---
title: "DAD curve procedure and isohyetal area data"
subject: "Engineering Hydrology"
chapter: 2
key: "semester_5/engineering_hydrology:dad-curve-procedure-isohyetal"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/dad-curve-procedure-isohyetal
site: BCT Engineering
---

# DAD curve procedure and isohyetal area data

Engineering Hydrology · Chapter 2: Precipitation

- Asked in: 2076 Chaitra, 2+4 marks
- Video: [DAD & IDF Curves | Engineering Hydrology](https://www.youtube.com/watch?v=lz55t5M8kOA) — APSEd
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/dad-curve-procedure-isohyetal
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-2

## Question

Write down the procedure of developing Depth Area Duration (DAD) curve. From an isohyetal map prepared for 12 hrs precipitation, following data are obtained.

| Isohyetal interval (cm) | Area enclosed (km$^2$) |
|---|---|
| >70 | 125 |
| 60-70 | 225 |
| 50-60 | 215 |
| 40-50 | 350 |
| <40 | 150 |

## Short answer

Procedure: prepare isohyetal maps for different durations, compute cumulative areas and average depths, plot DAD curves per duration and envelope curves. For the 12-h data, average depths over cumulative areas 125, 350, 565, 915, 1065 km² are 75.0, 68.6, 63.4, 56.4 and 53.4 cm respectively.

## Answer

#### Procedure for developing a DAD curve
1. Select a major storm and collect rainfall data of all gauges for the storm period.
2. Prepare isohyetal maps of the storm for different durations (for example 6, 12, 24, 48 h) using the maximum depth for each duration at every station.
3. For each duration, measure the area enclosed by each isohyet, starting from the storm centre outward, and compute the cumulative area.
4. Compute the average depth over each cumulative area = (sum of area × mean depth) / cumulative area.
5. Plot average depth (y) against area (x) for each duration, giving one curve per duration.
6. Do the same for several storms and draw the **envelope curves** of the maximum values. These are used for design.

#### Calculation for the 12-hour isohyetal data
Assumptions: the mean depth of the top class (> 70 cm) is taken as 75 cm and that of the lowest class (< 40 cm) as 35 cm. Other classes use the mid value.

| Class (cm) | Mean depth (cm) | Area (km²) | Cum. area (km²) | Cum. volume (cm·km²) | Avg depth over cum. area (cm) |
|---|---|---|---|---|---|
| > 70 | 75 | 125 | 125 | 9375 | 75.0 |
| 60-70 | 65 | 225 | 350 | 24000 | 68.6 |
| 50-60 | 55 | 215 | 565 | 35825 | 63.4 |
| 40-50 | 45 | 350 | 915 | 51575 | 56.4 |
| < 40 | 35 | 150 | 1065 | 56825 | 53.4 |

Plot the average depth against the cumulative area (125, 350, 565, 915, 1065 km²) for the 12-h duration; the depth falls from 75 cm to 53.4 cm as the area increases.

**Answer: Average 12-h depth over 125, 350, 565, 915 and 1065 km² = 75.0, 68.6, 63.4, 56.4 and 53.4 cm respectively.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is used to prepare the isohyetal maps for different durations?
   A. Mean depth at each station
   B. Maximum depth for each duration at every station
   C. Annual rainfall
   D. Only the storm centre gauge
   Answer: B. Maps for each duration use the maximum depth of that duration at every gauge. (Key point: Isohyetal maps)

2. The average depth over a cumulative area is:
   A. Sum of (area x mean depth) divided by cumulative area
   B. Largest isohyet value
   C. Sum of mean depths
   D. Cumulative area divided by volume
   Answer: A. Average depth = cumulative volume / cumulative area. (Key point: Average depth)

3. Which curves are used for design?
   A. The curve of the first storm only
   B. Envelope curves of maximum values from several storms
   C. The average of all storms
   D. The curve of the shortest duration
   Answer: B. DAD curves for several storms are enveloped to give maximum values for design. (Key point: Design use)

4. The average 12-h depth over the first 350 km2 is:
   A. 75.0 cm
   B. 70 cm
   C. 68.6 cm
   D. 65.0 cm
   Answer: C. (125x75 + 225x65)/350 = 24000/350 = 68.6 cm. (Key point: Result)

## Related in Engineering Hydrology

- [Average precipitation by isohyetal method](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aisohyetal-average-depth) — Engineering Hydrology, chapter 2, asked 2×
- [Intensity-duration curve and depth-area curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aidf-depth-area-curve) — Engineering Hydrology, chapter 2
- [Mass curve, hyetograph, depth-duration curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Amass-curve-hyetograph-mdd) — Engineering Hydrology, chapter 2
- [Hyetograph and intensity-duration curve, 6:00-12:00 storm](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahyetograph-intensity-duration-6to12) — Engineering Hydrology, chapter 2
- [Presentation of precipitation data, IDF curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Apresent-precip-data-idf) — Engineering Hydrology, chapter 2
- [IDF curve and DAD curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aidf-dad) — Engineering Hydrology, chapter 2, asked 2×

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