---
title: "Causes of rainfall inconsistency and correction"
subject: "Engineering Hydrology"
chapter: 2
key: "semester_5/engineering_hydrology:rainfall-inconsistency-causes-correction"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rainfall-inconsistency-causes-correction
site: BCT Engineering
---

# Causes of rainfall inconsistency and correction

Engineering Hydrology · Chapter 2: Precipitation

- Asked in: 2070 Chaitra, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rainfall-inconsistency-causes-correction
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-2

## Question

What can be the causes of inconsistency while recording the rainfall of a station? Explain how it can be corrected for the future use?

## Short answer

Inconsistency arises from gauge relocation, exposure alteration, instrument/observer change, or environment shift. Correct by double mass curve: plot cumulative station rainfall vs cumulative mean of consistent neighbours; a slope break indicates the change; adjust earlier data by multiplying by the ratio of recent to old slope.

## Answer

#### Causes of inconsistency
- Shifting of the gauge to a new location or elevation.
- Changes in the surroundings: new buildings or trees, or clearing of trees, changing exposure.
- Change in the type or condition of the instrument, or its errors (leaks, damaged funnel).
- Change in observer, observing time or procedure.
- Changes in the environment such as urbanisation, or errors in recording and transcription.

#### Correction for future use
1. Test the record with a **double mass curve**: plot cumulative annual rainfall of the test station against the cumulative mean of 10-25 nearby consistent stations.
2. If the points form a single straight line, the record is consistent. A break in slope shows inconsistency, and the year of break is noted.
3. Correct the earlier (or doubtful) data to the conditions of the recent period:

$$
P_{cx} = P_x \times \frac{M_c}{M_a}
$$

where $M_c$ is the slope of the segment to which the data are to be corrected (usually the recent one) and $M_a$ is the slope of the segment being corrected.

4. Use the adjusted series for frequency analysis, design and further studies.

## Check yourself

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

1. Which is a cause of inconsistency?
   A. Shifting the gauge to a new location
   B. Heavy rain
   C. Using a larger sample
   D. Long record length
   Answer: A. Gauge shift, changed exposure, instrument or observer changes cause inconsistency. (Key point: Cause of inconsistency)

2. Which test detects inconsistency?
   A. Single mass curve of the station
   B. Double mass curve against mean of 10-25 nearby consistent stations
   C. Hyetograph
   D. Thiessen polygon
   Answer: B. A break in slope in the double mass curve shows inconsistency. (Key point: Test method)

3. What is the correction formula?
   A. P_cx = P_x x Ma/Mc
   B. P_cx = P_x + Mc
   C. P_cx = P_x x Mc/Ma
   D. P_cx = P_x / Mc
   Answer: C. Mc is the slope to which data are corrected (recent), Ma the slope of the segment being corrected. (Key point: Correction formula)

## Related in Engineering Hydrology

- [Causes of inconsistency, mass curve applications](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arainfall-inconsistency-mass-curve) — Engineering Hydrology, chapter 2, asked 2×
- [24-hr maximum rainfall for 50-year return period](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-24hr-max-50yr-semilog) — Engineering Hydrology, chapter 6
- [Double mass curve, station X and 18 stations](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Admc-x-18) — Engineering Hydrology, chapter 2
- [Double mass curve, station X and 25 stations, 1941-1976](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Admc-x-25stn-36yr) — Engineering Hydrology, chapter 2
- [Double mass curve, station X and 15 stations, 2013-2022](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Admc-x-15stn-2013-2022) — Engineering Hydrology, chapter 2
- [Estimating missing precipitation at station B](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Amissing-rainfall-four-stations) — Engineering Hydrology, chapter 2

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