Unreliability in the assumption of soil properties, selection of elapse time, choice of time step size and atmospheric variables causes most inconsistencies in the simulation results. Therefore, time step size studied was based on root water up-take on a single Lime tree on a Boulder Clay. The effects of ¼, ½ and 1 day time step sizes on generated capillary potential at 0.0 m, 1.4 m and 3.0 m from the lime tree for elapse time of 30, 90, 190 and 270 days elapsed time were studied. A straightforward sink term for root water uptake was used and combined with two-dimensional axi-symmetric governing equation for unsaturated soil. The simulated capillary potential was directly proportional to the elapsed time. The time step sizes studied were found to give the same generated capillary potentials at same spacial distances. The idea was to investigate the generated capillary potentials at ¼, ½ and 1 days time steps sizes at a same spatial distance from the Lime tree on a Boulder Clay should be the same. Consequently, the difference in the results generated with the three initial time step sizes were far less than ±5%, which is satisfactory.
Key words: Unsaturated soil, boundary condition, capillary potential, spatial discretization, time discretization.