TIME STEP CONSIDERATIONS

TIME STEPS

time steps are the temporal equivalent of grid cells

time steps should be small when stresses change and increase in length to a constant, convenient size until the stresses change

STRESS PERIODS

stress periods are groups of time steps during which the stresses do not change

Here is a SCHEMATIC to help you visualize the concept:



Smaller time steps provide more accuracy, but one wonders how small is small enough?

It may be difficult to decide on the initial time step size. One can always simply try one step of a size that seems reasonable given the overall length of the simulation and look at the mass balance to see if it will be small enough. A rule of thumb is to calculate the maximum step size for an explicit formulation of the problem. In a two-dimensional plan view problem with a constant grid spacing in the x and y direction equal to "a":

Some codes such as MODFLOW request that you enter parameters describing the time period, number of steps and a multiplier to gradually increase steps. These parameter are related as follows. The equation can be rearranged to solve for any parameter. Here it is arrange to solve for initial step size:

The multiplier is typically 1.1 to 1.5