def Q_weir_rectangular_SIA(h1, h2, b, b1):
    r'''Calculates the flow rate across rectangular weir from
    the height of the liquid above the crest of the notch, the liquid depth
    beneath it, and the width of the notch. Model from [1]_ as reproduced in
    [2]_.

    Flow rate is given by:

    .. math::
        Q = 0.544\left[1 + 0.064\left(\frac{b}{b_1}\right)^2 + \frac{0.00626
        - 0.00519(b/b_1)^2}{h_1 + 0.0016}\right]
        \left[1 + 0.5\left(\frac{b}{b_1}\right)^4\left(\frac{h_1}{h_1+h_2}
        \right)^2\right]b\sqrt{g}h^{1.5}

    Parameters
    ----------
    h1 : float
        Height of the fluid above the crest of the weir [m]
    h2 : float
        Height of the fluid below the crest of the weir [m]
    b : float
        Width of the rectangular flow section of the weir [m]
    b1 : float
        Width of the full section of the channel [m]

    Returns
    -------
    Q : float
        Volumetric flow rate across the weir [m^3/s]

    Notes
    -----
    The following limits apply to the use of this equation:

    b/b1 ≤ 0.2
    h1/h2 < 2
    b > 0.15 m
    h1 > 0.03 m
    h2 > 0.1 m

    Examples
    --------
    >>> Q_weir_rectangular_SIA(0.2, 0.5, 1, 2)
    1.0408858453811165

    References
    ----------
    .. [1] Normen für Wassermessungen: bei Durchführung von Abnahmeversuchen
       an Wasserkraftmaschinen. SIA, 1924.
    .. [2] Blevins, Robert D. Applied Fluid Dynamics Handbook. New York, N.Y.:
       Van Nostrand Reinhold Co., 1984.
    '''
    h = h1 + h2
    Q = 0.544*(1 + 0.064*(b/b1)**2 + (0.00626 - 0.00519*(b/b1)**2)/(h1 + 0.0016))\
    *(1 + 0.5*(b/b1)**4*(h1/(h1 + h2))**2)*b*g**0.5*h**1.5
    return Q