Physical Environment | ISC Class 12 Geography Notes

This note covers India's locational setting in the Indian Ocean Rim and its advantages as a subcontinent, its extent and frontiers, the geological eras and the geological evolution of the Peninsular Plateau, the Himalayas and the Great Plains, the five physiographic divisions (the Himalayan mountain complex with its three parallel ranges and offshoots, the Indus-Ganga-Brahmaputra Plains, the Peninsular Plateau, the coastal plains and the islands), the Himalayan and Peninsular drainage systems with the names and sources of the main rivers, the factors and mechanism of the Indian monsoon, atmospheric pressure and the four seasons, temperature and rainfall graphs, droughts and floods with their mitigation programmes, and India's forests, forest area and cover, social forestry, forest conservation movements and the National Forest Policy of 1988.

What is India's locational setting in the Indian Ocean Rim?

India's position in the Indian Ocean

India occupies a central position in the Indian Ocean, projecting southward as a peninsula between the Arabian Sea on the west and the Bay of Bengal on the east.

This location gives India a long coastline and places it astride the sea routes that link West Asia, East Africa and Southeast Asia, which together with the island states of the ocean make up the Indian Ocean Rim.

Because it lies at roughly the midpoint of this rim, India is well placed for trade, shipping and strategic influence across the ocean, and its southernmost point lies close to the equator, giving it access to both tropical and sub-tropical latitudes within one country.

The rim countries include South Africa, Kenya and Mauritius in Africa, Oman, the UAE and Iran in West Asia, Sri Lanka and Bangladesh in South Asia, Malaysia, Singapore, Thailand and Indonesia in South-East Asia, and Australia. They cooperate through the Indian Ocean Rim Association, set up in 1997, of which India is a founder member. The location matters today for three reasons. Trade: most of India's foreign trade by volume moves by sea across this ocean. Shipping and energy: the sea lanes that carry oil from the Persian Gulf to East Asia pass close to India and its island groups. Security: India's navy and its island territories let it watch these lanes against piracy and keep them open, an aim India has set out since 2015 as SAGAR (Security and Growth for All in the Region).

The Tropic of Cancer and climatic zones

The Tropic of Cancer runs through the central part of India in an east-west direction, dividing the country into a northern sub-tropical and temperate belt and a southern tropical belt.

The land to the south of the Tropic, being nearer the equator, experiences high temperatures through the year with a small daily and annual range, while the area north of it experiences a more extreme climate with a greater range of temperature.

This single fact of latitudinal position underlies much of the variety in India's climate, vegetation and farming calendar.

What are the locational advantages of India as a subcontinent?

India is called a subcontinent because it is a large, clearly bounded landmass, set apart from the rest of Asia by the wall of the Himalayas in the north and enclosed by the Arabian Sea, the Bay of Bengal and the Indian Ocean on the other three sides.

This setting gives India several locational advantages. It has a long coastline on both flanks of the peninsula, so it can trade with West Asia and Africa across the Arabian Sea and with Southeast Asia across the Bay of Bengal.

The Himalayas form a strong natural frontier in the north, while the passes through the northwestern mountains have long served as gateways for movement and trade with Central Asia.

Its tropical and sub-tropical latitudes give a long growing season and a wide range of crops, and the monsoon winds that the peninsula draws in from the ocean supply most of the country's rainfall.

Its central place in the Indian Ocean, together with the island groups of the Andaman and Nicobar Islands and Lakshadweep, extends India's reach far into the ocean on both sides.

What are India's extent, frontiers and coastline?

India extends roughly from 8°4'N to 37°6'N latitude and from 68°7'E to 97°25'E longitude, giving it a north-south extent and an east-west extent of a similar order, each close to 3,000 km.

The total area of the country is close to 3.28 million km², making it one of the larger countries of the world by area.

India's land frontier is shared with seven neighbouring countries: Pakistan and Afghanistan in the northwest, China, Nepal and Bhutan along the Himalayan north, and Bangladesh and Myanmar in the east.

The total length of this land frontier is close to 15,200 km.

India also has a very long coastline, close to 7,516 km if the coasts of the mainland and the island territories (the Andaman and Nicobar Islands and Lakshadweep) are added together; the mainland coast alone is close to 6,100 km.

This long coastline, together with India's position astride the Indian Ocean trade routes, is why India has historically been described as commanding the "Eastern Seas": it can watch over and support shipping moving between the Suez route, the Persian Gulf, East Africa and Southeast Asia, and its many natural harbours on both coasts support this role.

How did India's geological structure evolve?

The Indian plate originally lay to the south of the equator and was joined to the Australian plate. Over a very long period it broke away, and the Indian plate moved steadily northward while the Australian plate drifted southeastward.

This northward movement, which continues even now, pushed the Indian plate against the Eurasian plate and threw up the Himalayas, and it still produces earthquakes and slow uplift in the mountain belt today.

Based on variations in geological structure, India is divided into three broad geological divisions that broadly follow the physical features of the country: the Peninsular Plateau, the Himalayas and other Peninsular mountains, and the Indus-Ganga-Brahmaputra Plains.

What are the names of the main Standard and Indian geological eras?

The Standard geological eras, from oldest to youngest, are the Archaean, the Proterozoic, the Palaeozoic, the Mesozoic and the Cenozoic.

The Archaean era is the age of the oldest crystalline rocks, and the very ancient gneisses and granites of the Peninsula belong to it. The Proterozoic era follows, and the Palaeozoic era includes the Cambrian period, after which the Peninsula stood as a rigid block.

The Mesozoic era includes the time when the area of the present Thar desert lay under the sea, and the Cenozoic era, made up of the Tertiary and Quaternary periods, is the time of the Himalayan uplift and the filling of the northern plains with alluvium.

The Indian geological eras are named after the same standard divisions, and Indian geology is commonly described in four broad eras: the Archaean era of the oldest gneisses and granites of the Peninsula, the Purana era of the older sedimentary and metamorphic rocks such as those of the Aravalis and the Cuddapah and Vindhyan systems, the Dravidian era of the great earth movements that folded and faulted the older rocks, and the Aryan era of the Himalayan uplift and the deposition of the Indo-Ganga-Brahmaputra alluvium.

The Peninsular Plateau

The Peninsula is made up of a great complex of very ancient gneisses and granites.

Since the Cambrian period it has stood largely as a rigid block, except where its western margin has been submerged beneath the sea and where tectonic activity has altered parts of it without disturbing the original basement rock.

As part of the Indo-Australian Plate, it has been subjected to vertical movements and block faulting; the rift valleys of the Narmada, the Tapi and the Mahanadi, and the Satpura block mountains, are examples of this.

The Peninsula is largely made of relict and residual mountains such as the Aravali hills, the Nallamala hills and the Mahendragiri hills, and its river valleys are shallow with low gradients.

During the Mesozoic era, the area of the present-day Thar desert, in the northwest of the Peninsular Plateau, lay under the sea; this is confirmed by wood fossils at Aakal, estimated to be about 180 million years old, and by marine deposits found around Brahmsar near Jaisalmer.

The Himalayas and other Peninsular mountains

Unlike the rigid Peninsular Plateau, the Himalayas and other Peninsular mountains are young, weak and flexible in structure. They are still subject to the interplay of internal (endogenic) and external (exogenic) forces, which has produced faults, folds and thrust planes.

Being tectonic in origin and dissected by fast-flowing young rivers, these mountains show gorges, V-shaped valleys, rapids and waterfalls, all signs that the rivers here are still in their youthful stage of erosion.

How did the Himalayas evolve geologically?

The Himalayas were formed by the collision of the northward-moving Indian plate with the Eurasian plate, and the mountain belt has grown through successive phases of uplift.

The Tethys Sea, which once lay between the two plates, was slowly squeezed and its sediments were folded and thrust upward to build the mountain ranges.

Because the Indian plate is still moving northward, the Himalayas continue to rise and are still subject to folding, faulting and earthquakes, which is why they are described as young, weak and flexible in structure.

The rivers that drain the rising mountains cut deep gorges at the same time as the uplift went on, and the eroded sediment was carried south to fill the depression of the northern plains.

The Indus-Ganga-Brahmaputra Plains

This is a geo-synclinal depression that reached its greatest development about 64 million years ago, during the third phase of Himalayan mountain building.

Since then it has been gradually filled by sediment brought down by the Himalayan and Peninsular rivers, and the average depth of this alluvial fill ranges between 1,000 and 2,000 m.

These three geological divisions differ sharply from each other: the Peninsular Plateau is ancient, rigid and made of hard crystalline rock, so its rivers have had millions of years to reach a mature, low-gradient stage, while the Himalayas are young, unstable and still being folded and faulted, so their rivers remain youthful, with steep gradients, gorges and rapids.

The plain between them is neither rock mass, being an entirely depositional feature built up by both mountain systems.

What are India's physiographic divisions?

Physiography is the outcome of structure, process and the stage of development reached by the land surface.

India shows great diversity in relief: the north has a vast expanse of rugged topography with mountain ranges, valleys and deep gorges; the south is a stable table land of highly dissected plateaus and denuded rocks with developed scarps; and the vast north Indian plain lies between the two.

On this basis, India is divided into five physiographic divisions: the Himalayan mountain complex, the Indus-Ganga-Brahmaputra Plains, the Peninsular Plateau, the Coastal Plains, and the Islands.

Each of these five divisions is studied with reference to its extent, altitude, slope and landform characteristics.

On an outline map of India, mark and shade the elevation bands given in the key (0-300 m, 300-600 m, 600-900 m and above 900 m), and mark the Himalayas, the Western and Eastern Ghats, the Aravali Range, the Vindhya and Satpura ranges, the Great Indian Desert, the Chota Nagpur plateau, Anaimudi and Mahendragiri peaks, and the Lakshadweep and Andaman and Nicobar island groups.

What are the three ranges and regional divisions of the Himalayan mountain complex?

The Himalayas consist of a series of parallel mountain ranges. The main ranges include the Greater Himalayan range (also called the central axial range), running for about 2,500 km from east to west, with a width varying between 160 and 400 km from north to south, together with its northwestern and northeastern offshoots.

The Himalayas are not merely a physical barrier between the Indian subcontinent and Central and East Asia; they are also a climatic, drainage and cultural divide, blocking cold winds from the north and forcing the monsoon to shed its moisture within the subcontinent.

The general orientation of the ranges is northwest to southeast in the northwestern part of India, but it changes direction further east: in the Darjiling and Sikkim region the ranges run east-west, in Arunachal Pradesh they run southwest to northwest, and in Nagaland, Manipur and Mizoram they run north-south.

What are the three parallel ranges of the Himalayas?

The Himalayan mountain complex is built of three parallel ranges, running one behind the other from north to south.

The northernmost is the Greater Himalayas, or Himadri, the highest and most continuous range, permanently snow-covered and the source of the great rivers.

South of it lies the Lesser Himalayas, or Himachal, a range of lower, more broken mountains with the famous hill stations and valleys.

The southernmost range is the Shiwaliks, the lowest and youngest foothills, made of loose sediment and separated from the Lesser Himalayas by valleys known as duns.

The ranges are arranged in an orthoclinal structure, meaning that the ranges run parallel to one another, each folded along the same general trend of the mountain arc.

The northwestern offshoots of the Himalayas include the Karakoram, the Ladakh, the Zaskar and the Pir Panjal ranges, while the northeastern offshoots include the Patkai Bum, the Naga hills, the Garo, Khasi and Jaintia hills and the Mizo hills.

How do the Western and Eastern Himalayas compare?

What are the regional divisions of the Himalayas?

On the basis of these regional variations, the Himalayas are divided into the Kashmir (or Punjab) Himalayas in the west, the Himachal, Uttaranchal (or Kumaon) Himalayas in the centre, the Nepal Himalayas, and the Assam Himalayas in the east, each with its own character of relief, drainage and vegetation.

Sketch a snow-covered Himalayan peak to represent the young, high and permanently snow-capped crest of the Greater Himalayan range.

What are the relief features and regional divisions of the Indus-Ganga-Brahmaputra Plains?

The northern plains are formed by alluvial deposits brought down by the Indus, the Ganga and the Brahmaputra.

They extend for about 3,200 km from east to west, with an average width between 150 and 300 km, and the maximum depth of alluvium ranges between 1,000 and 2,000 m.

From north to south, the plains are divided into distinct relief belts.

Definition: Bhabar, Tarai, Bhangar and Khadar. Bhabar is a narrow belt, about 8 to 10 km wide, lying parallel to the Shiwalik foothills, where streams coming down from the mountains deposit coarse rock and boulder material and sometimes disappear underground.

Tarai is the belt south of the Bhabar, about 10 to 20 km wide, where the streams re-emerge without a properly marked channel, creating marshy, swampy ground with luxuriant natural vegetation and varied wildlife.

South of the Tarai lie the alluvial plains proper, made up of the Bhangar (the belt of old alluvium) and the Khadar (the belt of new, more fertile alluvium, renewed by annual flooding).

The alluvial plains show mature river features such as sand bars, meanders, ox-bow lakes and braided channels; the Brahmaputra plains in particular are known for riverine islands and sand bars, and the rivers here form some of the world's largest deltas, for example the Sunderbans delta.

Otherwise, much of the plain is a featureless surface with a general elevation of only 50 to 150 m above sea level.

Haryana and Delhi mark the water divide between the Indus and the Ganga river systems, while the Brahmaputra flows from northeast to southwest before turning almost 90° southward at Dhubri to enter Bangladesh.

The fertile alluvial soils of these plains support crops such as wheat, rice, sugarcane and jute, and carry a very large population.

Regionally, the plains are divided into the Rajasthan plain, which contains the Great Indian Desert in its drier western part, the Punjab plain, the Ganga plain, and the Brahmaputra (Assam) plain.

Within the alluvial tracts, the strips of land lying between two rivers are called doabs (literally "two waters"), a term widely used in the Punjab and Ganga plains to describe the fertile interfluve farmland.

Sketch flooded paddy fields with people working in them and trees in the background, to represent the flat, fertile, intensively cultivated northern plain.

What are the relief features of the Peninsular Plateau?

Rising from about 150 m above the river plains to an elevation of 600 to 900 m, the Peninsular Plateau is an irregular triangle bounded by the Delhi ridge (an extension of the Aravalis) in the northwest, the Rajmahal hills in the east, the Gir range in the west and the Cardamom hills in the south, with a further extension in the northeast forming the Shillong and Karbi Anglong plateau.

It is made up of a series of patland plateaus such as the Hazaribagh plateau, the Palamu plateau, the Malwa plateau, the Coimbatore plateau and the Karnataka plateau, and is one of the oldest and most stable landmasses of India.

The Deccan Plateau, Central Highlands and Northeastern Plateau

On the basis of its prominent relief, it is divided into the Deccan Plateau, the Central Highlands and the Northeastern Plateau.

The Deccan Plateau is bordered by the Western Ghats in the west, the Eastern Ghats in the east, and the Satpura, Maikal and Mahadeo hills in the north.

The Western Ghats, known locally as the Sahyadri, the Nilgiri hills and the Anaimalai and Cardamom hills further south, are comparatively higher and more continuous than the Eastern Ghats, with an average elevation of about 1,500 m that increases from north to south;

Anaimudi (2,695 m) on the Anaimalai hills is the highest peak of the Peninsular plateau, followed by Dodabetta (2,637 m) on the Nilgiri hills, where the two Ghats meet. Most Peninsular rivers rise in the Western Ghats.

The Eastern Ghats, by contrast, are discontinuous, lower hills, heavily eroded by rivers such as the Mahanadi, the Godavari, the Krishna and the Kaveri, which cut through them to reach the sea.

The Central Highlands, bounded to the west by the Aravali range, carry the Satpura range of scarped plateaus at 600-900 m, and show metamorphic rocks such as marble, slate and gneiss; an extension as far west as Jaisalmer is covered by longitudinal sand ridges and crescent-shaped dunes called barchans.

The Northeastern Plateau, chiefly the Meghalaya plateau with its Garo, Khasi and Jaintia hill divisions, is an outlier of the main Peninsular block, separated from it by a fault created during the Himalayan uplift; it is rich in coal, iron ore, sillimanite, limestone and uranium.

What are the Malwa, Chotanagpur and Deccan plateaus?

The Malwa plateau lies in the northwestern part of the Peninsular Plateau, between the Aravalis and the Vindhyan range, and is built of lava flows that give it its dark soil.

The Chotanagpur plateau lies in the northeastern part of the Peninsula, with the Rajmahal hills on its north-eastern edge, and is a large reserve of mineral resources such as coal and iron ore.

The Deccan Plateau is the great triangular tableland of the south, bounded by the Western Ghats, the Eastern Ghats and the Satpura range, and it slopes gently from west to east.

Together these plateaus make up the broad, stable tableland of the Peninsula, with an average elevation of 600 to 900 m and a general slope from west to east, which is also shown by the eastward flow of most of its rivers.

Distinctive relief features

Distinctive relief features of the plateau include badland topography (seen in the Chambal ravines, where gullies have cut the surface into a maze of ridges and channels), tors, block mountains, rift valleys and bare rocky hummocky hills.

Sketch a rugged plateau surface with layered, eroded rocky cliffs to show the ancient, dissected character of the Peninsular plateau, in contrast with the youthful Himalayan relief.

Sketch a sandy desert landscape with dunes and camels crossing it, to represent the Great Indian Desert (Marusthali), lying northwest of the Aravalis, with low rainfall below 150 mm a year.

How do the Western and Eastern Coastal Plains differ?

India's coastal plains are divided into the western coastal plains and the eastern coastal plains, which are geomorphologically opposite types.

The western coastal plains are an example of a submerged coast: the city of Dwaraka, once part of the Indian mainland on the west coast, is believed to now lie under water.

Because of this submergence the western coast is narrow, and this narrowness provides natural depth for ports and harbours such as Kandla, Mazagaon, Marmagao, Mangalore and Cochin.

It is divided from north to south into the Kachchh and Kathiawar coast, the Konkan coast, the Goan coast and the Malabar coast, and it narrows in the middle while broadening towards the north and south.

Rivers crossing the western coastal plain do not form deltas. The Malabar coast has distinctive lagoons called Kayals (backwaters), used for fishing, inland navigation and tourism, including the Nehru Trophy Vallamkali boat race held every year in Punnamada Kayal, Kerala.

The eastern coastal plains, by contrast, are broader and are an example of an emergent coast, with well-developed deltas formed by eastward-flowing rivers entering the Bay of Bengal, including the deltas of the Mahanadi, the Godavari, the Krishna and the Kaveri.

Because the coast is emergent, it has fewer natural ports and harbours, and the continental shelf extends up to 500 km into the sea, making good harbour development harder.

What are the divisions of the Eastern Coastal Plain?

The eastern coastal plain is divided from north to south into three parts.

The Utkal coast lies in Odisha, along the deltas of the Mahanadi and other rivers, and is broad and fertile.

The Andhra coast lies further south, along the deltas of the Godavari and the Krishna, and is also a wide, delta-built plain.

The Coromandel coast lies in the far south, along Tamil Nadu, and is the southernmost part of the eastern plain.

All three divisions are broad, emergent plains built up by river deltas, in contrast with the narrow, submerged divisions of the western coast.

Sketch a coastal scene with palm trees and a water body to represent the low, flat coastal plain landscape typical of both coasts of India.

How do the Andaman and Nicobar Islands differ from Lakshadweep?

India has two main island groups, one in the Bay of Bengal and one in the Arabian Sea, and they differ considerably in origin and character.

Some of the highest points of the Andaman and Nicobar Islands are Saddle peak on North Andaman (738 m), Mount Diavolo on Middle Andaman (515 m), Mount Koyob on South Andaman (460 m) and Mount Thuiller on Great Nicobar (642 m).

Sketch a small forested island surrounded by sea water with a visible sandy beach, to represent an island of either group.

What are India's major rivers and drainage systems?

Definition: Drainage, drainage basin and watershed. The flow of water through well-defined channels is called drainage, and the network of such channels is a drainage system.

The area drained by a river and its tributaries is its drainage basin (or river basin for larger rivers); the line separating one drainage basin from another is the watershed, a term usually kept for the catchments of smaller rivulets, while larger catchments are called river basins.

On the basis of where they discharge, Indian rivers are grouped into the Arabian Sea drainage and the Bay of Bengal drainage, separated by the Delhi ridge, the Aravalis and the Sahyadris.

Nearly 77 per cent of India's drainage area, including the Ganga, the Brahmaputra and the Mahanadi, drains towards the Bay of Bengal, while 23 per cent, including the Indus, the Narmada and the Tapi, drains to the Arabian Sea.

On the basis of origin, rivers are classed as Himalayan or Peninsular.

The Himalayan rivers, including the Indus, the Ganga and the Brahmaputra systems, are fed both by melting snow and by rainfall, so they are perennial.

In their mountain courses they form deep gorges and V-shaped valleys, but on entering the plains they build flat valleys, ox-bow lakes, flood plains, braided channels and deltas, and they meander and frequently shift course; the Kosi, called the "sorrow of Bihar", is notorious for this.

Peninsular rivers, by contrast, generally have a fixed course, show few meanders and are largely non-perennial, drying up or shrinking in the dry season; the Narmada and the Tapi, which flow through rift valleys, are exceptions to this pattern.

1. During the Miocene period, about 5 to 24 million years ago, a single mighty river, called the Shiwalik or Indo-Brahma, flowed the entire length of the Himalaya from Assam to Punjab and on to Sind.
2. During the Pleistocene period, upheaval in the western Himalayas, including the uplift of the Potwar Plateau (the Delhi Ridge), created the water divide between the Indus and Ganga systems.
3. At about the same time, down-thrusting of the Malda gap between the Rajmahal hills and the Meghalaya plateau diverted the Ganga and the Brahmaputra towards the Bay of Bengal.
4. As a result, the single Indo-Brahma river was dismembered into three separate systems: the Indus and its tributaries in the west, the Ganga and its tributaries in the centre, and the Brahmaputra and its tributaries in the east.

The evolution of the Peninsular drainage followed a different course: the subsidence of the western flank of the Peninsula in the early Tertiary period disturbed the symmetry of its rivers; the upheaval of the Himalayas caused subsidence and trough faulting along the northern flank, which the Narmada and the Tapi now occupy without alluvial or deltaic deposits; and a slight tilt of the Peninsular block from northwest to southeast gave the whole Peninsular drainage its general orientation towards the Bay of Bengal.

What are the names and sources of the main rivers?

On an outline map, mark the Indus, Ganga, Brahmaputra, Narmada, Tapi, Godavari, Krishna, Mahanadi and Kaveri rivers, and draw the water divide line separating the Arabian Sea drainage from the Bay of Bengal drainage.

What factors control India's climate?

India's climate, though broadly monsoonal, varies greatly between regions. While the mercury occasionally touches 55°C in western Rajasthan in summer, it may fall to as low as minus 45°C around Leh in winter; while Cherrapunji and Mawsynram receive more than 1,080 cm of rainfall a year, Jaisalmer rarely receives more than 9 cm. Several factors, acting together, explain this diversity.

Latitude, mountains and distance from the sea

Latitude: the Tropic of Cancer divides India into a tropical southern zone, with high year-round temperatures and a small daily and annual range, and a sub-tropical northern zone, with a more extreme climate.

The Himalayan Mountains act as an effective climatic divide, shielding the subcontinent from cold winds from the Arctic and central Asia, and also trapping monsoon winds so that they shed their moisture within the subcontinent.

Distribution of land and water: because the ocean heats and cools more slowly than land, the differential heating of the Indian landmass and the surrounding Indian Ocean creates different pressure zones in different seasons, which drives the reversal of the monsoon winds.

Distance from the sea gives coastal regions an equable climate and interior regions an extreme one, so places such as Mumbai have little sense of seasonal extremes while Delhi, Kanpur and Amritsar experience sharp seasonal contrasts.

Altitude, relief and monsoon-specific mechanisms

Altitude lowers temperature with height, so Darjiling, on the same latitude as Agra, has a January temperature of only 4°C against Agra's 16°C.

Relief affects temperature, pressure, wind direction and rainfall; the windward slopes of the Western Ghats and Assam receive heavy rainfall in June-September while the leeward Deccan plateau stays comparatively dry.

Several additional mechanisms shape the monsoon specifically: the seasonal shift of the Jet streams (fast upper-air wind belts, the westerly jet withdrawing from north India and the easterly jet setting in along 15°N, a shift held responsible for the burst of the monsoon), the Southern Oscillation (the fluctuation of pressure between Tahiti in the east Pacific and Darwin in northern Australia, used to gauge the strength of the monsoon), and El Niño (a complex weather system appearing every three to seven years, in which a warm current off the coast of Peru temporarily replaces the cold Peruvian current, raising sea temperature there by about 10°C; in 1990-91 a strong El Niño event delayed the onset of the southwest monsoon over most of the country by five to twelve days).

Western disturbances (shallow cyclonic depressions from the Mediterranean) and tropical cyclones add further rainfall at particular seasons.

Definition: ITCZ. The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone at the equator where the trade winds of the two hemispheres converge and air tends to rise.

In July it shifts to about 20°N-25°N latitude over the Gangetic plain, sometimes called the monsoon trough; in winter it shifts back southward.

What are the atmospheric pressure conditions in India during the year?

In the cold weather season, by the end of December the sun shines vertically over the Tropic of Capricorn, and feeble high pressure conditions develop over the northern plain, with the isobars of 1019 mb and 1013 mb passing through northwest India and far south India respectively.

Air pressure over south India is slightly lower, so winds blow from the northwestern high pressure zone towards the low pressure zone over the Indian Ocean in the south.

In the hot weather season, as temperatures rise over the northern plains, pressure falls there and a trough of low pressure develops, while pressure over the surrounding seas remains higher.

In the southwest monsoon season, the low pressure over the northwestern plains becomes intense by early June and draws in the trade winds of the southern hemisphere, and the monsoon trough lies over the Gangetic plain.

In the retreating monsoon season, the low pressure over the land weakens as the sun moves southward, and pressure begins to rise again over northern India while the ocean to the south stays relatively warm.

How does the monsoon originate and progress, and what are India's seasons?

Onset and mechanism of the monsoon

1. During April and May, as the sun shines vertically over the Tropic of Cancer, the large landmass north of the Indian Ocean heats intensely, producing an intense low pressure cell over the northwestern subcontinent.
2. Because the ocean to the south heats more slowly, pressure there stays relatively high, and this low pressure cell attracts the southeast trade winds of the southern hemisphere across the equator between 40°E and 60°E longitude.
3. This shift causes the ITCZ to move north to about 20°N-25°N over the Gangetic plain, and the Coriolis force deflects the crossing trade winds so that they blow from southwest to northeast, becoming the southwest monsoon.
4. At the same time, the westerly jet stream withdraws from its position south of the Himalayas, and the easterly jet stream sets in along 15°N; this shift is held responsible for the sudden burst of the monsoon.
5. The southwest monsoon sets in over the Kerala coast by 1 June, reaches Mumbai and Kolkata between 10 and 13 June, and engulfs the entire subcontinent by mid-July.

During the rainy season, if rain fails for one or more weeks after a few days of good rain, this is called a break in the monsoon. In northern India, breaks occur when rain-bearing storms are infrequent along the monsoon trough; along the west coast, dry spells occur when winds blow parallel to the coast rather than onshore.

How do the rain-bearing systems work, and how does the monsoon retreat?

India has two main rain-bearing systems. The first is the series of tropical depressions that form over the Bay of Bengal and move north-west along the monsoon trough, bringing rain to the northern plains. The second is the Arabian Sea current of the south-west monsoon, which brings rain to the west coast; much of the rain on the Western Ghats is orographic, as the moist air is forced to rise over the hills. Because the axis of the monsoon trough shifts, the tracks of the depressions change and the amount of rain varies from year to year. Rainfall comes in spells and declines from the west coast inland and from the south-east towards the north-west across the northern plains.

The retreat of the monsoon follows the southward march of the sun. By the end of September the low-pressure trough over the Ganga plain weakens and moves south. The monsoon withdraws from western Rajasthan in the first week of September and from Rajasthan, Gujarat, the western Ganga plain and the Central Highlands by the end of that month. By early October the low pressure lies over the northern Bay of Bengal, by early November over Karnataka and Tamil Nadu, and by the middle of December it has left the Peninsula. The retreat brings clear skies and the hot, humid "October heat" in the north.

Through the year the winds and rainfall follow this cycle. In winter, dry north-easterly winds blow from the land, so most of India is dry, except the north-west, which gets light rain from western disturbances, and the Tamil Nadu coast, which gets rain from the north-east monsoon. In the hot season, pressure falls over the north and local storms such as the Loo, the Norwesters (Kalbaisakhi) and mango showers occur. From June to September the south-west monsoon winds bring most of the year's rain, heaviest on the west coast and in the north-east. In October and November the winds turn back to the north-east, and the eastern coast of the Peninsula has its wettest months. Temperatures differ sharply between north and south: the northern plains have a cold winter, with mean daily temperatures below 21°C, while the Peninsula, moderated by the sea, has no well-defined cold season.

What are the characteristics of the four main seasons?

Meteorologists recognise four seasons in India: the hot and dry season, the hot and wet season, the cool and dry season and the cool and wet season.

In the cool and dry season (roughly mid-November to February), mean daily temperatures in the northern plain stay below 21°C, and night temperatures in Punjab and Rajasthan may fall below freezing; excessive cold here is due to the continental climate of interior north India, snowfall in the nearby Himalayas and cold winds reaching India from the Caspian Sea and Turkmenistan around February.

Winter rainfall, where it occurs, comes mainly from weak temperate cyclones from the Mediterranean, which bring light but valuable rain (and snow in the lower Himalayas) that benefits rabi crops; the average winter rainfall in Delhi is around 53 mm.

In the hot and dry season (April-June), temperatures of 48°C are not uncommon in northwestern India, while the Peninsula, moderated by the surrounding ocean, stays between 26°C and 32°C; local storms of this season include Loo (hot, dry, oppressing winds of the northern plains), Mango showers (pre-monsoon showers in Kerala and coastal Karnataka that help mangoes ripen early) and Kalbaisakhi/Nor'westers (dreaded evening thunderstorms of Bengal and Assam, useful for tea, jute and rice).

The hot and wet season (June-September) brings the Arabian Sea and Bay of Bengal branches of the monsoon, which together give most of India's annual rainfall.

The cool and wet season (October-November) is marked by clear skies, high humidity and the "October heat" in the north, while the eastern Peninsula, affected by cyclonic depressions crossing from the Andaman Sea, receives its rainiest months here, often with destructive cyclones striking the deltas of the Godavari, Krishna and Kaveri.

For each city, plot monthly mean temperature as a line and monthly rainfall as bars.

Mumbai and Chennai should show a small temperature range with a coastal, equable pattern; Delhi should show a large annual temperature range with a sharp monsoon rainfall peak;

Jaisalmer should show very high summer temperatures with almost no rainfall through the year (its total rarely exceeds about 9 cm); Leh should show very low winter temperatures (down to about minus 45°C) and very little rainfall of any kind.

What causes droughts and floods, and how is India's forest cover conserved?

Droughts and floods: causes and mitigation

India's average annual rainfall is about 125 cm, but it shows great variability in time and space: some years and regions get far more than their average, others far less, and this variability, not the average itself, is the chief cause of droughts and floods.

A drought develops where the monsoon is weak, delayed or fails over a season, leaving reservoirs, soil moisture and crops short of water; areas most prone to this are the regions of low and inadequate rainfall, including parts of Rajasthan, Gujarat, the Deccan interior and the rain-shadow areas east of the Western Ghats.

A flood develops where rain-bearing systems bring intense or prolonged rain to river catchments, or where a monsoon branch bursts suddenly; the Ganga-Brahmaputra delta, the Odisha coast and the river plains of north India, which carry large volumes of snow-melt and monsoon water through low-gradient channels, are especially affected, and rivers such as the Kosi and the Brahmaputra are notorious for shifting course and causing floods.

Agricultural prosperity in India depends heavily on timely and well-distributed rainfall. Because the monsoon can fail in one region while flooding another in the same year, and because much monsoon water otherwise runs quickly to the sea, mitigation has concentrated on storing and redistributing water: building dams and reservoirs on both Himalayan and Peninsular rivers, developing irrigation canals, and planning schemes to transfer surplus water from water-surplus basins to water-deficit basins through inter-basin link canals.

What are the main drought and flood mitigation programmes?

Drought mitigation in India has been organised through a set of national programmes that store water, conserve soil moisture and provide relief when the monsoon fails.

The Drought Prone Areas Programme works in the dryland districts of the country to develop land and water resources, conserve soil moisture and create rural employment, so that drought-prone areas become less vulnerable to a weak monsoon.

The Desert Development Programme works in the hot desert districts of Rajasthan, Gujarat and Haryana, and in the cold desert of Ladakh, to check desertification and develop water resources.

The National Watershed Development Project for Rainfed Areas treats whole watersheds with soil and water conservation measures, so that rain is held in the soil instead of running off.

Flood mitigation has been organised through the Flood Management Programme, which funds embankments, drainage channels and flood protection works. Alongside these, the Central Water Commission issues flood forecasts and warnings for the major river basins.

Forests, forest policy and conservation

Forests are of great importance to people: they supply timber, fuelwood, fodder and non-timber produce, protect soil from erosion, regulate river flow and rainfall, and support a wide range of wildlife.

Misuse through over-felling, encroachment and shifting cultivation has reduced forest cover in many regions, and a gap between the recorded forest area (land legally notified as forest) and the actual forest cover (land actually under tree canopy, whether notified or not) is a recurring concern in forest management.

Human activity has had a heavy impact. Forests have been misused through felling for timber and fuelwood, clearing for farms, shifting cultivation, mining, dams and roads, overgrazing and encroachment, and the result has been soil erosion, floods, siltation of rivers and the loss of wildlife habitat. Yet forests have great potential for exploitation if they are used sustainably: timber and bamboo for building and paper, fuelwood and fodder, and minor forest produce such as tendu leaves, lac, gums, resins and medicinal plants, on which many tribal communities depend. The aim is to use this potential without destroying the forest, which is why the need for conservation, through protection, regeneration and social forestry, is stressed alongside its use.

In India, the forest area is about 7.7 lakh km², or roughly 23 per cent of the geographical area of the country, while the forest cover is about 7.1 lakh km², or roughly 21.7 per cent of the geographical area.

The National Forest Policy of 1988 set out to maintain environmental stability, conserve the natural heritage of the country, check soil erosion and denudation, increase forest and tree cover through massive afforestation, meet the needs of the rural and tribal population for fuelwood, fodder and minor forest produce, and encourage efficient use of forest produce while discouraging wasteful consumption.

Under the policy, Social Forestry was promoted to raise tree cover outside notified forests and to relieve pressure on natural forest: agro-forestry (growing trees along with crops on farmland), community forestry (raising trees on common village land for the shared benefit of the community), commercial farm forestry (growing trees as a cash crop for sale), non-commercial farm forestry (growing trees mainly for a household's own fuel, fodder and small timber needs) and urban forestry (raising and managing trees in and around towns and cities, in parks, along roads and on vacant land, to improve the urban environment).

Case study: the Chipko Movement

Two well-known movements have shaped public awareness of forest conservation in India. Van Mahotsav ("festival of trees") is an annual tree-planting campaign aimed at increasing public participation in afforestation, and it works alongside the Social Forestry schemes promoted under the National Forest Policy of 1988.

The Chipko Movement, which began in the hill villages of the central Himalayas, saw villagers physically embracing (the Hindi word "chipko" means "to stick to") trees marked for felling by contractors, in order to prevent their destruction.

The movement drew wide attention to the link between deforestation, landslides and floods in the hills, and strengthened the case for community control over local forest resources. It is widely cited as an example of how local, non-violent protest can influence forest policy and practice.

Taken together with the objectives of the National Forest Policy, which aims to check soil erosion, conserve the natural heritage of the country and meet the fuelwood, fodder and minor forest produce needs of rural and tribal communities, the Chipko Movement shows how community vigilance can directly support these aims on the ground, by physically preventing felling rather than merely following the policy's afforestation targets. It remains a reference point whenever the conservation and misuse of India's forests are discussed.

Case study: India as the "Mistress of the Eastern Seas"

India has long been described as commanding the "Eastern Seas" because of the position of its coastline astride the main shipping lanes of the Indian Ocean Rim, linking West Asia, East Africa and Southeast Asia.

Its coastline, close to 7,516 km when the mainland and the island territories are added together, and its many natural harbours, support this role: ports such as Kandla, Mazagaon, Marmagao, Mangalore and Cochin on the submerged, narrow western coast benefit from natural depth close to shore, while the broader, emergent eastern coast, despite having fewer natural harbours because its continental shelf extends up to 500 km into the sea, still gives access to the Bay of Bengal trade routes through the deltas of the Mahanadi, the Godavari, the Krishna and the Kaveri.

Because the mainland projects southward between the Arabian Sea and the Bay of Bengal, with the Andaman and Nicobar Islands and Lakshadweep extending India's maritime reach further still, the country is positioned to watch over and support shipping moving between the Suez route, the Persian Gulf, East Africa and Southeast Asia.

This combination of a long coastline, natural ports and a central position in the Indian Ocean Rim is the basis of India's traditional description as the "Mistress of the Eastern Seas", a description that is still used to explain India's strategic and trading advantage in the region today.

Going beyond the syllabus

These ideas extend the unit and are not needed for the questions on it.

What is global warming and how does it affect India?

Global average surface temperature has risen over the past century and a half, and projections suggest a further rise of about 2°C by the year 2100, driven mainly by large-scale industrialisation and the carbon dioxide and other greenhouse gases released by human activity.

Carbon dioxide, released to the atmosphere in large quantities by the burning of fossil fuel, is a major source of concern, but methane, chlorofluorocarbons and nitrous oxide, though present in much smaller concentrations in the atmosphere, are better absorbers of long wave radiation than carbon dioxide and so are more effective at enhancing the greenhouse effect.

One likely consequence is a rise in sea level, caused by the melting of glaciers and sea-ice, estimated on average at about 48 cm by the end of this century, which would increase the incidence of annual flooding and would raise the risk of flooding along India's long coastline.

Climatic change of this kind would also promote insect-borne diseases such as malaria, and would lead to a shift in climatic boundaries, making some regions wetter and others drier; agricultural patterns would shift, and both human population and the wider ecosystem would experience change as a result.

This places long-term climate change, driven by the build-up of greenhouse gases, alongside year-to-year rainfall variability as a concern for India's water resources and farming.

Glossary

• Peninsular Plateau — the ancient, rigid landmass of gneiss and granite forming peninsular India, stable since the Cambrian period.
• Bhabar — a narrow belt at the foot of the Shiwaliks where mountain streams deposit coarse material and may disappear underground.
• Tarai — the marshy, swampy belt south of the Bhabar where streams re-emerge without a clear channel.
• Bhangar and Khadar — the old (Bhangar) and new (Khadar) alluvial deposits of the northern plains, the Khadar being renewed by annual floods.
• Doab — the fertile strip of land lying between two rivers.
• Badland — a landscape of deep gullies and ridges cut by erosion, as in the Chambal ravines.
• Lagoon — a shallow coastal water body separated from the open sea by a sand bar, such as the Kayals of the Malabar coast.
• Delta — a fan-shaped deposit of sediment at a river mouth, typical of the eastern coastal rivers such as the Godavari and Kaveri.
• Drainage basin / watershed — the area drained by a river system; "watershed" is usually used for the catchment of a small stream, "river basin" for a large river.
• ITCZ — the Inter Tropical Convergence Zone, the equatorial low pressure belt where the trade winds of both hemispheres converge.
• Jet stream — a fast-moving band of upper-air wind whose seasonal shift is linked to the onset of the Indian monsoon.
• Southern Oscillation — the fluctuation of air pressure between Tahiti and Darwin used to gauge the strength of the monsoon.
• El Niño — a periodic warming of the eastern Pacific that can delay or weaken the Indian monsoon.
• Loo — hot, dry, oppressive wind of the northern plains in the hot weather season.
• Social forestry — raising trees outside notified forest, including agro-forestry, community forestry, commercial and non-commercial farm forestry.

Common errors and misconceptions

• Misconception: the Himalayas and the Peninsular Plateau are geologically similar because both are mountainous. Correct: the Peninsula is an ancient, rigid block stable since the Cambrian period, while the Himalayas are young, weak and still being folded and faulted.
• Misconception: all Indian rivers are perennial. Correct: Himalayan rivers, fed by both snow-melt and rain, are perennial, but most Peninsular rivers are non-perennial, with the Narmada and Tapi as exceptions that flow through rift valleys.
• Misconception: "watershed" and "river basin" mean exactly the same thing. Correct: watershed is generally used for the smaller catchment of a rivulet, while river basin is used for the larger catchment of a major river.
• Misconception: the western coast has many natural ports because it is a flat, open coastline. Correct: the western coast has natural ports precisely because it is a narrow, submerged coast with deep water close to shore.
• Misconception: the monsoon is a single simple wind system blowing from the sea to the land. Correct: it results from the shift of the ITCZ, the reversal of pressure between land and sea, the withdrawal of the westerly jet stream and the onset of the easterly jet stream acting together.
• Misconception: droughts and floods occur in entirely separate, unconnected regions of India. Correct: both result from the same underlying cause, the variability of monsoon rainfall, and can occur in different parts of the country in the same season.
• Misconception: "forest area" and "forest cover" mean the same statistic. Correct: forest area is land legally notified as forest, while forest cover is the land actually bearing tree canopy, and the two figures can differ.

Exam-style questions with model answers

Q1. State the latitude and longitude extent of India and name the water body that separates India from Sri Lanka. [1 mark]

1. India extends from 8°4'N to 37°6'N latitude and from 68°7'E to 97°25'E longitude, and the narrow waterbody that separates India from Sri Lanka is the Palk Strait.

Q2. The Bhabar region plays a significant role in the area's ecosystem, yet it is not considered suitable for agriculture. Justify the statement by giving two reasons. [2 marks]

1. The Bhabar is a narrow belt of coarse rock and boulder material at the foot of the Shiwaliks, where streams deposit their heavy load and then disappear underground, so the surface is dry and stony.
2. Because the streams sink below the surface, there is little water available at the surface for crops, and the coarse, porous soil holds little moisture, so the belt is not suitable for agriculture.

Q3. Peninsular Plateau is an old landmass of rocks whereas the Himalayas is a young mountain of rocks. Comment on the rock type in these two areas and discuss how it has influenced the evolution of the river valleys in these areas. [4 marks]

1. The Peninsular Plateau is made of very ancient, hard crystalline rocks such as gneisses and granites, which have stood as a rigid block since the Cambrian period.
2. The Himalayas are made of young, weak and flexible sedimentary and metamorphic rocks that are still being folded, faulted and thrust upward.
3. Because the Peninsular rocks are hard and stable, its rivers have had a long time to erode them into broad, shallow valleys with low gradients and a fixed course.
4. Because the Himalayan rocks are young and still rising, its rivers cut deep gorges and V-shaped valleys, form rapids and waterfalls, and remain in their youthful stage.

Q4. India is known as the "Mistress of the Eastern Seas". Justify the statement by giving two reasons. [2 marks]

1. India's long coastline of about 7,516 km, with natural harbours on both coasts, lets it watch over and support shipping across the Indian Ocean Rim.
2. Its central position astride the sea routes linking West Asia, East Africa and Southeast Asia, together with the Andaman and Nicobar Islands and Lakshadweep, extends its reach far into the eastern seas.

Q5. Explain the mechanism by which the southwest monsoon sets in over India. [4 marks]

1. During April and May, the sun shines vertically near the Tropic of Cancer and intensely heats the large landmass north of the Indian Ocean, creating a low pressure cell over northwestern India.
2. The ocean to the south heats more slowly, so pressure there remains comparatively high, and this pressure difference attracts the southeast trade winds of the southern hemisphere across the equator between 40°E and 60°E longitude.
3. As they cross the equator, the Coriolis force deflects these winds so that they blow from southwest to northeast, turning them into the southwest monsoon, while the ITCZ shifts north to about 20°N-25°N over the Gangetic plain.
4. At the same time, the westerly jet stream withdraws from its position south of the Himalayas and the easterly jet stream sets in along 15°N, and this shift is linked to the sudden burst of the monsoon, which reaches the Kerala coast by 1 June.

Q6. Distinguish between droughts and floods in India, with reference to their cause and the regions typically affected. [4 marks]

1. A drought occurs when the monsoon is weak, delayed or fails over a season, leaving an area short of the rainfall it normally needs for crops and water supply.
2. A flood occurs when rain-bearing systems bring intense or prolonged rainfall to a river catchment, or when a monsoon branch bursts suddenly, overwhelming river channels.
3. Droughts are typical of regions of low or inadequate rainfall, such as parts of western Rajasthan, Gujarat and the interior Deccan, including rain-shadow areas east of the Western Ghats.
4. Floods are typical of the Ganga-Brahmaputra delta, the Odisha coast and the river plains of north India, where large volumes of monsoon and snow-melt water move through low-gradient channels and rivers such as the Kosi and Brahmaputra frequently shift course.

Q7. Evaluate the role of the National Forest Policy of 1988 and social forestry in conserving India's forests. [7 marks]

1. The National Forest Policy of 1988 aims to maintain environmental stability, conserve the natural heritage of the country and check soil erosion and denudation.
2. It seeks to increase forest and tree cover through massive afforestation, recognising that forests regulate river flow, rainfall and soil protection.
3. It addresses the needs of rural and tribal communities for fuelwood, fodder and minor forest produce, rather than treating forests purely as a source of commercial timber.
4. Social forestry under the policy, through agro-forestry, community forestry, commercial farm forestry, non-commercial farm forestry and urban forestry, raises tree cover outside notified forests, which relieves pressure on existing natural forest.
5. Urban forestry in particular brings trees into towns and cities, improving the urban environment and adding to tree cover where notified forest cannot exist.
6. Movements such as Van Mahotsav build public participation in planting, while the Chipko Movement shows how community vigilance can directly prevent felling.
7. The continuing gap between recorded forest area and actual forest cover suggests that the policy's aims have been only partly achieved, and misuse and encroachment remain a concern despite these measures.

Key takeaways

• India's central position in the Indian Ocean and its long coastline of about 7,516 km give it locational advantages across the Indian Ocean Rim and as a subcontinent.
• India is divided into three geological regions: the ancient, rigid Peninsular Plateau, the young, flexible Himalayas, and the depositional Indus-Ganga-Brahmaputra Plains.
• The five physiographic divisions are the Himalayan mountain complex, the Indus-Ganga-Brahmaputra Plains, the Peninsular Plateau, the Coastal Plains and the Islands.
• The Himalayas have three parallel ranges, the Greater Himalayas, the Lesser Himalayas and the Shiwaliks, and are divided regionally into the Kashmir, Himachal-Uttaranchal, Nepal and Assam Himalayas.
• The northern plains show a sequence of relief belts from north to south: Bhabar, Tarai, Bhangar and Khadar.
• The Western Ghats are higher and more continuous than the Eastern Ghats, and most Peninsular rivers rise in the Western Ghats.
• Himalayan rivers are perennial, youthful and meandering in the plains; Peninsular rivers are mostly non-perennial with a fixed course.
• The monsoon's onset depends on the shift of the ITCZ, the reversal of land-sea pressure, and the withdrawal of the westerly jet stream and onset of the easterly jet stream.
• India's rainfall varies greatly in space and time, and this variability, not the annual average, causes both droughts and floods.
• Social forestry, including urban forestry, and movements such as Van Mahotsav and the Chipko Movement support the aims of the National Forest Policy of 1988.

Test yourself

What is the Peninsular Plateau, and since which geological period has it remained largely stable?

It is the ancient complex of gneisses and granites forming peninsular India, standing largely as a rigid block since the Cambrian period, apart from western submergence and some tectonic faulting.

Name the five physiographic divisions of India.

The Himalayan mountain complex, the Indus-Ganga-Brahmaputra Plains, the Peninsular Plateau, the Coastal Plains and the Islands.

What is the difference between Bhangar and Khadar?

Bhangar is the belt of older alluvium in the northern plains, while Khadar is the belt of newer alluvium nearer the river, renewed by annual flooding.

Why do the Himalayan rivers remain perennial while most Peninsular rivers do not?

Himalayan rivers are fed both by melting snow and by monsoon rainfall, so they keep flowing through the year, while Peninsular rivers depend mainly on monsoon rainfall alone.

What is the ITCZ, and where does it lie over India in July?

It is the Inter Tropical Convergence Zone, the equatorial belt where trade winds converge; in July it shifts to about 20°N-25°N over the Gangetic plain, forming the monsoon trough.

Name the four seasons recognised by meteorologists in India.

The hot and dry season, the hot and wet season, the cool and dry season and the cool and wet season.

What distinguishes the western coastal plain from the eastern coastal plain?

The western coast is narrow and submerged, with natural ports and no deltas, while the eastern coast is broader and emergent, with well-developed river deltas but fewer natural harbours.

What is social forestry, and name two of its forms?

It is the raising of trees outside notified forest land to relieve pressure on natural forest; two of its forms are agro-forestry and community forestry.