@staticmethod
def calc_pressure_head(lam, h_b):
    """Calculate pressure head.

        Pressure head is set using *lambda* and *h_b*, using an
        equation after Brooks-Corey (1964), following Rawls et al., 1992.

        Parameters
        ----------
        lam : float, optional
            Pore-size distribution index. Exponent that describes the
            distribution of pore sizes in the soil, and controls
            effective hydraulic conductivity at varying water
            contents, following Brooks and Corey (1964) [-].
        h_b : float (m), optional
            Bubbling pressure. Capillary pressure of the soil,
            controlling effective hydraulic conductivity at varying
            water contents, following Brooks and Corey (1964) [m]
        """
    return (2.0 + 3.0 * lam) / (1.0 + 3.0 * lam) * h_b * 0.5
