---
title: "Intensity-duration curve and depth-area curve"
subject: "Engineering Hydrology"
chapter: 2
key: "semester_5/engineering_hydrology:idf-depth-area-curve"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/idf-depth-area-curve
site: BCT Engineering
---

# Intensity-duration curve and depth-area curve

Engineering Hydrology · Chapter 2: Precipitation

- Asked in: 2069 Chaitra, 6 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/idf-depth-area-curve
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-2

## Question

Explain Intensity Duration Curve and Depth Area Curve.

## Short answer

Intensity-Duration curve shows maximum average rainfall intensity decreasing with increasing duration; derived from a storm's mass curve for design. Depth-Area curve shows average storm depth decreasing with area, obtained from isohyetal maps, used to convert point rainfall to areal estimates for large catchments.

## Answer

#### Intensity-Duration Curve
It shows how the **maximum average rainfall intensity** of a storm changes with **duration**. The intensity is high for short durations and falls for long durations.

- Prepared from a recorded storm: for each duration $D$ the greatest rainfall in any period of length $D$ is found from the mass curve, and $i_{max} = \text{depth}/D$.
- Plotted as intensity (y) against duration (x). With frequency (return period) added it becomes the IDF curve.

```text
 i
  |*
  |  *
  |     *
  |         *
  +--------------- D
```

**Uses:** design storm for drainage, culverts, small dams and rational method calculations.

#### Depth-Area Curve
It shows how the **average depth of rainfall** of a storm decreases as the **area** increases. Rainfall is greatest at the storm centre and falls off outward.

- Prepared from an isohyetal map: the average depth inside each isohyet and the area enclosed are computed, and plotted as depth (y) against area (x). Curves for different durations form the DAD curve.

```text
 Depth
  |*
  |  *
  |     *
  |         *
  +--------------- Area
```

**Uses:** to convert point rainfall to areal rainfall (areal reduction), design of large catchments, estimating probable maximum precipitation (PMP).

## Check yourself

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

1. How does maximum average intensity vary with duration?
   A. Rises with duration
   B. Constant
   C. Zero for short
   D. High for short, falls for long
   Answer: D. Peak bursts are short. (Key point: Intensity-duration shape)

2. What does adding return period to the intensity-duration curve give?
   A. IDF curve
   B. DAD curve
   C. Hyetograph
   D. Mass curve
   Answer: A. IDF = intensity-duration-frequency. (Key point: Adding frequency)

3. How does average depth change with area?
   A. Increases
   B. Decreases as area increases
   C. Constant
   D. Equal everywhere
   Answer: B. Rain is greatest at the storm centre. (Key point: Depth-area curve)

4. From what is a depth-area curve prepared?
   A. Mass curve
   B. Tipping bucket
   C. Isohyetal map
   D. Double mass curve
   Answer: C. Average depth inside each isohyet and enclosed area are plotted. (Key point: Source of depth-area curve)

## Related in Engineering Hydrology

- [Intensity-duration curve from storm record](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aintensity-duration-30min-5hr) — 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
- [Intensity-duration graph from mass curve ordinates](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aintensity-duration-15-180min) — Engineering Hydrology, chapter 2
- [Hyetograph and intensity-duration curve, Lumle tipping bucket](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Atipping-bucket-lumle) — 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
- [DAD curve procedure and isohyetal area data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Adad-curve-procedure-isohyetal) — Engineering Hydrology, chapter 2

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