Salinity distribution in paddy root zone under subsurface drainage

One-dimensional convective dispersion solute transport model was solved by Crank–Nicholson finite difference scheme and was applied over the flow domain of subsurface drained paddy fields laid with tile drain at different spacings. The flow domain was divided into a number of stream tubes to predict salinities at different distances from drain centre, at different depths from the ground surface and at different times after the initiation of the operation of the subsurface drainage system. The sub division of the flow domain into a number of stream tubes was done for two purposes, viz. (i) to enable estimation of pore water flow velocity more appropriately with respect to the flow area within a stream tube and (ii) to enable comparison of predicted salinities with the observed values which were available within some of the stream tubes. The initial and the boundary conditions in solving one dimensional equation were based on the field investigated salinity values. In the solute transport model there are two essential input parameters, viz. the pore water velocity and the dispersion coefficient. The pore water velocity was calculated by dividing the Darcian velocity by the drainable porosity. The dispersion coefficient was fitted by trial and error till the average absolute deviation between the predicted and the observed salinities became minimum.