---
title: "Flood hydrograph, DRH, UH, baseflow separation"
subject: "Engineering Hydrology"
chapter: 5
key: "semester_5/engineering_hydrology:define-hydrograph-drh-uh-baseflow"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/define-hydrograph-drh-uh-baseflow
site: BCT Engineering
---

# Flood hydrograph, DRH, UH, baseflow separation

Engineering Hydrology · Chapter 5: Hydrograph Analysis

- Asked in: 2078 Kartik, 8 marks
- Video: [Flood Hydrograph and Base Flow - Hydrology - Water Resource Engineering 1](https://www.youtube.com/watch?v=2wr0FPrvuKU) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/define-hydrograph-drh-uh-baseflow
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-5

## Question

Define flood hydrograph, Direct Run-off Hydrograph (DRH) and Unit Hydrograph. Write the different methods of base flow separation in hydrograph analysis.

## Short answer

Flood hydrograph: discharge vs time during flood. DRH: surface runoff only after subtracting baseflow. UH: DRH from 1 cm effective rainfall uniformly over basin in duration D. Baseflow separation methods: straight-line, fixed-base length (N=0.83A^0.2 days), variable-slope (extend recession curves), semi-log master recession curve.

## Answer

#### Flood hydrograph
A **flood (storm) hydrograph** is the graph of discharge against time at a river section during a flood caused by a storm. It has a rising limb, a crest (peak) and a recession (falling) limb, and it includes both direct runoff and base flow.

```text
 Q |       peak
   |       /\
   | rising/  \ recession
   |   ___/    \___
   |__/  base flow  \____
   +--------------------- t
```

#### Direct runoff hydrograph (DRH)
The **DRH** is the hydrograph of the surface runoff (and interflow) only. It is obtained by subtracting the base flow from the storm hydrograph. The area under the DRH equals the volume of direct runoff, which is the effective rainfall depth multiplied by the basin area.

#### Unit hydrograph
The **unit hydrograph** is the DRH due to 1 cm (1 mm) of effective rainfall, uniformly distributed over the catchment, at a constant rate during a specified duration (the $D$-hour UH).

#### Methods of base-flow separation
1. **Straight-line method:** a horizontal or sloping straight line from the start of the rise to the point on the recession limb where direct runoff ends.
2. **Fixed-base length method:** the end of direct runoff is taken $N = 0.83A^{0.2}$ days after the peak ($A$ in km$^2$). The pre-storm recession is extended forward to the time of the peak, and this point is joined to the end point by a straight line.
3. **Variable-slope (three-point) method:** the pre-storm recession is extended forward to the time of the peak, and the post-storm base flow recession is extended backward to the inflection point on the recession limb. The two points are joined by a straight line.
4. **Master recession curve (semi-log) method:** the point on the semi-log recession plot where the slope changes marks the end of direct runoff.

The area between the hydrograph and the base-flow line gives the direct runoff volume.

## Check yourself

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

1. A flood (storm) hydrograph contains:
   A. Only surface runoff
   B. Only base flow
   C. Both direct runoff and base flow
   D. Only rainfall
   Answer: C. It is total discharge against time and so includes base flow. (Key point: Flood hydrograph)

2. The DRH is obtained by:
   A. Adding base flow to the storm hydrograph
   B. Subtracting base flow from the storm hydrograph
   C. Dividing the storm hydrograph by area
   D. Multiplying by phi
   Answer: B. Removing base flow leaves surface runoff (and interflow). (Key point: DRH)

3. The area under the DRH equals:
   A. Total rainfall x area
   B. Infiltration volume
   C. Effective rainfall depth x basin area
   D. Base flow volume
   Answer: C. Direct runoff volume equals effective rainfall depth multiplied by basin area. (Key point: Area under DRH)

4. In the master recession (semi-log) method the end of direct runoff is where:
   A. The log Q vs t plot becomes a straight line (slope changes)
   B. Q is maximum
   C. Q = 0
   D. Rain starts
   Answer: A. Base flow recession is a straight line on semi-log paper; the departure point marks the end of direct runoff. (Key point: Semi-log method)

## Related in Engineering Hydrology

- [Storm hydrograph, DRH, baseflow separation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Astorm-drh-baseflow-sketches) — 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
- [Factors affecting flood hydrograph, baseflow separation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aflood-hydrograph-factors-baseflow) — Engineering Hydrology, chapter 5
- [Flood hydrograph from 4-h UH, 50 m3/s base](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aflood-hydrograph-4h-uh-3-5) — Engineering Hydrology, chapter 5, asked 2×
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- [Flood hydrograph from 4-h UH, three 2-hr rainfalls](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aflood-hydrograph-4h-uh-3-2) — Engineering Hydrology, chapter 5, asked 2×