When performing non-linear flow calculations, results can be influenced by the particular finite element mesh that is being used (mesh alignment). This is particularly the case when the Standard data set for flow parameters is selected for the material around the phreatic level (pore water pressure = 0) with default parameters. The standard data set is based on ‘Coarse’ sand. The corresponding Relative Permeability curve and Saturation curve show a rapid decrease for negative values of the suction head, ψ. In particular the rapid decrease in permeability will make it difficult for water to penetrate into the next element. As soon as water has entered a new element, it can fill that element rather easily, but then it is again difficult to enter into the next element. This is what causes the element alignment in flow calculations.
A way to overcome this problem is, rather than using the Standard data set based on ‘Coarse’ sand, to use ‘Fine’ material instead. Note that this will cause the elements in the unsaturated zone to be more saturated than with ‘Coarse’ material. Hence, the unsaturated zone has more storage capacity, but it will make the solution less dependent on the mesh.