def ndwi_gao(b8, b11):
    """
    Normalized Difference Water Index (Gao, 1996).

    .. math:: NDWI = (b8 - b11)/(b8 + b11)

    :param b8: NIR.
    :type b8: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns NDWI: Index value

    .. Tip::

        Gao, B. 1996. NDWI - A normalized difference water index for remote \
        sensing of vegetation liquid water from space. Remote Sensing of \
        Environment 58(3), 257-266. doi:10.1016/s0034-4257(96)00067-3.

    """

    NDWI = (b8 - b11)/(b8 + b11)
    return NDWI