Duration

Definition

Peak flow duration is the median number of days that a flow event stays over the flow thresholds for the 10, 5, and 2 year recurrence intervals (2nd, 5th, 10th, and 20th percentile exceedance) of annual peak flow over the period of record. This metric is measured in number of days.

Steps

1. Convert raw data from a single column of dates and a single column of flows into a matrix with columns organized by water year. Each column starts with the beginning of the water year (i.e. 10/1) and ends with the end of water year (9/30).

2. Calculate the 10%, 20%, and 50% exeedance values of annual peak flow over the entire period of record.

   for column_number, _ in enumerate(matrix[0]):
        flow_data = matrix[:, column_number]
        peak_flows.append(np.nanmax(flow_data))
    for percentile in peak_percentiles:
        peak_exceedance_values.append(np.nanpercentile(peak_flows, 100 - percentile))

3. In each water year, create an object each time the flow passes the exceedance value.

   """Init current flow object"""
        for percent in exceedance_percent:
            exceedance_object[percent] = []
            exceedance_duration[percent] = []
            current_flow_object[percent] = None
   
        """Loop through each flow value for the year to check if they pass exceedance threshold"""
        for row_number, flow_row in enumerate(matrix[:, column_number]):
   
            for percent in exceedance_percent:
                if bool(flow_row < exceedance_value[percent] and current_flow_object[percent]) or bool(row_number == len(matrix[:, column_number]) - 1 and current_flow_object[percent]):
                    """End of an object if it falls below threshold, or end of column"""
                    current_flow_object[percent].end_date = row_number + 1
                    current_flow_object[percent].get_max_magnitude()
                    exceedance_duration[percent].append(current_flow_object[percent].duration)
                    current_flow_object[percent] = None

4. For each day flow value is over the exceedance value, the object adds the flow value to an array using the add_flow method. Duration is then calculated by finding the length of flow attribute.

   class FlowExceedance:
   
      def __init__(self, start_date, end_date, duration, exceedance):
          self.start_date = start_date
          self.end_date = end_date
          self.duration = duration
          self.flow = []
          self.exceedance = exceedance
   
      def add_flow(self, flow_data):
          self.flow.append(flow_data)