The Flow-Duration Curve

The flow-duration curve is a graphical representation that shows the relationship between stream discharge and the percentage of time that a given flow is equalled or exceeded. It is an essential tool for hydrologists and water resource engineers as it provides valuable information about the frequency and duration of various flow rates in a river or stream.

Understanding the Components

To understand why option D is the correct answer, let's break down the components of the flow-duration curve:

Stream Discharge: Stream discharge refers to the volume of water flowing through a river or stream at a specific point in time. It is typically measured in cubic meters per second (m³/s) or cubic feet per second (cfs).

Percentage of Time: The percentage of time refers to the duration or frequency of a particular flow rate occurring. This can be calculated by dividing the number of occurrences of a specific flow by the total number of observations and multiplying by 100.

The Flow-Duration Curve:

The flow-duration curve is constructed by plotting the stream discharge on the y-axis and the percentage of time on the x-axis. The data points are arranged in descending order, starting with the highest flow rate and ending with the lowest.

Interpreting the Curve:

The flow-duration curve provides valuable insights into the flow characteristics of a river or stream. Here's how it can be interpreted:

1. High Flows: The upper portion of the curve represents high flow rates. This indicates the percentage of time that the stream discharge is equalled or exceeded by a specific flow rate. For example, the curve may show that the stream discharge is equalled or exceeded 10% of the time by a flow rate of 1000 m³/s.

2. Base Flow: The base flow refers to the flow rate that is sustained during dry periods or low-flow conditions. It represents the minimum stream discharge observed over an extended period. The curve shows the base flow as it approaches 0% on the x-axis.

3. Low Flows: The lower portion of the curve represents low flow rates. This indicates the percentage of time that the stream discharge is equalled or exceeded by a specific flow rate. For example, the curve may show that the stream discharge is equalled or exceeded 90% of the time by a flow rate of 100 m³/s.

Conclusion

In summary, the flow-duration curve is a graphical representation of the relationship between stream discharge and the percentage of time that a given flow is equalled or exceeded. It provides valuable information about the frequency and duration of various flow rates, including high flows, base flow, and low flows. By analyzing this curve, hydrologists and water resource engineers can make informed decisions regarding water resource management, flood control, and infrastructure design.