Consider a two-dimensional flow through isotropic soil along $x$ direction and $z$ direction. If $h$ is the hydraulic head, the Laplace’s equation of continuity is expressed as

- $\dfrac{\partial h}{\partial x}+ \dfrac{\partial h}{\partial z} = 0 \\$
- $\dfrac{\partial h}{\partial x}+ \dfrac{\partial h}{\partial x} \dfrac{\partial h}{\partial z}+ \dfrac{\partial h}{\partial z} = 0 \\$
- $\dfrac{\partial^2 h}{\partial x^2}+ \dfrac{\partial^2 h}{\partial z^2} = 0 \\$
- $\dfrac{\partial^2 h}{\partial x^2}+ \dfrac{\partial^2 h}{\partial x \partial z} + \dfrac{\partial^2 h}{\partial z^2} = 0$