Flow Duration Curve Explained

A cumulative frequency curve is a graph that shows the total number or proportion of observations that fall below or above a certain value. In hydrology, such curves are used to represent streamflow characteristics.

The question describes a specific type of cumulative frequency curve related to discharge in a stream or river. It asks for the name of the curve that shows the percent of time a specified discharge is equaled or exceeded over a given period.

Let's look at the options provided:

• Hydrograph: A hydrograph is a graph showing the rate of flow (discharge) versus time. It plots discharge on the y-axis and time on the x-axis. This shows how discharge varies over time, not the percentage of time a certain discharge is exceeded.
• Mass duration curve: This term is not a standard hydrological curve. There is a mass curve, which is a cumulative plot of inflow or outflow volume over time, often used in reservoir design.
• Power duration curve: This type of curve is related to hydropower, showing the amount of power available for a certain percentage of time. While related to flow, it directly represents power, not just discharge.
• Flow duration curve: A flow duration curve (FDC) is a cumulative frequency curve that shows the percentage of time that a specified flow rate (discharge) was equaled or exceeded during a specific period. It is plotted with discharge on the y-axis and the percentage of time the discharge was equaled or exceeded on the x-axis (often on a logarithmic scale). This definition precisely matches the description given in the question.

Therefore, the cumulative frequency curve that shows the percent of time specified discharge were equaled or exceeded during a given period is known as a Flow duration curve.