In an open system percolation model, the pressure head distribution was under positive pressure in the plow
layer but changed to negative pressure in the plowsole layer and upper subsoil layer. However in a closed
system percolation system, the pressure head distribution had positive pressure in all layers. Positive and
negative pressure was controlled by ground water level. We postulated that Cd in rice plants for the negative
hydraulic pressure model was higher than that with positive hydraulic pressure model. According to Sukthai
P. et al. (2005) Cd uptake was higher by rice plants with positive hydraulic pressure than those with negative
hydraulic pressure.