12.4. Derivation of DAD Curves

To develop DAD relationships, the following are required:

• Storm event data (preferably from extreme or major storms)
• Rain gauge network across the region
• Time-series rainfall data at each gauge

To derive DAD curves, it is essential to gather relevant data. This includes:

1. Storm Event Data: This is data collected from significant storm events, particularly extreme storms, as they provide critical insights into rainfall behavior during severe weather. Historical records of major storms help in establishing patterns.
2. Rain Gauge Network: A well-distributed network of rain gauges is crucial. These instruments measure rainfall at specific locations, enabling hydrologists to assess spatial rainfall distribution across the region.
3. Time-Series Rainfall Data: This data is collected over time and is needed for each gauge to understand changes and trends in rainfall. It provides a timeline of rainfall events and assists in analyzing their frequency and intensity.

Think of the data requirements like preparing for a big event, like a concert. To have a successful concert, you need:

1. Concert history (like storm event data) to know which bands perform best.
2. A sound system and microphones (like a rain gauge network), ensuring music reaches everyone in the audience.
3. A schedule of past events (time-series data) to plan effectively for the next one, considering what worked and what didn’t.

One of the most common methods for calculating areal average rainfall is the Isohyetal method, where:

• Isohyets (lines of equal rainfall) are drawn over the storm area.
• The area between isohyets is computed.
• The volume of rainfall is estimated and averaged over the area. This gives averaged depths for sub-areas, which are then used to create cumulative depth-area-duration data points.

The Isohyetal method is a technique used in DAD curve derivation that provides a visual representation of rainfall distribution. The process consists of:

1. Drawing Isohyets: Isohyets are lines on a map that connect points of equal rainfall. By drawing these lines across the storm area, hydrologists can visualize how much rain fell in different locations.
2. Calculating Area Between Isohyets: The areas bordered by these lines are measured. Each area represents a section of the storm region with a specific rainfall depth.
3. Estimating Rainfall Volume: After defining these areas, the total volume of rainfall is estimated and averaged over the entire area to find an average rainfall depth. This step is crucial for calculating how much water is contributed to the region by the storm.

Imagine looking at a large map of a city after a heavy snowfall. You draw lines around regions that received equal amounts of snow. By calculating the area for each region, you can estimate how much total snow fell. This is similar to the Isohyetal method: drawing isohyets provides a clear picture of rainfall distribution, which helps in understanding the storm's impact on different areas.

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