---
title: "Causes of inconsistency, mass curve applications"
subject: "Engineering Hydrology"
chapter: 2
key: "semester_5/engineering_hydrology:rainfall-inconsistency-mass-curve"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rainfall-inconsistency-mass-curve
site: BCT Engineering
---

# Causes of inconsistency, mass curve applications

Engineering Hydrology · Chapter 2: Precipitation

- Asked in: 2082 Bhadra, 3+3 marks; 2078 Bhadra, 3+3 marks (asked 2 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rainfall-inconsistency-mass-curve
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-2

## Question

What are common causes of inconsistencies in rainfall data? Please demonstrate applications of mass curves with appropriate illustrative examples.

## Short answer

Inconsistencies arise from gauge relocation, exposure changes, instrument or observer changes. Mass curves: rainfall mass curve gives intensity and hyetograph; double mass curve detects inconsistency and corrects earlier data by the ratio of recent to earlier slopes; reservoir mass curve determines storage capacity.

## Answer

#### Causes of inconsistency in rainfall data
A record is inconsistent when the conditions of observation changed during the record period. Common causes:
- Shifting of the rain gauge to a new site, or change of its elevation.
- Change in the surroundings (new buildings, trees, deforestation) that changes exposure.
- Change of the type of gauge or instrument, or its faults (leaks, damage).
- Change in observation procedure, observer, or time of observation.
- Errors in transcription, instrument calibration, or missing data filled wrongly.
- Changes in environment such as urbanisation or land use.

#### Applications of mass curves
**1. Rainfall mass curve (accumulated rainfall vs time):** Slope at any time gives the intensity. From it, the hyetograph and maximum intensity for various durations can be obtained.

*Example:* Cumulative rain 0, 1.0, 3.0, 4.5 cm at 0, 30, 60, 90 min. Intensity in 30-60 min = (3.0 - 1.0)/0.5 = 4 cm/h, the greatest.

**2. Double mass curve:** Plot cumulative rainfall of the test station against cumulative average of nearby stations.
- Straight line means consistent.
- A break in slope shows change of regime. The data before the break are corrected by

$$
P_{c} = P_{x}\times\frac{M_c}{M_a}
$$

where $M_c$ is the slope of the recent segment (to which the data are corrected) and $M_a$ is the slope of the earlier segment.

*Example:* If the slope before 1990 is 1.2 and after is 0.9, rainfall before 1990 is multiplied by 0.9/1.2 = 0.75 to make it consistent with the present site.

**3. Other uses:** The reservoir mass curve (Rippl diagram) of cumulative inflow gives the storage required, and mass curves of runoff are used to find yield and detect inconsistent streamflow records.

## 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 rain gauge to a new site
   B. Rain heavier than average
   C. A wet year
   D. More rainy days
   Answer: A. Change in observation conditions. (Key point: Cause)

2. Slope of a rainfall mass curve gives:
   A. Total depth
   B. Intensity
   C. Area
   D. Evaporation
   Answer: B. Slope = rain per time. (Key point: Mass curve slope)

3. Between 30 and 60 min cumulative rain goes 1.0 to 3.0 cm. Intensity is:
   A. 2 cm/h
   B. 1 cm/h
   C. 4 cm/h
   D. 6 cm/h
   Answer: C. 2 cm per 0.5 h. (Key point: Example)

4. Slopes 1.2 before and 0.9 after 1990. Factor for older data:
   A. 1.2/0.9 = 1.33
   B. 1.2 x 0.9
   C. 0.9 + 1.2
   D. 0.9/1.2 = 0.75
   Answer: D. Mc/Ma. (Key point: Correction example)

## Related in Engineering Hydrology

- [Causes of rainfall inconsistency and correction](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arainfall-inconsistency-causes-correction) — Engineering Hydrology, chapter 2
- [Phi-index and rainfall excess duration from mass curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aphi-index-mass-curve-3-8cm) — Engineering Hydrology, chapter 3
- [Double mass curve method for consistency](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Admc-method-explain) — Engineering Hydrology, chapter 2, asked 5×
- [Mass curve, hyetograph, depth-duration curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Amass-curve-hyetograph-mdd) — Engineering Hydrology, chapter 2
- [Intensity-duration graph from mass curve ordinates](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aintensity-duration-15-180min) — Engineering Hydrology, chapter 2
- [Phi-index from mass curve, 3.5 cm runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aphi-index-3-5cm-runoff) — Engineering Hydrology, chapter 3

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