> For the complete documentation index, see [llms.txt](https://eflow.gitbook.io/ffc-readme/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://eflow.gitbook.io/ffc-readme/functional-flow-calculator/metrics/peak-magnitude-flows/duration.md).

# 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.

   ```python
   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.

   ```python
   """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.

   ```python
   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)
   ```