposes these crops to high temperatures, high evaporation, and elevated soil and water salinity, which reduce crop growth and yield. In addition, unseasonal heavy rainfall exacerbates the situation by causing waterlogging, which may compound crop yield losses. Facilitating surface drainage can enable earlier crop establishment and mitigate the effects of unseasonal waterlogging. Field experiments were conducted at Dacope, Khulna, using sunflower to evaluate the effects of drainage depths (no drainage, 15 and 30 cm) across multiple locations, with three replicates. Artificial waterlogging was induced at the vegetative stage. The 30 cm drainage depth significantly increased yield and removed excess water more rapidly than a 15‐cm depth. Undrained plots had the lowest yields and highest excess water. The 30 cm drain also reduced salinity at 15–30 cm by up to 39% during waterlogging events and in soil water by up to 15% compared with shallower or no drainage. While a 15‐cm drain supports early crop establishment, a 30‐cm drain offers greater protection against waterlogging and maximizes sunflower yield, improving net farm income in the Ganges Delta.