def sol_rad_island(et_rad):
    """
    Estimates incoming solar (or shortwave) radiation [MJ m-2 day-1]
    (radiation hitting a horizontal plane after scattering by the atmosphere)
    for an island location using FAO equation 51. An island is defined as a
    land mass with width perpendicular to the coastline <= 20 km. Use this
    method only if radiation data from elsewhere on the island is not
    available. NOTE: This method is only applicable for low altitudes (0-100 m)
    and monthly calculations.

    Arguments:
    et_rad  - extraterrestrial radiation [MJ m-2 day-1]
    """
    solar_rad = (0.7 * et_rad) - 4.0
    return solar_rad