Causes of inconsistency, mass curve applications

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.