To establish representative low flows and floods (high flows) for a river course a hydrologist will analyse the frequency of daily flows in the form of a flow duration curve. A flow duration curve is a plot of discharge versus the amount of time that a certain discharge was exceeded. For most studies the data needed for this analysis is the daily average flow (the average discharge or flow for each day), calculated using instantaneous discharge data as shown in this example.

Calculate Daily Average Flow

1. Ensure the Variables required for the calculation Discharge can be viewed in your datasheet and two variables to output to, Daily Average Flow and Percent of time exceeded exist in your list of Variables.
2. Right-click on the datasheet and select Calculation.
3. Select Master Variable A as Discharge.
4. Enter the formula: DAILYAVGA
5. Select the Output variable Daily Average Flow at the desired level.
6. Select the Conversion Chart tab, where you can review the calculation input variables and the result of the proposed calculation.
7. Similarly, select the Conversion Result tab to review the data in grid view, before completing the calculation.
8. Check the Database box in the Update section.
9. Then click Update. Once the calculation has run you will have calculated the Daily Average Flow using the instantaneous discharge data.
10. (Optional) To continue to calculate Percent of time exceeded as directed below, please keep the Calculation window open.

Calculate Percent of Time Exceeded

To calculate the percent of time exceeded requires the use of the RANK function. RANKDSC is used to sort (rank) the data series from the largest value to the smallest value.

The formula (RANKDSCA/(n + 1))*100 will rank the average daily discharge for the period of record from the largest value to the smallest value, involving a total of n values, where n equals the number of points or events for the period of flow records being analysed. Using this formula, you are also ranking the point as a percentage.

In the Calculation window:

1. Change Master Variable A to Daily Average Discharge.
2. Enter the following formula (RANKDSCA/(n + 1))*100       where n is the total number of indexed records.
3. Select the Output Variable Percent of time exceeded.
4. Select theCalculation Chart tab, where you can review the calculation input variables and the result of the proposed calculation.
5. Similarly, select theCalculation Result tab to review the data in grid view, before completing the calculation.
6. Check the Database box in the Update section.
7. Then click Update. Once the calculation has run you will have calculated the Percent of time exceeded.
8. Close the Calculation window.

Create a Flow Duration Curve

1. Click Chart in the View group of the Menu ribbon.
2. Drag the Percent of Time Exceeded variable on to the x-axis.
3. Drag the Daily Average Flow variable on the y-axis. Your data should be ready on the chart momentarily.