def choose_dimensions(valid_dims, overrides={}):
    """For each dimension, choose a single valid option (except for 'time', where
    we use the wildcard '*' to get the whole time-series.)
    If not specified, choose the first valid option for each dimension.

    Parameters
    ----------
    valid_dims : dict
        map of lists of valid dimension values
    overrides : dict, optional
        selected dimensions

    Returns
    -------
    dict
        final choice of dimensions
    """
    # By default, choose first valid item for all dimensions; then override where needed:
    chosen_dimensions = {k: next(iter(v.keys())) for k, v in valid_dims.items()}
    # get whole time-series, not just a single point in time:
    chosen_dimensions["time"] = "*"
    chosen_dimensions.update(overrides)
    return chosen_dimensions