How can I estimate the step size for my MODFLOW model?

  • Scroll to top of this topic Scroll to Top of Page

  • Print the current topic Print Topic

  • No expanding elements on this page Show/Hide Expanders

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.)