A rough estimate of the required time step size for a transient model can be determined using the following equation.
Δt = Sa2/4Kb = SSba2/4Kb = SSa2/4K
Where S = storativity (dimensionless), SS = specific storage (1/L), a = length of cell side, K = hydraulic conductivity, b = cell thickness (Anderson and others, 2015, p. 319). The time step size varies with cell dimensions and aquifer properties. You can use one of the following formulas in a 3D data set to see how the required time step size varies. The formulas will return 0 where no time step size can be estimated. the time step size can be estimated on active cells in simulated layers that are not specified head cells and where the transmissivity or hydraulic conductivity is greater than zero.
LPF or UPW package
With no convertible layers
If((((Active and SimulatedLayer) and not(Modflow_Specified_Head)) and (Kx > 0.)), ((BlockAreaTop * Specific_Storage) / (4. * Kx)), 0.)
With at least one convertible layer
If((((Active and SimulatedLayer) and not(Modflow_Specified_Head)) and (Kx > 0.)), If(ConfinedLayer, ((BlockAreaTop * Specific_Storage) / (4. * Kx)), ((BlockAreaTop * Specific_Yield) / ((4. * Kx) * LayerHeight))), 0.)
BCF package
With no convertible or unconfined layers
If((((Active and SimulatedLayer) and not(Modflow_Specified_Head)) and (Transmissivity > 0.)), ((BlockAreaTop * Confined_Storage_Coefficient) / (4. * Transmissivity)), 0.)
With at least one convertible or unconfined layer
If(((Active and SimulatedLayer) and not(Modflow_Specified_Head)), If(ConfinedLayer, If((Transmissivity > 0.), ((BlockAreaTop * Confined_Storage_Coefficient) / (4. * Transmissivity)), 0.), If((Kx > 0.), ((BlockAreaTop * Specific_Yield) / ((4. * Kx) * LayerHeight)), 0.)), 0.)
HUF package
With no convertible layers
If((((Active and SimulatedLayer) and not(Modflow_Specified_Head)) and (Kx > 0.)), ((BlockAreaTop * HUF_SS) / (4. * Kx)), 0.)
With at least one convertible layer
If((((Active and SimulatedLayer) and not(Modflow_Specified_Head)) and (Kx > 0.)), If(ConfinedLayer, ((BlockAreaTop * HUF_SS) / (4. * Kx)), ((BlockAreaTop * HUF_SY) / ((4. * Kx) * LayerHeight))), 0.)
