For transient groundwater model calculations, a temporal discretization (step size between the time steps to be calculated) must be selected. The size of a suitable time step size depends to a large extent on the time course of the event to be modelled. For example, the modelling of a flood wave may require time steps of the order of hours to a day, while time steps of a few days to a month are permissible for the calculation of annual variations in the groundwater level. The modelling of short-term changes in abstraction at extraction wells, on the other hand, may require time steps in the order of minutes shortly after a significant change in the extraction rate, whereby in a subsequent phase with unchanged extraction rates, the time steps can incrementally be made larger.

In order to avoid influencing the model results through excessively large time steps, a sensitivity analysis can be carried out at the start of a project, where relatively large time step sizes is continiously reduced until no difference between the model results of two subdequent runs is recognisable. A further reduction in the time step size would increase the computational effort without improving the model results.

An implicit Euler method is used in SPRING for the time discretisation of the transient flow equation:

The transient boundary conditions are treated fully implicitly by default (θ= 1.0), i.e, to calculate the state for time t_n+1 the boundary conditions of time t_n+1 are used.

With rate boundary conditions, only an explicit treatment of the transient boundary conditions is possible (θ= 0.0), i.e. the boundary conditions of time t_n are used to calculate the state for time t_n+1..

Special case: transport calculation