# iMOD User Manual version 4.4 (html)

#### 12.35Timestep

Just a small nodal spacing is desirable, one would like to use small time steps to obtain an accurate solution as well. A good order-of-magnitude is to estimate the critical time step $\Delta t_c$ with a formulae given by de Marsily (1986). For a homogeneous and isotropic aquifer $\Delta t_c$ can be estimated by:

\begin{align*} \Delta t_c=S \frac {\left ( \Delta x \times \Delta y \right )^2}{4T} \end{align*}

whereby $\Delta x$ and $\Delta y$ are the cell sizes of the model (m), $S$ (-) is the porosity values and $T$ is the transmissivity value (m$^2$/day). The result of the equation is given in the following figure: