1. In India, the rainfall is predominantly dictated by the monsoon climate. The monsoon in India arises from the reversal of the prevailing wind direction from Southwest to Northeast and results in three distinct seasons during the course of the year. The Southwest monsoon brings heavy rains over most of the country between June and October, and is referred to commonly as the ‘wet’ season.
2. Moisture laden winds sweep in from the Indian Ocean as low-pressure areas develop over the subcontinent and release their moisture in the form of heavy rainfall. Most of the annual rainfall in India comes at this time with the exception of in Tamil Nadu, which receives over half of its rain during the Northeast monsoon from October to November.
3. The retreating monsoon brings relatively cool and dry weather to most of India as drier air from the Asian interior flows over the subcontinent. From November until February, temperatures remain cool and precipitation low. In northern India, it can become quite cold, with snow occurring in the Himalayas as weak cyclonic storms from the west settle over the mountains.
4. Between March and June, the temperature and humidity begin to rise steadily in anticipation of the Southwest monsoon. This pre-monsoonal period is often seen as a third distinct season although the post-monsoon in October also presents unique characteristics in the form of slightly cooler temperatures and occasional light drizzling rain.
5. These transitional periods are also associated with the arrival of cyclonic tropical storms that batter the coastal areas of India with high winds, intense rain and wave activity. Rainfall and temperature vary greatly depending on season and geographic location.
6. Further, the timing and intensity of the monsoon is highly unpredictable. This results in a vastly unequal and unpredictable distribution over time and space. In general, the northern half of the subcontinent sees greater extremes in temperature and rainfall with the former decreasing towards the north and the latter towards the west.
7. Rainfall in the Thar Desert and areas of Rajasthan can be as low as 200mm per year, whereas on the Shillong Plateau in the Northeast, average annual rainfall can exceed 10,000 mm per year. The extreme southern portion of the country sees less variation in temperature and rainfall. In Kerala, the total annual rainfall is of the order of 3,000 mm.

Measurement of Rainfall

Rainfall is the source of water used for irrigation purposes and, therefore, knowledge of its amount, character, season or periods and the effects produced by it is of prime importance to all whose duty is to design, carry out, improve, or maintain irrigation works. The amount if precipitation expressed as the depth in centimetres (or inches) which falls on a level surface, and is measured by rain gauge. The following are main types of rain gauges used for measurement of rainfall:

1. Non-automatic rain gauges: This is also known as non-recording rain gauge. Symon’s rain gauge is an example of non-automatic rain gauge
2. Automatic rain gauges: These are integrating type recording rain gauges and are of three types:
   • Weighing bucket rain gauge.
   • Tipping bucket type rain gauge.
   • Float type rain gauge.

1. Symon’s Rain Gauge:

• Symon’s rain gauge is most common type of non-automatic rain gauge and is used by Meteorological Department of Government of India.
• As shown in figure below it consists of cylindrical vessel 127 mm in diameter with a base enlarged to 210 mm diameter.
• The funnel shank is inserted in the neck of a receiving bottle which is 75 to 100 mm diameter. A receiving bottle of rain gauge has a capacity of about 75 to 100 mm of rainfall and during heavy rainfall this quantity frequently exceeds.
• The rain should be measured 3 to 4 times in a day of heavy rainfall lest the receiver should overflow.
• A cylindrical graduated measuring glass is furnished with each instrument, which reads to 0.2 mm. The rainfall should be estimated to the nearest 0.1 mm. • The rain gauge is set up in a concrete block 60 cm X 60 cm X60 cm.

$$\text{Figure 16: Symon's Rain Gauge}$$

The following important points should be kept in mind while selecting the site for a rain gauge station: - The site where a rain gauge is set up should be an open place.

• The distance between the rain gauge and the nearest object should be at least twice the height of the object. In no case should it be nearer to the obstruction than 30m

• The rain gauge should never be situated on the side or top of a hill if a suitable site on the ground can be found.

• In the hills, where it is difficult to find level space, the site for the gauge should be chosen where it is best shielded from high winds, and where the wind does not cause eddies.

• A fence, if erected to protect the gauge from the cattle etc. should be so located that distance of the fence is not less than twice its height.

2. Weighing Bucket Type Rain Gauge:

1. Self-recording gauges are used to determine rates of rainfall over short period of time. The common type of self-recording gauge is the weighing bucket type.
2. The weighing bucket gauge essentially consists of a receiver bucket supported by a spring over a balance or any other weighing mechanism.
3. The movement of the bucket due to its increasing weight is transmitted to a pen which traces the record on a clock driven chart.

$$\text{Figure 17. Weighing Bucket Type Rain Gauge}$$

Figure 18. Photographic View of Weighing Bucket Type Rain Gauge

3. Tipping Bucket Type Rain Gauge

1. A Steven’s tipping bucket type rain gauge consists of 300 mm diameter sharp edge receiver. At the end of the receiver is provided a funnel.
2. A pair of bucket are provided at pivoted under the funnel in such a way that when one bucket receives 0.25 mm of precipitation it tips, discharging its content into a container bringing the other bucket under the funnel.
3. Tipping of the bucket completes an electric circuit causing the movement of a pen to mark on clock driven revolving drum which carries a record sheet.
4. The electric pulses generated due to the tipping of the buckets is recorded at the control room far away from the rain gauge.

$$\text{Figure 19. Tipping Bucket Type Rain Gauge}$$

Figure 20. Photographic View of Tipping Bucket Type Rain Gauge

4. Float or Syphon Type Rain Gauge.

1. The working of a float or syphon type rain gauge is similar to the weighing bucket type rain gauge.
2. A funnel receives the rain water which is collected in a rectangular container. A float is provided at the bottom of the container.
3. The float is raised as the water level rises in the container, its movement being recorded by a pen moving on a clock work.
4. When the water level in the container rises so that the float touches the top, the siphon comes into operation, and releases the water; thus all the water in the box is drained out.

$$\text{Figure 21. Float or (Syphon) Type Rain Gauge}$$

Variation of Rainfall.

1. Rainfall measurement is commonly used to estimate the amount of water falling over the land surface, part of which infiltrates into the soil and part of which flows down to a stream or river.
2. For a scientific study of the hydrologic cycle, a correlation is sought, between the amount of water falling within a catchment, the portion of which that adds to the ground water and the part that appears as streamflow. Some of the water that has fallen would evaporate or be extracted from the ground by plants.
3. In the figure below, a catchment of a river is shown with four rain gauges, for which an assumed recorded value of rainfall depth has been shown in the table.
4. It is on the basis of these discrete measurements of rainfall that an estimation of the average amount of rainfall that has probably fallen over a catchment has to be made. Three methods are commonly used, which are discussed in the following section.

Figure 22. A hypothetical catchment showing 4 Rain Gauge Station

Adequacy of Rain Gauges

If there are already some raingauge stations in a catchment, the optimal number of stations that should exist to have an assigned percentage of error in the estimation of mean rainfall is obtained by statistical analysis as Optimum number of rain gauges = N= $(C_v/ϵ)^2$ where N = optimal number of stations, ϵ= allowable degree of error in the estimate of the mean rainfall and $C_v$= coefficient of variation of the rainfall values at the existing m stations (in per cent). If there are m stations in the catchment each recording rainfall values $P_1$, $P_2$, $P_i$, $P_m$ in a known time, the coefficient of variation Cv is calculated as:

Coefficient of variation = $C_v$= $\frac{(100\times \sigma_{m-1})}{\bar{P}}$ in %

Standard Deviation =$σ_(m-1)$= $\sqrt{\frac{\sum^m_{i=1}(P_i-\bar{P})^2}{m-1}}$

Mean Rainfall = $\bar{P}$= $ \frac{\sum^m_{i=1}P_i}{m}$

$P_i$ = precipitation magnitude in the $i_th$ station In calculating N, it is usual to take ϵ =10%. It is seen that if the value of ϵ is small, the number of rain gauge stations will be more. According to WMO recommendations, at least 10% of the total rain gauges should be of self-recording type.

Average Rainfall Depth

The time of rainfall record can vary and may typically range from 1 minute to 1 day for non – recording gauges, recording gauges, on the other hand, continuously record the rainfall and may do so from 1 day 1 week, depending on the make of instrument. For any time, duration, the average depth of rainfall falling over a catchment can be found by the following three methods.

• The Arithmetic Mean Method
• The Thiessen Polygon Method
• The Isohyetal Method

1. The Arithmetic Mean Method
   • The simplest of all is the Arithmetic Mean Method, which taken an average of all the rainfall depths as shown in Figure below.
   • Average rainfall as the arithmetic mean of all the records of the four rain gauges, as shown below: $$ \frac{15+12+8+5}{4}=10.0\ mm$$

$$\text{Figure 23. Representation of rainfall recorded in four rain gauges}$$

1. The Thiessen Polygon Method
   • This method, first proposed by Thiessen in 1911, considers the representative area for each rain gauge. These could also be thought of as the areas of influence of each rain gauge, as shown in figure

Figure 24. Rainfall measurements by Thiessen Polygon Method (a) Rainfall recorded (b) Area of influence

• These areas are found out using a method consisting of the following three steps:

  1. Joining the rain gauge station locations by straight lines to form triangles
  2. Bisecting the edges of the triangles to form the so-called “Thiessen polygons”
  3. Calculate the area enclosed around each rain gauge station bounded by the polygon edges (and the catchment boundary, wherever appropriate) to find the area of influence corresponding to the rain gauge.

For the given example, the “weighted” average rainfall over the catchment is determined as, $$\frac{55\times15+70\times12+80\times8+35\times5}{(55+70+80+35)}=10.33\ mm $$ 3. The Isohyetal Method

• This is considered as one of the most accurate methods, but it is dependent on the skill and experience of the analyst.
• The method requires the plotting of isohyets as shown in the figure and calculating the areas enclosed either between the isohyets or between an isohyet and the catchment boundary.
• The areas may be measured with a planimeter if the catchment map is drawn to a scale.
• For the problem shown in figure 25, the following may be assumed to be the areas enclosed between two consecutive isohyets and are calculated as under:

Area I = 40 km2

Area II = 80 km2

Area III = 70 km2

Area IV = 50 km2

Total catchment area = 240 $km^2$

• The areas II and III fall between two isohyets each. Hence, these areas may be thought of as corresponding to the following rainfall depths:

Area II: Corresponds to (10 + 15)/2 = 12.5 mm rainfall depth

Area III: Corresponds to (5 + 10)/2 = 7.5 mm rainfall depth

• For Area I, we would expect rainfall to be more than 15mm but since there is no record, a rainfall depth of 15mm is accepted. Similarly, for Area IV, a rainfall depth of 5mm has to be taken. Hence, the average precipitation by the isohyetal method is calculated to be

$$\frac{40\times15+80\times12.5+70\times7.5+50\times5}{240}=9.9\ mm$$

*Isohyets: Lines drawn on a map passing through places having equal amount of rainfall recorded during the same period at these places (these lines are drawn after giving consideration to the topography of the region).