The use of numerical groundwater model to improve effectiveness of subsurface drainage system in irrigated field

Abstract

The research 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 SEAWAT model was used to optimize subsurface drainage design to
ensure that the salt concentration of the groundwater at the base of the root zone does not
exceed pre determined levels instead of the conventional approach of maintaining the
groundwater at a predetermined water table level. The study was carried out on a
conceptual uniform homogenous block of irrigated flat field of shallow water table depth
of 0.5 m and salt concentration of 7200 mg/l with impermeable layer at 20 m deep and
impermeable field boundaries. Using the model, spacings were designed to be used as
design criteria for subsurface drainage system to maintain salt concentrations of 6000,
5000 and 4000 mg/l at the base of the root zone and water table depth of 0.8 m from the
soil surface. The results showed that over a wide range of irrigation water quality and
aquifer hydraulic conductivity the optimum drain spacing using SEAWAT was,
depending on irrigation water quality and aquifer hydraulic conductivity, wider by
between 3 and 50 % and the amount of drain discharge reduced by 1 and 27 % than
would be recommended using conventional 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.

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