$$ \frac{\overline{R}_r}{K} \cdot \frac{\overline{L}}{\overline{D}} \cdot \frac{\overline{L}}{z_{\text{max}}} \left\{ \begin{array}{ll} > m & \text{topography controlled} \\ < m & \text{recharge controlled} \end{array} \right. $$

Where the terms have the following meaning:

• $R_r$ is the average anual recharge rate
• $K$ is the aquifer's hydraulic conductivity
• $L$ is the average distance between surface waters
• $D$ is the average aquifer thickness
• $z_{\text{max}}$ is the maximum distance between the surface water levels and terrain elevation
• $m$ is either 8 for one dimensional groundwater flow, or 16 for radial-symmetric groundwater flow (in a circular aquifer).

In cases of a relatively low permeability combined with enough recharge by precipitation the groundwater is likely to follow the surface. However, for example in karst areas, where extensive networks of caves and tunnels in soluble rock cause a very effective drainage over long distances, the groundwater will follow the surface on a much more regional scale. Also, areas with high relief groundwater at least isn't parallel to the surface, and for example in sand dunes, the groundwater level is usually relatively flat compared to the dune.