---
title: "Base flow separation methods"
subject: "Engineering Hydrology"
chapter: 5
key: "semester_5/engineering_hydrology:baseflow-separation"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/baseflow-separation
site: BCT Engineering
---

# Base flow separation methods

Engineering Hydrology · Chapter 5: Hydrograph Analysis

- Asked in: 2079 Baisakh, 5 marks
- Video: [Chapter 9 Baseflow seperation](https://www.youtube.com/watch?v=w7E0B7MVZXs) — Seema Shah-Fairbank
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/baseflow-separation
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-5

## Question

How can the base flow be separated in hydrograph analysis? Explain with figure.

## Short answer

Baseflow separation methods: straight-line from rise start to end of direct runoff; fixed-base length N=0.83A^0.2 days after peak, with pre-storm curve extended; variable-slope extending pre- and post-storm recession curves to peak and inflection point; semi-log master recession curve.

## Answer

Base flow separation divides the storm hydrograph into **direct runoff** and **base flow** so that the direct runoff hydrograph (DRH) can be analysed. The common methods are:

```text
 Q |        peak
   |       /\
   |  A   /  \  N days
   | ____/    \____B___C
   |      \ ___/  ..
   |       (1)(2)(3)
   +------------------------ t
```

**1. Straight-line method (Method 1):** join the point A where the rising limb begins to the point B on the recession limb where direct runoff ends (a horizontal or sloping straight line). It is simple and is used for small catchments and perennial streams.

**2. Fixed-base length method (Method 2):** the end of direct runoff is taken $N$ days after the peak,
$$
N = 0.83\,A^{0.2}
$$
($A$ in km$^2$, $N$ in days). Extend the pre-storm base flow recession curve forward from A to a point below the peak, and join this point to the point B at $N$ days after the peak by a straight line.

**3. Variable-slope method (Method 3):** extend the pre-storm base flow recession curve forward to the time of the peak (A to D). Extend the post-storm base flow recession curve backward to the point of inflection C on the recession limb. Join D to C by a straight line.

**4. Master recession curve / semi-log method:** plot the recession limb on semi-log paper ($\log Q$ against $t$). The point where the straight (base flow) recession line begins to depart from the plotted curve is the end of direct runoff.

All methods are arbitrary, so the same method should be used for all storms of the catchment. Volume of direct runoff = area between the hydrograph and the base flow line.

## Check yourself

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

1. Why is base flow separated from a storm hydrograph?
   A. To get the direct runoff hydrograph for analysis
   B. To find the infiltration capacity
   C. To increase the peak
   D. To convert it to a synthetic UH
   Answer: A. Subtracting base flow leaves the DRH, whose area gives direct runoff volume. (Key point: Purpose)

2. In the fixed-base length method, the end of direct runoff is taken at N days after the peak, where N is:
   A. 0.83 A^0.2 (A in km^2)
   B. 0.83 A^2
   C. A^0.83
   D. 0.2 A^0.83
   Answer: A. N = 0.83 A^0.2 with A in km^2 and N in days. (Key point: Fixed-base length)

3. In the variable-slope method the base flow line is drawn by joining:
   A. The start of rise to the peak
   B. The pre-storm recession extended to the peak time and the recession limb extended back to the inflection point
   C. The peak to the end of the hydrograph horizontally
   D. Two points N days apart on the rising limb
   Answer: B. Points D (under the peak) and C (inflection) are joined by a straight line. (Key point: Variable-slope method)

4. Since all separation methods are arbitrary, one should:
   A. Use a different method for each storm
   B. Use the same method for all storms of a catchment
   C. Always use the semi-log method
   D. Ignore base flow
   Answer: B. Consistency keeps the derived DRHs comparable. (Key point: Consistency)

## Related in Engineering Hydrology

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