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

# Storm hydrograph, DRH, baseflow separation

Engineering Hydrology · Chapter 5: Hydrograph Analysis

- Asked in: 2075 Chaitra, 3+3 marks
- Video: [HIE: Module 3 - Hydrograph & its components and Base flow separation](https://www.youtube.com/watch?v=KxMslGG4uAc) — SAVI SVIT 6CV2018
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/storm-drh-baseflow-sketches
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-5

## Question

Define storm hydrograph, direct runoff hydrograph and baseflow. Explain the methods to separate base flow from storm hydrograph with clear sketches.

## Short answer

Storm hydrograph is total discharge vs time. Direct runoff hydrograph is surface runoff after removing baseflow (groundwater contribution). Baseflow separation methods: straight line from start to end of direct runoff; fixed-base length (N=0.83A^0.2 days after peak); variable-slope joining extended recessions; master recession curve on semi-log plot.

## Answer

#### Definitions
- **Storm hydrograph:** the graph of total discharge against time at a river section during and after a storm. It consists of the rising limb, crest and recession limb, and it includes base flow.
- **Direct runoff hydrograph (DRH):** the hydrograph of surface runoff (plus interflow), obtained from the storm hydrograph by removing the base flow.
- **Base flow:** the part of the stream flow that comes from groundwater and delayed sub-surface flow. It keeps the stream flowing between storms.

#### Methods of base flow separation
```text
 Q |        /\ peak
   |       /  \
   |    A /    \       B = end of direct runoff
   |  ---/------\------B----
   |   (1) straight line AB
   +--------------------------- t
```
**1. Straight-line method:** join the point A, where the rising limb starts, to the point B on the recession limb where direct runoff ends, by a straight line (horizontal when base flow is level). It is simple and suits perennial streams.

```text
 Q |        /\  peak
   |       /  \
   |     A/    \
   |  ---/__    \__ B      (2) A'-B with
   |   A' \ \__________ N days after peak
   +-------------------------- t
```
**2. Fixed-base length method:** the end of direct runoff B is taken $N$ days after the peak, $N = 0.83A^{0.2}$ ($A$ in km$^2$). Extend the pre-storm base flow recession forward from A to a point A' below the peak, and join A' to B by a straight line.

```text
 Q |         /\ peak
   |        /  \
   |       /    \
   |   .--D      C   <- inflection
   |  /  extended recession
   |_/____________________ t
```
**3. Variable-slope method:** extend the pre-storm recession forward to a point D under the peak, extend the post-storm base flow recession backward to the inflection point C on the recession limb, and join D and C. It is considered the most realistic.

**4. Master recession (semi-log) method:** on a semi-log graph the base flow recession is a straight line. The point where the plot of the falling limb merges into this line is the end of direct runoff.

## Check yourself

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

1. Base flow is the part of stream flow that comes from:
   A. Surface runoff
   B. Groundwater and delayed sub-surface flow
   C. Direct rainfall
   D. Snowfall only
   Answer: B. It sustains the stream between storms. (Key point: Base flow)

2. In the straight-line method the base flow line joins:
   A. Start of rise to the point where direct runoff ends
   B. The peak to the origin
   C. Inflection point to the peak
   D. Two points on the rising limb
   Answer: A. A line AB from the start of rise to the end of direct runoff. (Key point: Straight-line method)

3. Which method is considered most realistic?
   A. Straight-line
   B. Fixed-base
   C. Variable-slope
   D. None
   Answer: C. The variable-slope method follows the recession behaviour with points D and C. (Key point: Variable-slope)

4. The fixed-base method places the end of direct runoff:
   A. At peak time
   B. N = 0.83 A^0.2 days after peak
   C. At 2 days always
   D. At the inflection point
   Answer: B. N depends on area A in km^2. (Key point: Fixed-base)

## Related in Engineering Hydrology

- [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
- [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
- [Derive 4-hr UH from triangular direct runoff hydrograph](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Atriangular-dro-4hr-uh) — Engineering Hydrology, chapter 5
- [Direct runoff hydrograph analysis, 4250-acre basin](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Atrapezoid-dro-4250-acre) — Engineering Hydrology, chapter 5
- [Base flow separation methods](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Abaseflow-separation) — Engineering Hydrology, chapter 5