---
title: "Presentation of precipitation data, IDF curve"
subject: "Engineering Hydrology"
chapter: 2
key: "semester_5/engineering_hydrology:present-precip-data-idf"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/present-precip-data-idf
site: BCT Engineering
---

# Presentation of precipitation data, IDF curve

Engineering Hydrology · Chapter 2: Precipitation

- Asked in: 2070 Asar, 3+2+3 marks
- Video: [Rainfall Statistics, Intensity-Duration-Frequency (IDF) Curves - Part 1](https://www.youtube.com/watch?v=JXSwKmfMZJo) — HydroMohsen
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/present-precip-data-idf
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-2

## Question

In what way can you present the precipitation data? What are the benefits of each method? Explain the method of drawing Intensity Duration Frequency (IDF) curve.

## Short answer

Data is presented as tables, hyetographs, mass curves, intensity-duration, DAD, isohyetal maps, and IDF curves. IDF method: from recording gauge, extract annual maximum depths for various durations, fit a probability distribution to get depths for selected return periods, convert to intensity, and plot intensity vs duration on log-log scale.

## Answer

#### Ways of presenting precipitation data
| Method | Description | Benefit |
|---|---|---|
| Tables | Daily, monthly, annual values | Exact numbers; easy reference |
| Hyetograph | Bar chart of rainfall depth/intensity vs time | Shows storm pattern and peak intensity; input to runoff analysis |
| Mass curve | Cumulative rainfall vs time | Slope gives intensity; can derive hyetograph and max depth |
| Intensity-duration curve | Max average intensity vs duration | Design storm for small structures |
| Depth-Area-Duration curve | Average depth vs area for various durations | Design storm for large basins, PMP |
| Isohyetal map | Contours of equal rainfall | Spatial distribution; mean areal rain |
| Rainfall frequency / IDF curve | Intensity vs duration for return periods | Probability-based design |

#### Drawing an IDF curve
1. Collect long-term rainfall records from an autographic (recording) gauge.
2. For each year, find the maximum rainfall depth for durations such as 5, 10, 15, 30, 60, 120, 360 and 1440 min (annual maximum series).
3. For each duration, fit a probability distribution (Gumbel, log-Pearson III) and get the depth for chosen return periods (2, 5, 10, 25, 50, 100 years).
4. Convert depth to intensity: $i = P/D$.
5. Plot intensity (y, usually log scale) against duration (x, log scale) with one curve for each return period.
6. If needed, fit $i = \dfrac{K T^x}{(D+a)^n}$.

```text
 i
  |\
  | \\   100-yr
  |   \\\  25-yr
  |      \\\ 5-yr
  +---------------- D
```

The curve is read for a design duration (usually the time of concentration) and chosen return period to get the design intensity.

## Check yourself

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

1. What is a benefit of the mass curve?
   A. Slope gives intensity; hyetograph and max depth can be derived
   B. It shows spatial distribution
   C. It gives return periods
   D. It gives exact daily values only
   Answer: A. A mass curve plots cumulative rainfall; its slope is intensity. (Key point: Mass curve benefit)

2. What does an isohyetal map show?
   A. Rainfall vs time
   B. Spatial distribution and mean areal rainfall
   C. Intensity vs return period
   D. Daily totals
   Answer: B. Contours of equal rainfall show spatial distribution. (Key point: Isohyetal map use)

3. What data are used to draw an IDF curve?
   A. Annual total rainfall
   B. Monthly means
   C. Annual maximum depths for various durations from a recording gauge
   D. Isohyets
   Answer: C. Annual maximum series for each duration are collected from autographic records. (Key point: IDF data basis)

4. After finding depths for each duration, what is done next?
   A. Plot depths directly vs year
   B. Average all depths
   C. Draw isohyets
   D. Fit a distribution, get depths for return periods, convert to intensity i = P/D, plot
   Answer: D. Gumbel/log-Pearson III gives depth per return period; i = P/D; plot with one curve per T. (Key point: IDF steps)

## Related in Engineering Hydrology

- [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
- [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
- [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, 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 curve and depth-area curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aidf-depth-area-curve) — 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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