Understanding the Discharge Frequency Curve

A discharge frequency curve is a graphical tool used in hydrology to represent the relationship between stream flow (discharge) and the frequency or probability with which a given flow rate is equaled or exceeded over a long period.

The curve essentially helps hydrologists and engineers understand how often certain flow magnitudes occur in a stream or river. This information is crucial for various applications, such as flood risk assessment, water resource management, and the design of hydraulic structures like bridges and dams.

Components of the Discharge Frequency Curve Plot

Based on the definition and common practice in hydrology, a discharge frequency curve is plotted with:

• Stream Flow (Discharge): This is typically plotted on the vertical axis (y-axis). It represents the volume of water passing a specific point in the stream per unit time, often measured in cubic meters per second (m³/s) or cubic feet per second (ft³/s).
• Probability of Exceedance: This is typically plotted on the horizontal axis (x-axis). It represents the percentage of time (or the probability) that a given discharge value is equaled or exceeded. Alternatively, it might represent the recurrence interval (the average time between events of a certain magnitude), which is the inverse of the probability.

Analyzing the Options

Let's examine the given options:

• Option 1: yield and discharge - While related to water resources, yield and discharge represent different concepts. Yield often refers to the amount of water that can be reliably supplied from a source over a period. This is not the standard plot for a discharge frequency curve.
• Option 2: Penetration test - This refers to a soil mechanics test (like the Standard Penetration Test - SPT) which measures the resistance of soil to penetration. This is completely unrelated to stream flow or frequency curves.
• Option 3: stream flow and percentage probability of flow is equalled or exceeded - This option directly matches the standard definition and practice of plotting a discharge frequency curve, relating the magnitude of stream flow to the probability of its exceedance.
• Option 4: rainfall and discharge - A plot between rainfall and discharge over time is typically called a hydrograph (if showing flow response to rainfall) or used in rainfall-runoff analysis. While related, it is not the specific discharge frequency curve which focuses on flow magnitude and its long-term probability/frequency.

Therefore, the correct description of the plot for a discharge frequency curve is between stream flow and the percentage probability of flow being equaled or exceeded.