def siwsi(b8a, b11):
    """
    Shortwave Infrared Water Stress Index \
    (Fensholt and Sandholt, 2003).

    .. math:: SIWSI = (b8a - b11)/(b8a + b11)

    :param b8a: NIR narrow.
    :type b8a: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns SIWSI: Index value

    .. Tip::

       Fensholt, R., Sandholt, I. 2003. Derivation of a shortwave \
       infrared water stress index from MODIS near- and shortwave \
       infrared data in a semiarid environment. Remote Sensing of \
       Environment 87, 111-121. doi:10.1016/j.rse.2003.07.002.
    """

    SIWSI = (b8a - b11)/(b8a + b11)
    return SIWSI