25 Distribution of Precipitation over India

Pulak Guhathakurta

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1. Learning outcomes
2. Introduction
3. Seasonal and annual distribution of precipitation
3.1. Winter
3.1.1. Rainfall
3.1.2. Rainy days
3.1.3. Variability
3.1.4. Weather systems
3.2. Pre-monsoon
3.2.1. Rainfall
3.2.2. Rainy days
3.2.3. Variability
3.2.4. Weather systems
3.3. South West monsoon
3.3.1. Rainfall
3.3.2. Rainy days
3.3.3. Variability
3.3.4. Weather systems
3.4. Post monsoon
3.4.1. Rainfall
3.4.2. Rainy days
3.4.3. Variability
3.4.4. Weather systems
3.5. Annual
3.5.1. Rainfall
3.5.2. Rainy days
3.5.3. Variability
4. Summary

 

 

  1. Learning outcomes
  • After studying this module, you shall be able to:
  • know the various aspects of rainfall distribution like normals, rainy days and variability know the spatial distribution of rainfall over India during different seasons of the year
  1. Introduction

India is largely an agrarian economy and rainfall received each year holds key to the success of various agricultural operations, yield or total production and societal well being. The country is so vast that it often happens to be seen in a same year while one part of the country is facing drought some other parts witnessing floods with the total rainfall for the country in its long term normal range. That is information on total annual rainfall for the country as a whole is inadequate from operational point of view. Besides, for sustainable planning of natural resources especially water across regions of the country and for policy making, it is very much essential to have detail knowledge of the typical rainfall distribution pattern spatially, within the annual calendar and the inter annual variabilities. This module discusses the aforesaid issues.

  1. Seasonal and annual distribution of precipitation The Meteorological seasons over India are:
  • Winter : January – February
  • Hot Weather (Pre Monsoon): March – May
  • Southwest Monsoon : June – September
  • Post Monsoon : October – December

3.1. Winter season (January-February)

 

Major portion of the country gets less than 5 % of annual rainfall with western parts reporting less than 1% of annual rainfall in this season. Jammu and Kashmir and adjoining Punjab, south eastern parts of Tamilnadu and Arunachal Pradesh show rainfall between 10-25 percentage of annual rainfall with one or two pockets of having greater than 25% of annual rainfall.

 

3.1.1.   Rainfall

 

Fig. 1 depicts the rainfall during the winter season. Rainfall is light during this season. The rainiest region in this season is located at North Assam, Arunachal Pradesh , Meghalaya and central region of Jammu and Kashmir where average rainfall reaches more than 20cm. South eastern parts of Tamil Nadu also gets considerable amount of rainfall ranging from 10 -15 cm. For the rest of the country rainfall is mainly less than 1cm over western parts and within 2.5 cm over central parts and within 5cm over most of the eastern region. Thus the rainfall during the winter season accounts for only 3.5 % of the annual rainfall.

 

3.1.2.   Rainy days

 

Fig. 2 depicts the frequency of rainy days (when a station receives 2.5 mm or more amount of rainfall during a day, it is termed as a rainy day). The frequency of rainy days is greater than 10 over central Kashmir. Arunachal Pradesh and adjoining north Assam reports greater than 15 days frequency and this frequency gradually decrease towards the south. Rest of India has the frequency not more than 5 days with major portion getting less than 2days only.

 

3.1.3.   Variability

 

Fig. 3 depicts the coefficient of rainfall variation. The variability of rainfall is very high over the entire country being more than 80 to 100% except northern and north eastern parts of India where the coefficient of variability is mostly between 50 to 80 %. In some areas of Jammu and Kashmir and north eastern parts of Assam and Arunachal Pradesh it accounts less than 40%.

3.1.4.     Weather systems

 

Western disturbances (WD) are synoptic scale weather systems which occur in middle latitude westerlies during the winter season. These systems approach north-west India, especially northern part of India north of 30o N. Movement of W.D. is mainly in ENEly direction. Sometimes a trough from W.D. penetrates into Arabian Sea and the anticyclone over central parts slightly shifts eastwards and the system will get the fresh supply of moisture from the Arabian Sea.

 

On some occasions, the anticyclone shifts to the Bay of Bengal along with the movement of W.D., in this case the trough also extends from W.D. into Arabian Sea. In this case, the W.D. gets the fresh supply of moisture not only from the Arabian Sea but from the Bay of Bengal also. Activity of W.D. depends upon the upper level divergence and the fresh moisture supply either from the Arabian Sea or the Bay of Bengal. Also due to the present of easterly wave over Bay of Bengal southern peninsular India gets rainfall during this season.

 

3.2.  Hot weather or Pre monsoon (March-May)

 

This is the summer or hot weather season. Most parts of central and western India receives rainfall within 5% of annual rainfall in this season. The regions of Jammu and Kashmir, eastern and north eastern India, major portion of south peninsula along with east coast belt receive rainfall between 5-15 % of annual rainfall whereas some regions of north east India and Jammu & Kashmir are having seasonal rainfall 15-30% of annual rainfall.

 

3.2.1.   Rainfall

 

Fig. 4 depicts the normal rainfall pattern during the summer season. Northwestern parts of India get least rainfall and amount gradually increases as we go east and southeastwards. The highest rainfall regions (100cm or more) are Meghalaya and eastern parts of Arunachal Pradesh. Most of the parts of northeast India except Manipur receive rainfall between 30 to 100 cm while some parts of Jammu and Kashmir and Kerala receive rainfall between 30 to 50cm. In the eastern parts mostly over Gangetic West Bengal, Bihar, Jharkhand, Orissa and Karnataka, Marathwada and southern parts of Konkan & Goa, south Tamilnadu seasonal rainfall is 10 to 30 cm.

In terms of all India rainfall this season accounts for only 11.1 % of the annual rainfall.

 

3.2.2. Rainy days

 

Fig. 5 depicts the frequency of rainy days. Frequency of rainy days is minimum ( less than 2) in Gujarat and adjoining west Madhya Pradesh and west Rajasthan while it is maximum (more than 40) over Cherrapunji and adjoining areas and extreme west Arunachal Pradesh. Frequency of rainy days is within 5 to 20 over most parts of eastern and peninsular and northern India. For the rest of the country frequency is between 2 to 5 days.

 

3.2.3.   Variability

 

Fig. 6 depicts the coefficient of rainfall variation. North west and central India accounts for more than 100% variation of rainfall. Coefficient of variability is within 50% over Northeast India while over extreme eastern parts of Arunachal Pradesh and Assam and some northern areas of Assam and Nagaland variability is even less than 30%. Jammu and Kashmir, Bihar, WB, Orissa and Peninsular India except either coastal belt shows CV between 30-60%.

 

3.2.4.   Weather systems

 

Intense convective activity (like thunderstorms) over India generally occurs during this season. The convective activity over NE India is generally known as Nor westers, and more locally as Kalabaisakhi. This activity starts in the month of March and reaches its maximum in May. The Convective Activity over NW India is called as “ANDHIS “. The activity starts in the month of April and it continues in June also. This activity occurs due to intense surface heating and instability. There are two types of dust storms or sand storms which are occurring over north-west India namely, the Convective type and the Pressure Gradient type. The convective activity over central India is generally associated with the low pressure systems or cyclonic circulations. During later part of May the ITCZ comes close to south peninsula and at this time the thunderstorms are mainly over the southernmost parts of peninsula.

 

3.3. South West monsoon season (June- September)

This is the principal season of rainfall for entire country and pattern resembles mostly with annual rainfall. Major portion of India gets more than 80% of annual in this season. Some areas of  Gujarat region and adjoin Maharashtra are even getting more than 95 % of annual rainfall in this season. Seasonal rainfall over south east Tamilnadu are of lowest percentage of annual rainfall (less than 5%). Most parts of Tamilnadu gets 20 to 40 % of annual rainfall during this season. Extreme northern parts of the country get 20% to 40 % of annual rainfall in this season. In NE India, it ranges mostly from 50 to 70%.

 

3.3.1.   Rainfall

 

Fig. 7 depicts the rainfall during the southwest monsoon season. Highest monsoon rainfall of more than 250 cm observed over Meghalaya , smaller areas in northeast Assam and Konkan and Goa. In most parts of eastern, north eastern and central India normal monsoon rainfall is within 100cm to 150 cm. The lowest seasonal rainfall is noticed over southern most parts of Tamilnadu and northern most parts of Jammu & Kashmir of amount less than 2.5 cm. Interiors of south peninsula receives not more than 40 cm of rainfall. Rainfall over extreme northwestern parts of country i.e. over west Rajasthan is less than 40 cm. Rainfall between 40 cm to 100 cm is noticed over interior Maharashtra, east Rajasthan, west Madhya Pradesh, most parts of Uttar Pradesh and Andhra Pradesh.

 

In terms of all India rainfall this season accounts for 74.7 % of the annual rainfall.

 

3.3.2.   Rainy days

 

Fig. 8 depicts the frequency of rainy days. During this season out of 122 days, the regions of north east India and Konkan & Goa receive more than 75 rainy days. In major areas of Paschim Banga, Jharkhand, Chhattisgarh and narrow belt along west coast frequency of rainy days is in the range 50 – 75. Lowest frequency of rainy days (less than 15) is over west Rajasthan, south Tamilnadu and extreme northern India.

 

3.3.3. Variability

 

Fig. 9 depicts the coefficient of rainfall variation. In this major rain giving season, variability is within 40% over most parts of the country with lowest variability of less than 15 % being observed over few areas of Tripura and Arunachal Pradesh around Itanagar. CV is within 15 % to 20 % over major parts of Tripura, Mizoram, some areas of Manipur, Nagaland, Assam, Arunachal Pradesh, Chhattisgarh and Konkan & Goa. Maximum variability of more than 100% is seen over extreme western Rajasthan and northern parts of Jammu and Kashmir. CV is more than 40% over most areas of Rajasthan, Jammu & Kashmir, Tamilnadu and some areas of eastern Meghalaya.

 

3.3.4.   Weather systems

 

Synoptic disturbances in the form of depressions and lows form in the Bay and move inland. Similarly off-shore troughs and vortices in the lower layers and mid-tropospheric levels over northern Arabian Sea produce heavy precipitation during the season. These disturbances and various other factors produce fluctuations in the amount and spatial distribution of rains in the country. Depending upon the position of the monsoon trough, which normally runs from Ganganagar to Kolkata to head Bay, a monsoon can be strong when monsoon trough is south of the normal position. It can be weak monsoon when monsoon trough is north of the normal position and it can be a break monsoon when monsoon trough runs along the foot hills of Himalayas or not seen at all.

 

3.4.  Post monsoon season (October – December)

 

Prominent feature of this season is the northeast monsoon over south peninsular region. More than 40% of annual rainfall receive in this season over east coast belt from south of Visakapattanam to extreme southern portion of Tamilnadu. Even some regions of Tamilnadu receive more than 70 % of annual rainfall during this season. Rainfall over other parts of peninsular India i.e. Karnataka, Rayalaseema, Kerala and coastal Andhra Pradesh which receive rain due to northeast monsoon is within 20 to 40 % of annual rainfall. Most of the remaining parts of the country receive less than 10 % of annual rainfall during this season.

 

3.4.1.   Rainfall

 

Fig. 10 depicts the rainfall during the post monsoon season. Major portion of central and western India falls under 2.5-7.5 cm. Area covering South peninsula up to 13 deg N which elongates as a strip along east coast narrowing down to Orissa coast receives rainfall ranging from 30 – 75 cm. Pattern shows gradual decrease of rainfall towards NW India and towards south Jammu and Kashmir where it touches 2.5 cm mark with Gujarat reporting less than a cm. Arunachal Pradesh, Tripura, Meghalaya and Mizoram receive greater than 30cm. Parts of Kerala, Jammu and Kashmir and most of NE India receives rainfall anywhere within 30 -75 cm.

In terms of all India rainfall this season accounts for only 10.7 % of the annual rainfall.

 

3.4.2.   Rainy days

 

Fig. 11 depicts the frequency of rainy days. Frequency of rainy days is more than 5 over south of 20 deg N. This increases towards south and SE wards with SE Tamilnadu having frequency more than 20 days. In the areas north of 20 deg N frequency of rainy days confine to less than 5 only.

 

Small areas with frequency greater than 10 are seen over Jammu and Kashmir. In NE India freq is in the range of 10-15 days.

 

3.4.3.   Variability

 

Fig. 12 depicts the coefficient of rainfall variation. Major portion of India except Peninsula and NE India has large variation more than 80%. CV is 40 to 50% in the peninsular India south of 17deg N and in NE India.

3.4.4.   Weather systems

 

North- East (winter) monsoon is characterized by a dry continental air mass blowing from the vast Siberian High pressure area from December to March. The precipitation includes snowfall during winter monsoon. This is also known as retreating monsoon. Much of India experiences infrequent and relatively feeble precipitation during the retreating monsoon. An exception to this rule occurs along the southeastern coast of India and for some distance inland. When the winds blow from the northeast across the Bay of Bengal, it picks up a significant amount of moisture, which is subsequently released after moving back onto the peninsula. Thus, from October to December the coast of Tamil Nadu state receives at least half of its annual precipitation i.e. roughly 1,000 mm of rainfall. Another type of winter precipitation occurs in northern India, due to the weak cyclonic storms originating in the Mediterranean basin. In the Himalayas, these storms bring weeks of drizzling rain and cloudiness and are followed by waves of cold temperatures and snowfall. The state of Jammu and Kashmir in particular receives much of its precipitation from these storms.

 

3.5.       Annual rainfall

 

3.5.1.   Rainfall

 

Fig. 13 depicts the annual rainfall. As most of rainfall of the country is received during SW Monsoon season the pattern resembles that of monsoon period except the region of Tamilnadu and adjoining areas. Rainfall is more than 100cm in areas east of 80 deg E.

 

Rainfall exceeds 250 cm along the west coast, areas over Assam and Meghalaya. Over Western Ghats and most of northeast India, West Bengal and adjoin region rainfall exceeds 150 cm. East coast gets more than 100 cm with pockets of more than 150 cm in Orissa coast. North most of Jammu and Kashmir reports less than 15 cm. Areas of rainfall more than 100 cm dominate over Central India followed by regions of more than 75cm.

 

3.5.1.   Rainy days

 

The areas with maximum frequency are located along the west coast, northeast regions with pockets of 125 days (Fig. 14). East of central India falls under less than 50 days and there is marked decrease further NW wards. The lowest freq in peninsula is in Maharashtra and Karnataka on the lee side of Western Ghats. The annual frequency of rainy days over west Rajasthan, Kutch and north of Jammu and Kashmir is the lowest in the country.

 

3.5.2.   Variability

 

Fig. 15 depicts the coefficient of rainfall variation. In the entire country except NW region, north Jammu and Kashmir, CV lies below 40%. NE India covered by 15- 30% with pockets of 40-50% in some parts of Assam and Meghalaya. West coastal belt and Peninsular India falls under region of variability less than 25% with some exceptions like SE Tamilnadu, central Maharashtra and some interior areas.

  1. Summary
  • Rainfall has high spatial and temporal variability.
  • The country wise average rainfall during the winter season accounts for only 3.5 % of the annual total; the rainiest region is located in North Assam, Arunachal Pradesh, Meghalaya and central part of Jammu and Kashmir (average rainfall > 20 cm). South eastern parts of Tamil Nadu also gets considerable amount of rainfall ranging from 10 -15 cm.
  • All India rainfall during pre-monsoon season accounts for only 11.1 % of the annual total. The highest rainfall regions (100 cm or more) are Meghalaya and eastern parts of Arunachal Pradesh. Most of the parts of northeast India except Manipur receive rainfall between 30 to 100 cm while some parts of Jammu and Kashmir and Kerala receive rainfall between 30 to 50 cm.
  • In terms of all India rainfall, monsoon season accounts for 74.7 % of the annual total. Highest monsoon rainfall (> 250 cm) is observed over Meghalaya, smaller areas in northeast Assam and Konkan and Goa. In most parts of eastern, north eastern and central India normal monsoon rainfall is within 100 cm to 150 cm..
  • Prominent feature of post monsoon season is the northeast monsoon over south peninsular region. More than 40% of annual rainfall received in this season is over east coast belt from south of Visakhapatnam to extreme southern portion of Tamilnadu. Even some regions of Tamilnadu receive more than 70 % of annual total. Rainfall over other parts of peninsular India i.e. Karnataka, Rayalaseema, Kerala and coastal Andhra Pradesh which receive rain due to northeast monsoon is within 20 to 40 % of annual rainfall. In terms of all India rainfall this season accounts for only 10.7 % of the annual rainfall.
  • The Annual rainfall exceeds 250 cm along the west coast and areas over Assam and Meghalaya. Over Western Ghats and most of northeast India, West Bengal and adjoining region rainfall exceeds 150 cm. East coast gets more than 100 cm with pockets of more than 150 cm in Orissa coast. North most of Jammu and Kashmir reports less than 15 cm. Areas of rainfall more than 100 cm dominate over Central India followed by regions of more than 75 cm.
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