Understanding Discharge Curves

In the study of hydrology and water resources, it is often important to understand how the flow rate, or discharge, of a river or stream varies over time. Different types of graphical representations are used to illustrate these variations for different purposes. The question asks about a specific type of cumulative frequency curve related to discharge.

Flow Duration Curve Definition

A cumulative frequency curve that shows the percentage of time a specified discharge was equalled or exceeded during a given period is known as a flow duration curve. This curve is a fundamental tool in hydrology used to characterize the flow variability of a river basin. It plots discharge values on the y-axis and the percentage of time that these discharge values were equalled or exceeded on the x-axis. For example, a point on the curve might show that a discharge of 50 cubic meters per second is equalled or exceeded 30% of the time.

• The flow duration curve is a cumulative frequency curve.
• It relates discharge (flow rate) to the percentage of time that flow rate is met or surpassed.
• It is useful for understanding the reliability of water availability and assessing potential for hydropower, water supply, and pollution dilution.

Analyzing Other Options

Let's consider why the other options do not fit the description:

• Mass duration curve: This term is not standard terminology in hydrology in the way described by the question. While mass curves (like the Rippl diagram) are used in water storage analysis, they typically plot cumulative inflow or net inflow over time, not the frequency of a specific discharge.
• Hydrograph: A hydrograph is a plot showing the discharge of a river or stream over time. The y-axis is discharge, and the x-axis is chronological time (e.g., hours, days, months). It shows the sequence of flow events, such as floods and low flows, but it is not a cumulative frequency curve showing the percentage of time a discharge is equalled or exceeded.
• Power duration curve: A power duration curve is typically used in the context of power generation, often specifically hydropower. It shows the amount of power that can be generated or demanded for a certain percentage of time. While related to flow duration curves for hydropower potential assessment, it plots power (often in kilowatts or megawatts) versus time percentage, not discharge versus time percentage directly.

Based on the definitions, the only option that matches the description of a cumulative frequency curve showing the percentage of time a specific discharge was equalled or exceeded is the flow duration curve.