DROUGHTS AND FLOOD IN INDIA
The erratic nature of the monsoon rains occasions droughts that occur once every five years on an average. Droughts affect about 35 per cent of the country's area receiving less than 75 cm of rainfall on an average and with poor irrigation facilities. The regions identified as drought-prone areas are: 0.37 million sq km area to the east of the Western Ghats along Krishna river at a distance of about 80 km from the coast; the semiarid and desert regions extending between Ahmedabad, Jullundhur and Kanpur; the Coimbatore and Tirunelveli areas in Tamil Nadu; Kutch and Saurashtra; Kalahandi, Orissa; Purulia, West Bengal; and the Mirzapur plateau.
The worst drought experienced by India occurred in 1877. The rainfall departure in 1877 was -79 per cent which had a spread of over 66.8 per cent of the area. In recent times droughts have occurred in 1979, 1982, and as close as 2000. The drought of 1979 had an adverse impact on about 200 million people in Eastern Rajasthan, Himachal Pradesh, Punjab and Andhra Pradesh.
The drought of 1982 was characterised by monsoon departure of -13.7 per cent and having an adverse affect on 46.4 per cent of the area and almost 100 million people of Rajasthan, Punjab and Himachal Pradesh. A 'phenomenal' drought took place in 1987 when the departure of rainfall was -19.3 per cent and the area suffering from deficient rainfall was 64.3 per cent. Among the victims were about 285 million people and 168 million cattle in 15 States and 6 Union Territories. Saurashtra, Kutch, Diu, the western part of Rajasthan, Delhi and Haryana suffered tremendously.
The occurrence of drought does not always have a link with the occurrence of rainfall in a particular region. In recent years, Cherrapunji which receives the highest amount of rainfall in the world was also facing drought conditions due to lack of water harvesting methods.
In the first quarter of the year 2000, large parts of the country were hit by drought. Some fourteen states reported drought or drought-like conditions of varying magnitude. The worst hit were Rajasthan (in which 2.6 crore people in 23,000 villages in 26 districts were affected), Andhra Pradesh (3 crore people in 17,000 villages in 18 districts) and Gujarat (2.5 crore people in 8,000 villages in 17 districts). Parts of Madhya Pradesh, Orissa, Maharashtra, Manipur, Mizoram and Tripura also came under some stress, as did some districts of Himachal Pradesh, Jammu and Kashmir, Karnataka and West Bengal which reported severe scarcity of water.
The irregularity of the monsoons has made India one of the worst flood-affected countries in the world. Heavy rainfall, cyclones, the overflowing of river waters, an elevation of the riverbeds, changes in the course of rivers and poor drainage facilities cause floods every year in about 25 to 50 per cent of the 2.42-crore-hectare land identified as prone to floods. But river floods cause about 60 per cent of floods in India-the two great river systems of the Ganga and Brahmaputra being mainly responsible. Floods are generally identified with the basins of the Himalayan rivers, the north-western rivers-the Beas, Ravi, Chenab, Jhelum, Sutlej, and those of central India. States most affected by floods are Uttar Pradesh, Bihar, Orissa, Assam, Karnataka, Madhya Pradesh, West Bengal and Gujarat. In the South, the deltas of the Godavari and Krishna have also suffered floods from time to time.
Floods in these areas also result from incessant rains and the occurrence of high tides at the time when the rivers are in spate. Some other causes of floods are the deforested catchment areas of the rivers, and the offshore bars which choke the mouths of the rivers and obstruct the free flow of water into the sea. Under various schemes of flood control protection has been provided through creation of storage dams, drainage channels and by making embankments.
The National Flood Control Programme was launched in 1954. It provides protection works in all flood-prone areas. The National Flood Control Policy comprises three phases: (i) immediate phase extending over a period of 2 years and comprising collection of basic hydrologic data, construction of embankments, urgent spurs, rivetments, improvement of river channels and raising of villages above flood level, (ii) short-term phase covering the next 4 to 5 years consisting of improvement of surface drainage, establishment of flood warning systems, shifting or raising of villages above the flood level, construction of building channel diversions, more embankment, and construction of raised platforms to be used during times of flood emergency; (iii) long-term phase which envisages schemes such as construction of dams or storage reservoirs for flood protection and soil conservation in the catchments of various rivers, and digging larger channel diversions.
Flood forecasting systems in India have been set up at Surat on the Tapti, Baruch on the Narmada, Varanasi, Buxar, Patna, Axambad-all on the Ganga, Dibrugarh and Gauhati on the Brahmaputra, Jalpaiguri on the Teesta, Delhi on Yamuna, Bhubaneshwar on the Subarnarekha and Burha Balang, Brahmani and Baitarni in Orissa, and the Sahibi for Rajasthan, and Gandhinagar on Chambal.
Friday, October 30, 2009
SIGNIFICANCE OF MONSOON RAINS
SIGNIFICANCE OF MONSOON RAINS
The monsoons are the single largest factor affecting the Indian economy as agriculture is directly dependent upon the monsoon rains. Four-fifths of the country's cropped area depend upon these rains. The remaining area is also irrigated with rainwater, stored in tanks and reservoirs. The cropping pattern is also determined by the quantity of rainfall. And so, the erratic nature of the monsoon greatly affects the agricultural output. Rainfall is important for generation of hydro-electricity which makes up for two-fifths of the total power generation.
The monsoons are the single largest factor affecting the Indian economy as agriculture is directly dependent upon the monsoon rains. Four-fifths of the country's cropped area depend upon these rains. The remaining area is also irrigated with rainwater, stored in tanks and reservoirs. The cropping pattern is also determined by the quantity of rainfall. And so, the erratic nature of the monsoon greatly affects the agricultural output. Rainfall is important for generation of hydro-electricity which makes up for two-fifths of the total power generation.
MECHANISM OF MONSOONS
MECHANISM OF MONSOONS
High temperatures in the months of May and June result in the formation of a low pressure zone along the Indo-Gangetic plains. The axis of this 'monsoon trough', oriented from the north-west to the south-east, shows periodic movements that play an important role in predictions of the monsoons.
The explanations given by Dr P. Koteshwaram' and Prof F. Flohn identify an area of rising air motion formed by the atmosphere over the Tibet plateau. Due to the rotation of the earth, the ascending air is deflected to the Indian west coast. as a return current from a southwesterly direction. The air gets laden with moisture as a result of the evaporation process when it moves across the Indian Ocean.
High temperatures in the months of May and June result in the formation of a low pressure zone along the Indo-Gangetic plains. The axis of this 'monsoon trough', oriented from the north-west to the south-east, shows periodic movements that play an important role in predictions of the monsoons.
The explanations given by Dr P. Koteshwaram' and Prof F. Flohn identify an area of rising air motion formed by the atmosphere over the Tibet plateau. Due to the rotation of the earth, the ascending air is deflected to the Indian west coast. as a return current from a southwesterly direction. The air gets laden with moisture as a result of the evaporation process when it moves across the Indian Ocean.
THE RETREATING MONSOON
THE RETREATING MONSOON
The period is from mid-September to mid-December. It marks transitional weather conditions between hot-wet and cool-dry weather. The south-westerly monsoon winds gradually retreat. Owing to the prevalence of high temperature-about 24°C-there is a period of hot weather called 'October Heat'. Due to westerly jet-stream, cyclonic rainfall is experienced in October. Violent tropical cyclone is also formed in Bay of Bengal and Arabian Sea. It causes great destruction in Godavari and Krishna deltaic regions.
The retreating monsoon is responsible for winter rainfall in Coromandal coast (Tamil Nadu) due to exclusive local relief factors.
The period is from mid-September to mid-December. It marks transitional weather conditions between hot-wet and cool-dry weather. The south-westerly monsoon winds gradually retreat. Owing to the prevalence of high temperature-about 24°C-there is a period of hot weather called 'October Heat'. Due to westerly jet-stream, cyclonic rainfall is experienced in October. Violent tropical cyclone is also formed in Bay of Bengal and Arabian Sea. It causes great destruction in Godavari and Krishna deltaic regions.
The retreating monsoon is responsible for winter rainfall in Coromandal coast (Tamil Nadu) due to exclusive local relief factors.
Effect of Relief
Effect of Relief The rain-laden winds from Arabian Sea bring orographic rainfall in Konkan coast and Western Ghats (about 250 em). The leeward side receives only 3775 em rainfall as it falls in the rain-shadow area. Thus TamilNadu gets no rain.
Distribution of Rainfall Except for Thar, north-west region of Jammu and Kashmir and some areas of Tamil Nadu, the rest of the country receives rainfall from southwest monsoon. 200 cm and above-Western Ghats and North-eastern region. 100-200 cm-West Bengal, Orissa, Bihar, Jharkhand, Andhra Pradesh, Kashmir foothills, Tamil Nadu coast. Less than 100 cm-Rajasthan, Punjab, western Uttar Pradesh, western Madhya Pradesh and Deccan plateau.
Variability of Annual Rainfall One of the important characteristic features of the monsoon is its variability. There may be a deviation by 20 to 50 per cent in the actual rainfall of a place in a year.
The variability of rainfall may be calculated with the help of the following formula:
C = (Standard deviation/Mean) x 100
where C is coefficient of variation.
The value of C indicates the deviation from the mean value of rainfall. In India, the coefficient of variation generally ranges between 15 and 30 per cent. The coefficient of ~ariation in places such as Mangalore, Sikkim, Arunachal Pradesh, Nagaland, Manipur and Mizoram is less than 15 per cent. On the other hand, it may be as high as 30 per cent in the interior regions of Maharashtra, Andhra Pradesh and Karnataka. So is the case with southern Gujarat, western Madhya Pradesh and central parts of Uttar Pradesh. The variability of rainfall over Rajasthan and Gujarat is over 40 per cent, while the desert areas of Rajasthan and parts of Kutch and Gujarat may have a rainfall variability ranging between 50 and 80 per cent.
Variability of rainfall plays a crucial role in Indian agriculture. Areas having high variability of rainfall suffer from a chronic water scarcity. Crop failures are common in such areas. These areas also experience spells of drought frequently.
Distribution of Rainfall Except for Thar, north-west region of Jammu and Kashmir and some areas of Tamil Nadu, the rest of the country receives rainfall from southwest monsoon. 200 cm and above-Western Ghats and North-eastern region. 100-200 cm-West Bengal, Orissa, Bihar, Jharkhand, Andhra Pradesh, Kashmir foothills, Tamil Nadu coast. Less than 100 cm-Rajasthan, Punjab, western Uttar Pradesh, western Madhya Pradesh and Deccan plateau.
Variability of Annual Rainfall One of the important characteristic features of the monsoon is its variability. There may be a deviation by 20 to 50 per cent in the actual rainfall of a place in a year.
The variability of rainfall may be calculated with the help of the following formula:
C = (Standard deviation/Mean) x 100
where C is coefficient of variation.
The value of C indicates the deviation from the mean value of rainfall. In India, the coefficient of variation generally ranges between 15 and 30 per cent. The coefficient of ~ariation in places such as Mangalore, Sikkim, Arunachal Pradesh, Nagaland, Manipur and Mizoram is less than 15 per cent. On the other hand, it may be as high as 30 per cent in the interior regions of Maharashtra, Andhra Pradesh and Karnataka. So is the case with southern Gujarat, western Madhya Pradesh and central parts of Uttar Pradesh. The variability of rainfall over Rajasthan and Gujarat is over 40 per cent, while the desert areas of Rajasthan and parts of Kutch and Gujarat may have a rainfall variability ranging between 50 and 80 per cent.
Variability of rainfall plays a crucial role in Indian agriculture. Areas having high variability of rainfall suffer from a chronic water scarcity. Crop failures are common in such areas. These areas also experience spells of drought frequently.
SEASONS IN INDIA
SEASONS IN INDIA
On the basis of monsoonal variations there are four seasons in India.
THE COLD WEATHER SEASON It starts in early December. The mean January temperature in Chennai and Thiruvananthapuram is about 24°-25°C, while in the Northern Plains it is about 10°-15°C. A high pressure area is developed over the plains in the north-west. Sometimes weather is disturbed by cyclonic depressions which originate over the Mediterranean and travel eastwards and reach north-west India. These western disturbances bring rainfall in Punjab region. The Tamil Nadu coastland also receives rainfall during this season from retreating northeastern winds.
THE HOT-WET WEATHER SEASON It starts in March and continues upto June. The Inter-Tropical Convergence (ITC) belt shifts from south to north. Along with it moist equatorial air is drawn from the Indian Ocean due to the anticyclone located around Malagasy. The highest temperature in March is about 38°C in the Deccan plateau. In April, Gujarat and Madhya Pradesh become more hot. The day temperature ranges from 38°C to 43°C. It increases upto 48°C in north-west India in May-June.
Due to increase in temperature the air pressure decreases. A low pressure belt develops by the end of May in Thar desert. Mango showers (pre-monsoon showers) occur in the Malabar Coast. A high pressure belt located in Deccan plateau checks the advance of the monsoon in northern regions. Assam and Bengal receive some rainfall through evening thunderstorms called knl baisakhi. Loa is the dry local wind that occurs in,the north-western region.
THE SOUTH-WEST MONSOON SEASON Due to the intense low pressure belt developed in the north-western plains, the south-west monsoon enters the Indian subcontinent. It blows from the Bay of Bengal and the Arabian Sea.
The normal dates of the onset of monsoon are-May 20 in Andaman and Nicobar Islands, May 29 in Kerala, June 15 at Kolkata and July 1 at Delhi. Cherrapunji (1,070 em)
in Khasi hills receives the maximum amount of rainfall in the world due to local relief factors. Due to the Himalayan and Purvanchal Hills, the rain-laden winds move in northwest direction and cause rainfall in the plains (due to easterly current).
Easterly Depression means a series of depressions which originate in Bay of Bengal. It travels in north-westerly direction and causes rainfall.
On the basis of monsoonal variations there are four seasons in India.
THE COLD WEATHER SEASON It starts in early December. The mean January temperature in Chennai and Thiruvananthapuram is about 24°-25°C, while in the Northern Plains it is about 10°-15°C. A high pressure area is developed over the plains in the north-west. Sometimes weather is disturbed by cyclonic depressions which originate over the Mediterranean and travel eastwards and reach north-west India. These western disturbances bring rainfall in Punjab region. The Tamil Nadu coastland also receives rainfall during this season from retreating northeastern winds.
THE HOT-WET WEATHER SEASON It starts in March and continues upto June. The Inter-Tropical Convergence (ITC) belt shifts from south to north. Along with it moist equatorial air is drawn from the Indian Ocean due to the anticyclone located around Malagasy. The highest temperature in March is about 38°C in the Deccan plateau. In April, Gujarat and Madhya Pradesh become more hot. The day temperature ranges from 38°C to 43°C. It increases upto 48°C in north-west India in May-June.
Due to increase in temperature the air pressure decreases. A low pressure belt develops by the end of May in Thar desert. Mango showers (pre-monsoon showers) occur in the Malabar Coast. A high pressure belt located in Deccan plateau checks the advance of the monsoon in northern regions. Assam and Bengal receive some rainfall through evening thunderstorms called knl baisakhi. Loa is the dry local wind that occurs in,the north-western region.
THE SOUTH-WEST MONSOON SEASON Due to the intense low pressure belt developed in the north-western plains, the south-west monsoon enters the Indian subcontinent. It blows from the Bay of Bengal and the Arabian Sea.
The normal dates of the onset of monsoon are-May 20 in Andaman and Nicobar Islands, May 29 in Kerala, June 15 at Kolkata and July 1 at Delhi. Cherrapunji (1,070 em)
in Khasi hills receives the maximum amount of rainfall in the world due to local relief factors. Due to the Himalayan and Purvanchal Hills, the rain-laden winds move in northwest direction and cause rainfall in the plains (due to easterly current).
Easterly Depression means a series of depressions which originate in Bay of Bengal. It travels in north-westerly direction and causes rainfall.
Climatic Regions of India According to Dr. Trewartha's scheme
Dr Trewartha's scheme has been most satisfactory of all classifications of the Indian climatic regions. He presented a modified form of Koppen's classification. Dr. Trewartha's classification divides India into four major regions of the A, B, C and H types. The A type refers to tropical rainy climate where high temperatures are consistent. The B type stands for a dry climate with high temperatures but little rainfall. The C type indicates a region with dry winters where low temperatures range between 0 °C and 18°C. The H type indicates a mountain climate. The A, B, and C types are further sub-divided.
There are seven climatic types.
Tropical Rain Forest (Am) Found in the westcoastal plains, the Sahyadris and parts of Assam, this climate is responsible for high temperatures that do not fall below 18.2°C even during winters. Temper tures rise to 29°C in April and May which are t] hottest months. Rainfall, though seasonal, is heav that is, about 200 cm annually during May-Nover ber.
Tropical Savanna (Aw) This climate is prevale in most of the peninsular areas except the semi-ar, zone in the leeside of the Western Ghats. It brinl the mean monthly temperature to above 18.2 c during the winters and to about 32°C in summer The maximum temperature may shoot up to 46 0 and sometimes even to 48°C. Rainfall is from Jur to September, although in the south, it may contim upto December end. Annual rainfall varies from ~ cm in the west to 152 cm in the east. As Tamil Nad has a more equitable temperature and experienc. more rainfall during October-December, it is class tied under the sub-type of Tamil Nadu Aw.
Tropical Semi-Arid Steppe Climate (BS) TI rainshadow belt runs southwards from centri Maharashtra to Tamil Nadu in the leeside of tt Western Ghats and the Cardamom Hills. It includE Karnataka state, the interior of Tamil Nadu, wester Andhra Pradesh and central Maharashtra. Temperc ture varies from 20°C to 23.8°C (December) an 32.8 °C (May) which are the coldest and hotte: months respectively. Annual rainfall is unreliabll varying from 40 to 75 em. So it is known as tl1 famine zone of India.
Tropical and Sub-Tropical Steppe (BSh) This climate is prevalent from Punjab to Kutch across the Thar desert and over northern Gujarat and western Rajasthan. Temperature vades from 12°C Uanuary) to 35°C in June. January and June are the coldest and hottest months of the year I,"espectively. The maximum temperature rises up to 49°C. The annual rainfall, varying from 30.5 to 63.5 em, is also highly erratic.
Tropical Desert (BWh) This climate extends over the western parts of Barmer, Jaisalmer and Bikaner districts of Rajasthan and parts of Kutch. Mean monthly temperature is uniformly high (about 34.5 °c) throughout the region during May and June, the hottest months. During winters, temperature decreases towards north. The rainfall is scanty with an annual average of 30.5 em, with some parts receiving a low 12.7 em. Moreover, it is also very erratic. Rains are mostly in the form of cloud bursts, taking place mostly in July-September when the south-west monsoons may penetrate the region.
Humid Sub-Tropical Climate with Dry Winters (Caw) The climate. covers a large area to the south of the Himalayas, east of the tropical and sub-tropical steppe and north of the tropical savanna from Punj~b to Assam. It also extends into Rajasthan east of the Aravalli range. Winters are mild to severe. Summers are extremely hot in the western part but quite mild in the east. May-June are the hottest months. The annual rainfall in the area varies from 63.5 em to more than 254 em, most of it received during the south-west monsoon season. Rainfall is more towards the east and north where the atmosphere is humid. Winters are dry except for a little rain received from the westerly depressions.
Mountain Climate(H) Such type of climate is seen in mountainous regions which rise above 6,000 m or more such as the Himalayas. There is a sharp contrast between the temperatures of the sunny and shady slopes. Inversion of temperature and variability of rainfall are felt progressively as one ascends to altitudes ~gher than 1,500 m.
The Trans-Himalayan belt situated to the northern side of the western Himalayas has a climate which is dry and cold. Vegetation is sparse and stunted. Winters are very cold and rainfall is scanty. Daily as well as annual range of temperature is high.
In the Himalayas where the southern slopes are protected from cold, northerly winds accessible to the south-west monsoons, there is heavy rainfall on the slopes which are at a height of 1,069-2,286 m above sea level.
There are seven climatic types.
Tropical Rain Forest (Am) Found in the westcoastal plains, the Sahyadris and parts of Assam, this climate is responsible for high temperatures that do not fall below 18.2°C even during winters. Temper tures rise to 29°C in April and May which are t] hottest months. Rainfall, though seasonal, is heav that is, about 200 cm annually during May-Nover ber.
Tropical Savanna (Aw) This climate is prevale in most of the peninsular areas except the semi-ar, zone in the leeside of the Western Ghats. It brinl the mean monthly temperature to above 18.2 c during the winters and to about 32°C in summer The maximum temperature may shoot up to 46 0 and sometimes even to 48°C. Rainfall is from Jur to September, although in the south, it may contim upto December end. Annual rainfall varies from ~ cm in the west to 152 cm in the east. As Tamil Nad has a more equitable temperature and experienc. more rainfall during October-December, it is class tied under the sub-type of Tamil Nadu Aw.
Tropical Semi-Arid Steppe Climate (BS) TI rainshadow belt runs southwards from centri Maharashtra to Tamil Nadu in the leeside of tt Western Ghats and the Cardamom Hills. It includE Karnataka state, the interior of Tamil Nadu, wester Andhra Pradesh and central Maharashtra. Temperc ture varies from 20°C to 23.8°C (December) an 32.8 °C (May) which are the coldest and hotte: months respectively. Annual rainfall is unreliabll varying from 40 to 75 em. So it is known as tl1 famine zone of India.
Tropical and Sub-Tropical Steppe (BSh) This climate is prevalent from Punjab to Kutch across the Thar desert and over northern Gujarat and western Rajasthan. Temperature vades from 12°C Uanuary) to 35°C in June. January and June are the coldest and hottest months of the year I,"espectively. The maximum temperature rises up to 49°C. The annual rainfall, varying from 30.5 to 63.5 em, is also highly erratic.
Tropical Desert (BWh) This climate extends over the western parts of Barmer, Jaisalmer and Bikaner districts of Rajasthan and parts of Kutch. Mean monthly temperature is uniformly high (about 34.5 °c) throughout the region during May and June, the hottest months. During winters, temperature decreases towards north. The rainfall is scanty with an annual average of 30.5 em, with some parts receiving a low 12.7 em. Moreover, it is also very erratic. Rains are mostly in the form of cloud bursts, taking place mostly in July-September when the south-west monsoons may penetrate the region.
Humid Sub-Tropical Climate with Dry Winters (Caw) The climate. covers a large area to the south of the Himalayas, east of the tropical and sub-tropical steppe and north of the tropical savanna from Punj~b to Assam. It also extends into Rajasthan east of the Aravalli range. Winters are mild to severe. Summers are extremely hot in the western part but quite mild in the east. May-June are the hottest months. The annual rainfall in the area varies from 63.5 em to more than 254 em, most of it received during the south-west monsoon season. Rainfall is more towards the east and north where the atmosphere is humid. Winters are dry except for a little rain received from the westerly depressions.
Mountain Climate(H) Such type of climate is seen in mountainous regions which rise above 6,000 m or more such as the Himalayas. There is a sharp contrast between the temperatures of the sunny and shady slopes. Inversion of temperature and variability of rainfall are felt progressively as one ascends to altitudes ~gher than 1,500 m.
The Trans-Himalayan belt situated to the northern side of the western Himalayas has a climate which is dry and cold. Vegetation is sparse and stunted. Winters are very cold and rainfall is scanty. Daily as well as annual range of temperature is high.
In the Himalayas where the southern slopes are protected from cold, northerly winds accessible to the south-west monsoons, there is heavy rainfall on the slopes which are at a height of 1,069-2,286 m above sea level.
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