Measuring Rainfall: How is Rainfall Measured in India?

Read this article to learn about: How is Rainfall Measured in India?

Among all forms of precipitation, rainfall is the common one. It is necessary to know the methods for measuring rainfall and analysing the rainfall data. The amount of rainfall is expressed as depth in centimeters (or millimeters) which falls on a level surface. Intensity of rainfall is the rate at which it falls at any one time. It is expressed as cm per hour or mm per hour.

Rainfall is measured by means of rain gauges.

Rain gauges are classified as-

(i) Non-recording type gauges and

(ii) Recording type gauges.

In the non-recording type rain gauge, the total rainfall in a particular period can be obtained. Observations are taken at the end of 24 hours period or at lesser intervals during heavy rains and the rainfall in the previous period are recorded. The Symons rain gauge (also adopted by the Indian Meteorological Department) is a common non- recording type of rain gauge.

This consists of a cylindrical vessel of 12.7 cm (5 inches) in diameter with a suitable base. On the top, a funnel of exactly 12.7 cm internal diameter is inserted. The shank of the funnel is put in a receiving bottle capable of taking 10 cm of rainfall. The rainfall collected in the bottle is measured with the help of a graduated measuring cylinder furnished with each rain gauge.

In the recording type of rain gauge, the amount of rain that falls with respect to time is suitably recorded on a graph paper. With such a record, it is possible to calculate the intensity of rainfall for any particular time interval. Three types of recording mechanisms are used in these gauges.

These are –

(i) Float,

(ii) Weighing, and

(iii) Tipping bucket.

In the float type mechanism a funnel collects the rainfall and the same is collected in a container. A float is provided in the container and the float rises as the water level in the container rises. The movements of the float are recorded by a pen on a recording drum which in turn is rotated by a clock work.

When the float in the container comes to the maximum level, a siphon comes into operation emptying the container. The float comes down and the process is repeated during the occurrence of the rainfall.

By analysing the chart on which the movements of the float are recorded it is possible to know the total rainfall as well as the rainfall in any particular period. In most of the rain gauges, it will be necessary to change the chart once in a day and in some, once in a week.

In the weighing type mechanism the rainfall is collected in a receiving bucket which is supported on a weighing mechanism like a spring or a lever. Addition of water in the bucket due to rainfall increases its weight which is transmitted by a recording mechanism.

In the tipping bucket mechanism, a bucket receives the rainfall. The bucket is divided into two compartments and so arranged that when one compartment is full, the bucket tips, empties and the second compartment comes into position to receive the water.

When it is full it tips back into original position and the process continues. Arrangement is also made in this gauge to collect the total amount of water from tipping bucket. Thus, the total rainfall can also be known.

The tipping bucket device has certain drawbacks.

These are –

(i) The tipping of the bucket can be designed for particular rainfall intensity and high or lower intensities will give incorrect readings,

(ii) At the time of high rainfall intensities, the tipping of the bucket is so fast that the resulting recording graph may be having too close lines making it difficult to get accurate readings, and

(iii) During the small interval taken for the tipping of the bucket no rainfall is recorded.

Location of Rain Gauges:

Errors in recording of the rainfall can result by improper location of the rain gauges.

The following general rules should be observed while locating the rain gauges:

1. The opening of the rain gauge should be at least 75 cm above the ground level.

2. The rain gauge should be located in an open space free from obstructions. In case of buildings or trees nearby, the distance of the rain gauge from such obstructions should be at least twice the height of the obstruction.

3. Uneven topography should be avoided and the rain gauge should be located on a level ground. Upward or downward wind movement often occurring on uneven topography may affect the amount of rainfall caught by the rain gauge.

Analysis of Rainfall Data:

The rainfall data collected at a rain gauge station can be analysed for different purposes.

A few simple cases are discussed here:

1. Mean Annual Rainfall:

The mean annual rainfall of any rain gauge station is calculated by taking simple average of the annual rainfall of several consecutive years. Greater the number of years for which data is available, more representatives is the value of the mean obtained. Mean monthly or mean weekly rainfalls can similarly be calculated.

2. Rainfall Intensity:

The rate at which rainfall occurs is known as intensity and is expressed as cm/hr, mm/hr or in/hr. In case of non-recording rain gauges, only the average rainfall in a day can be obtained. In case of automatic rain gauges, using the chart obtained from the rain gauge, it is possible to calculate the intensity of rainfall occurring for any selected time interval.

A typical recording rain gauge chart is shown in Fig. 3.4. Table 3.1 shows how the chart is analysed. The time and the amount of rain are selected from representative points where the rainfall rate changes significantly.

Relation between Intensity and Duration of Rainfall:

Considering an individual storm, if P cm rain occurs in a period of T hours, the mean intensity of rainfall I will be P/T cm per hour. In a particular storm, the intensity of rainfall will not be same throughout. If a small period t is suitably selected within the total time, the intensity of rainfall in that period can be much higher than the mean intensity (Fig. 3.5).

While calculating the runoff from small areas, the value of this maximum intensity for different periods is required. The intensity of rainfall is an inverse function of its duration i.e. longer the duration of the rainfall, less will be the mean intensity. It has been reasonably well established that the relation between t, the intensity of rainfall for a small interval to the mean intensity I and rainfall duration T is as follows –

The rainfall records at different places are analysed and the maximum intensities of rainfall, when the time interval is equal to one hour, are calculated. Such values are known as the one hour rainfall of that region (I₀). This is a characteristic of the whole region and applies to a vast area having the same weather conditions. Knowing the one hour rainfall, using Eq. 3.2 the maximum intensity possible for any other interval can be calculated.

Generally, so far as small agricultural watersheds are concerned I₀ can be taken as 10 cm to 12.5 cm for regions of intense and prolonged rainfall and 5 cm to 7.5 cm for other areas. However, accurate results can be obtained by analysing the rainfall records collected over a long period.

Frequency of Rainfall:

Soil conservation structures, constructed to carry runoff should be designed to handle the maximum expected runoff. The maximum expected runoff can be calculated if the maximum expected rainfall is known. The maximum rainfall that is expected depends upon the period that is taken into consideration.

Such period is usually expressed as the frequency. The frequency of occurrence of a rainfall, also known as the recurrence interval, is defined as the period of years during which one storm of a given duration and intensity can be expected to occur.

For example – a 10 year frequency for one hour rain is that magnitude of hourly rainfall which can be expected to be equalled or exceeded once in 10 years. It does not mean that this magnitude of rainfall will occur at a regular interval of 10 years, but it only means that there is every chance for that rainfall to occur once in 10 years.

The probability of occurrence of an event is inversely related to the return period, as-

To calculate the maximum expected rainfall for a particular recurrence interval, probability concepts are used. Using the available data various probability distribution functions can be developed. One of the approaches is an empirical probability distribution developed using a method called plotting position.

In this, the data are arranged by order of magnitude and an empirical cumulative probability of either a “less than” a certain value (smallest to largest) or “greater than” a certain value type (largest to smallest) is calculated.

The probability of a value being greater than is called the exceedance probability. One of the commonly used methods is the Weibull plotting position given as –

When the ranking is from highest to the lowest, P is the probability of values being equal to or greater than the ranked value, denoted by P (X ≥ x). The other plotting position formulae commonly used are given in Example 1.

Example 1:

Monthly rainfall data for the month of October have been collected for the past 15 years at a given location. Estimate the probabilities of exceedence using different plotting position formulae.

It can be stated that using Weibull formula the probability of rainfall being at least 130 mm is 50 out of 100 or 5 years out of ten and 100 mm is some 8 years out of ten.

Usually greater than 25 data points are recommended. For a small number of data points (< 25), the Foster formula gives a larger value than the Weibull formula and smaller than California formula.

The exceedence formula is recommended for estimating the largest values (upper limit) and the California formula for estimating the lowest values (lower limit). As the sample size increases, it is seen that the probability estimates by the four plotting formulae tend to converge.

The intensity, duration and frequency of rainfall are interrelated and such relation depends upon the locality under consideration.

General relationships are given in the form-

Tejwani et al. (1975) analysed the rainfall data for different rainfall stations in India and gave the 24 hours rainfall for different frequencies.

Rainfall records at a particular location are analysed to develop intensity—duration—frequency relations as shown in Figs. 3.6 and 3.7.

Average Depth of Rainfall over an Area:

For any storm the rainfall over a large area will not be the same. If sufficient number of rain gauges is located spread over the entire area, each rain gauge will record certain depth of rainfall. To calculate the average rainfall for the entire area, three methods are available.

These are:

1. Arithmetic mean,

2. Thiessen method, and

3. Isohyetal method

1. Arithmetic Mean:

As the name indicates, in this method, the average rainfall is obtained by dividing the sum of the depths recorded at all stations in the area by the number of stations. This method gives reasonably accurate results provided the rain gauges are distributed all over the area and at the same time the rainfall varies in a regular manner. This is not the usual case and as such more accurate methods as described below should be adopted.

2. Thiessen Method:

In this location of the rain gauges is plotted on a map of the area and the stations are connected by means of straight lines. Perpendicular bisectors are constructed on each of the lines, such that each of the rain gauge station is enclosed in a certain area (Fig. 3.8). For example – station C is enclosed by the polygon O₁O₂O₇O₈.

If P₁, P₂, P₃ … are the amounts of rainfall recorded in each of the rain gauges and A₁, A₂, A₃, are the areas of the polygons enclosing them, the average rainfall P, over the given area A is given by-

3. Isohyetal Method:

In this method, after plotting the location of the rain gauge stations and the amounts of rainfall at each of the stations, isohyetals (lines of equal rainfall) are drawn by interpolation (Fig. 3.9). The area between the successive isohyetals is determined.

Planimeter is an instrument which can be conveniently used for measuring the areas of such irregular figures. The same formula as used in the Thiessen’s method is used, where A₁, A₂…….. now represent the areas between two successive isohyetals and P₁, P₂, ……… represent the average rainfall of the area.

The Thiessen’s method and the isohyetal method give more accurate information than the simple arithmetic mean. Data from rain gauge stations slightly beyond the area under consideration can be used in both these methods.

Example 2:

Calculate the equivalent depth of rainfall for the basin for a storm whose recorded depths are shown in Fig. 3.8.

Solution: