A Theoretical Study of Subsurface Drainage Model Simulation of Drainage Flow and Leaching in Salt Affected Irrigated Fields

A three-dimensional variable-density groundwater flow model, the SEAWAT model, was used to assess the influence of subsurface drain spacing, evapotranspiration and irrigation water quality on salt concentration at the base of the root zone, leaching and drainage in salt affected irrigated land. The study was carried out on a conceptual uniform homogenous irrigated field of shallowwatertable depth of 0.5m and aquifer salt concentration of 7200 mg/l with an impermeable layer at 10 m depth and impermeable field boundaries. The model was run for 10 years with an irrigation rate (applied recharge) of 8 mm/d and salt concentration of 1,500 mg/l, over a range of drain spacings. During the 10-year drainage period, the simulated concentrations at the base of the root zone and the discharge rates were the same at all the spacing when evapotranspiration was not included. However, upon inclusion of evapotranspiration, the simulated concentration at the base of the root zone ranged from about 5,200 to about 6200 mg/l, the discharge rate ranged from 2.3 to 1.9 mm/d. When the applied recharge concentration was changed to 1,000 mg/l and 700mg/l, but with all the other parametersmaintained, the simulated concentration at the base of the root zone ranged from 3,700 to 4,400 mg/l, and from 2,800 to 3200 mg/l for the different spacing, respectively.