NUMERICAL MODELLING IN IMPROVING SUBSURFACE DRAINAGE SYSTEM FOR SALT CONCENTRATION CONTROL

The study demonstrates that three-dimensional variable-density groundwater flow models such as the SEAWAT model can be effectively used for design of subsurface drainage systems for controlling salt concentration in the root zone on salt affected irrigated land. The study was carried out on Mankessim Irrigation Project site in Ghana of initial shallow water table depth of 0.5 m and salt concentration of 6800 mg/l with assumed impermeable layer at 10 m deep and impermeable field boundaries. The results showed that over a wide range of irrigation water quality and aquifer hydraulic conductivity, the optimum drain spacing using SEAWAT model was wider by between 3 and 50 % and the amount of drain discharge reduced by 1 and 27 % than were calculated using conventional (Hooghoudt) design equations. It was concluded that Three-Dimensional Variable-Density Groundwater Flow models are better for designing effective drainage systems than conventional drain spacing design equations such as Hooghoudt.