def vapour_pressure_deficit(svp, vp):
    """
    Computes the vapour pressure deficit :math:`\\Delta_e` in [mbar]

    .. math ::
        \\Delta_e=e_s-e_a

    Parameters
    ----------
    svp : float
        saturated vapour pressure
        :math:`e_s`
        [mbar]
    vp : float
        actual vapour pressure
        :math:`e_a`
        [mbar]

    Returns
    -------
    vpd : float
       vapour pressure deficit
       :math:`\\Delta_e`
       [mbar]

    Examples
    --------
    >>> from ETLook import meteo
    >>> meteo.vapour_pressure_deficit(12.5, 5.4)
    7.1
    >>> meteo.vapour_pressure_deficit(vp=5.4, svp=12.3)
    6.9

    """
    vpd = svp - vp
    vpd[vpd < 0] = 0
    return vpd
