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

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