def get_iwp_label_key( iwp_label ):
    """
    Retrieves a key that locates the supplied IWP label within the underlying
    dataset.  The key returned locates the label both temporarly and spatially.

    Takes 1 argument:

      iwp_label - IWP label to locate.

    Returns 1 value:

      label_key - Tuple identifying iwp_label's location within a dataset.  Comprised
                  of (time step index, z index).

    """

    return (iwp_label["time_step_index"], iwp_label["z_index"])