def calc_gw_movement_indicator(pgw, bid_name, thresholds):
    """Calculate movement average of groundwater table relative 
    to lowest trigger threshold (in mAHD). Negative values indicate lowest 
    trigger level was exceeded.
    """
    first = pgw.head(1)[bid_name].iloc[0]
    last_level = pgw.tail(1)[bid_name].iloc[0]
    lowest_threshold = min(thresholds.keys())

    return (last_level - lowest_threshold) / (first - lowest_threshold)