def _convert_units(cube):
    """Convert unit of cube, used only for water variables.

    From kg m-2 s-1 to kg m-2 month-1 or kg m-2 day-1.
    Note that the unit kg m-2 s-1 is equivalent to mm s-1.
    """
    mip = cube.attributes['mip']

    if mip == 'Amon':
        cube.convert_units('kg m-2 month-1')  # equivalent to mm/month
    elif mip == 'day':
        cube.convert_units('kg m-2 day-1')  # equivalent to mm/day
    return cube