---
title: "Define unit hydrograph and its uses"
subject: "Engineering Hydrology"
chapter: 5
key: "semester_5/engineering_hydrology:define-uh-uses"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/define-uh-uses
site: BCT Engineering
---

# Define unit hydrograph and its uses

Engineering Hydrology · Chapter 5: Hydrograph Analysis

- Asked in: 2076 Asoj, 5 marks
- Video: [Unit Hydrograph Introduction - Definition](https://www.youtube.com/watch?v=1kKth86mxPU) — HydrologyVideos
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/define-uh-uses
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-5

## Question

Define unit hydrographs and explain the uses of hydrograph.

## Short answer

A unit hydrograph is the direct runoff hydrograph from 1 cm effective rainfall uniformly over a catchment in a specified duration. Uses include estimating flood hydrographs and peaks for design and forecasting, deriving synthetic UH for ungauged basins, studying land-use change impacts, and reservoir routing.

## Answer

#### Unit hydrograph
A **unit hydrograph (UH)** is the direct runoff hydrograph resulting from **one unit (1 cm or 1 mm) of effective rainfall** generated uniformly over the drainage basin at a uniform rate during a specified period (the unit duration $D$, e.g. the 3-h UH).

It is based on the assumptions of linearity and time invariance, so hydrographs of other storms of the same duration are found by scaling and superposition.

```text
 Q |      /\
   |     /  \     area = 1 cm x A
   |    /    \__
   |___/        \____
   +------------------- t
        |<--- Tb --->|
```

#### Uses of the hydrograph (unit hydrograph) in engineering
1. **Estimation of flood hydrograph and peak flow** for any storm on the catchment, by multiplying the UH by the effective rainfall and adding baseflow.
2. **Design flood** for dams, spillways, culverts, bridges and flood-control works, using the design storm.
3. **Flood forecasting and warning** from observed rainfall.
4. **Synthetic UH (Snyder, SCS)** for ungauged basins, derived from catchment properties.
5. **Studying the effect of land-use change** (urbanization, deforestation) on runoff.
6. **Deriving UH of other durations** by the S-curve method.
7. **Reservoir and flow routing studies** which need inflow hydrographs.
8. **Water-resources planning,** such as estimating storm runoff volume and time to peak.

## Check yourself

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

1. A UH is based on the assumptions of:
   A. Linearity and time invariance
   B. Constant infiltration and no evaporation
   C. Steady flow
   D. Uniform channel
   Answer: A. These allow scaling and superposition for other storms of the same duration. (Key point: UH definition)

2. A key use of the UH is to:
   A. Estimate the flood hydrograph and peak from effective rainfall
   B. Measure rainfall
   C. Compute evaporation
   D. Calibrate rain gauges
   Answer: A. Multiplying the UH by effective rainfall and adding baseflow gives the flood hydrograph. (Key point: Main use)

3. For ungauged basins the UH is obtained from:
   A. Synthetic methods like Snyder or SCS
   B. Straight-line separation
   C. S-curve
   D. Direct measurement
   Answer: A. Synthetic UHs use catchment properties. (Key point: Ungauged basins)

4. A UH of a different duration is derived using:
   A. Phi-index
   B. S-curve method
   C. Fixed-base method
   D. Rating curve
   Answer: B. The S-curve lagging method converts a UH to another duration. (Key point: Other durations)

## Related in Engineering Hydrology

- [Unit hydrograph, assumptions and limitations](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Auh-assumptions-limits) — Engineering Hydrology, chapter 5, asked 2×
- [Unit hydrograph and UH for multiple storms](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Auh-define-multiple-storm) — Engineering Hydrology, chapter 5, asked 2×
- [Basin area from 4-h unit hydrograph ordinates](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aarea-of-basin-from-4h-uh) — Engineering Hydrology, chapter 5
- [Unit hydrograph, assumptions, base flow separation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Auh-assumptions-baseflow-methods) — Engineering Hydrology, chapter 5
- [Flood hydrograph, DRH, UH, baseflow separation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Adefine-hydrograph-drh-uh-baseflow) — Engineering Hydrology, chapter 5
- [2-hr UH and flood hydrograph from storm](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Auh-2h-from-4h-three-storms) — Engineering Hydrology, chapter 5