The rainfall distribution pattern over the Ganga basin decreases from the

1. Mechanism of the South-West Monsoon: The Two Branches (basic)

To understand why India’s rainfall is divided into two distinct "branches," we must first look at the massive physical engine that drives it. During the summer, the landmass of northern India heats up intensely, causing the Inter-Tropical Convergence Zone (ITCZ) to shift northward to about 20°N-25°N latitude. This creates a powerful thermal low-pressure zone over the Ganga plains, which acts like a giant vacuum, pulling in winds from the high-pressure areas of the southern Indian Ocean INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.30.

As these Southeast Trade winds cross the Equator, the Coriolis Force (caused by the Earth's rotation) deflects them to the right. They transform into the moisture-heavy Southwest Monsoon winds. However, as this massive air mass approaches India, it hits a physical obstacle: the tapering shape of the Indian Peninsula. This triangular landmass acts as a wedge, splitting the single monsoon current into two distinct paths: the Arabian Sea Branch and the Bay of Bengal Branch Geography of India, Chapter 4: Climate of India, p.16.

The two branches behave quite differently due to the topography they encounter:

• Arabian Sea Branch: This is the more powerful branch. It strikes the Western Ghats (specifically the Malabar Coast of Kerala) around June 1st. It moves northward, reaching Mumbai by June 10th INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.31.
• Bay of Bengal Branch: This branch moves toward the northeast. Interestingly, it doesn't just keep going east; the Arakan Hills in Myanmar deflect a major portion of this branch toward the Indian subcontinent. Consequently, it enters West Bengal and Bangladesh from the south and southeast, eventually moving up the Ganga plains and the Brahmaputra valley INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.37.

Feature	Arabian Sea Branch	Bay of Bengal Branch
Entry Point	Western Coast (Kerala/Mumbai)	Northeast/East Coast (Bengal/Myanmar)
Key Influence	Western Ghats	Arakan Hills & Himalayas
Temperature	Relatively cooler (Somalian Current)	Relatively warmer (Supports Cyclones)

Sources: INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.30, 31, 37; Geography of India, Chapter 4: Climate of India, p.16, 28

2. Factors Affecting Rainfall Distribution: Relief and Continentality (basic)

To understand why it rains heavily in some parts of India and barely at all in others, we must look at two fundamental geographic "rules": Relief (the shape of the land) and Continentality (the distance from the sea). These factors act like a filter, determining how much moisture actually reaches a specific region.

1. Relief (Orographic Rainfall): This is perhaps the most powerful local factor. When moisture-laden monsoon winds encounter a physical barrier, such as a mountain range, they are forced to rise. As this air ascends, it expands due to lower pressure and cools down—a process known as adiabatic cooling. Once the air reaches its dew point, clouds form, and heavy rain falls on the windward side (the side facing the wind). Conversely, as the air descends on the other side, it becomes warmer and drier, creating a rain-shadow area or leeward side Physical Geography by PMF IAS, Hydrological Cycle, p.339. This is why the Himalayan foothills (Tarai) receive significantly more rain than the plains further south—the mountains literally "squeeze" the moisture out of the clouds.

2. Continentality (Distance from the Sea): Think of the monsoon winds as a sponge soaked in water. As the "sponge" moves inland from the Bay of Bengal or the Arabian Sea, it continuously loses water through rainfall. Because the sea takes longer to heat up and cool down than land, coastal areas enjoy a moderate climate and high humidity INDIA PHYSICAL ENVIRONMENT, Climate, p.29. However, as you move deeper into the interior of the landmass, the winds have already spent most of their moisture. This creates a clear spatial gradient: rainfall is highest at the coast and gradually decreases as you move toward the heart of the continent.

In the context of the Ganga basin, these two factors work together. The rainfall is highest in the east (closer to the Bay of Bengal) and decreases as you move west (Continentality). Simultaneously, it is higher in the north along the Himalayas and decreases as you move south toward the plateau (Relief).

Factor	Mechanism	Impact on Rainfall
Relief	Physical barriers (Mountains) force air to rise and cool.	Heavy rain on windward slopes; dry "rain-shadows" on leeward sides.
Continentality	Increasing distance from the moisture source (the ocean).	Progressive decrease in rainfall intensity from the coast to the interior.

Sources: Physical Geography by PMF IAS, Hydrological Cycle, p.339; INDIA PHYSICAL ENVIRONMENT, Climate, p.29; Geography of India (Majid Husain), Climate of India, p.3

3. The Himalayan Barrier and the Tarai Belt (intermediate)

To understand the rainfall patterns of North India, we must first look at the Himalayas not just as a mountain range, but as a colossal climatic divide. Standing like a 3,200 km-long wall, the Himalayas perform two critical functions for the Indian climate. First, they act as an invincible shield, blocking the frigid, bone-chilling winds of the Arctic and Central Asia from entering the subcontinent. Second, and most importantly for our topic, they serve as a physical barrier that traps the moisture-laden South-West monsoon winds, forcing them to shed their water within the Indian territory INDIA PHYSICAL ENVIRONMENT, Chapter 4, p.29.

When the Bay of Bengal branch of the monsoon travels up the Ganga Valley, it eventually strikes the foothills of the Himalayas. Here, a process called orographic lifting occurs: the air is forced to rise along the mountain slopes, cooling down as it ascends, which leads to heavy condensation and precipitation. This creates a distinct North-to-South rainfall gradient across the Ganga plains. While the southern parts of the plain (near the Peninsular plateau) receive moderate rain, the northern strip lying at the base of the mountains — known as the Tarai belt — receives significantly higher rainfall, often ranging between 100 to 200 cm Geography of India, Chapter 4, p.31.

The Tarai region is a low-lying, marshy tract characterized by thick forests and high humidity. Because it sits directly in the path of the rising monsoon air, it enjoys a much higher rainfall average than the plains further south. In fact, as you move from the foothills (North) toward the interior of the peninsula (South), the rainfall generally decreases. This is in addition to the East-to-West decrease we see as winds move inland from the Bay of Bengal Geography of India, Chapter 4, p.25.

Sources: INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.29; Geography of India, Chapter 4: Climate of India, p.31; Geography of India, Chapter 4: Climate of India, p.25

4. Climatic Classification: Koppen's Scheme for the Ganga Basin (intermediate)

To understand the climate of the Ganga Basin, we look to the most widely used system in geography: Koppen's Climatic Classification. This is an empirical system, meaning it is based on observed data—specifically mean monthly temperature and precipitation Geography of India, Majid Husain, Climate of India, p.33. For the vast Indo-Gangetic plain, Koppen assigned the code Cwg. Understanding this three-letter code is the secret to mastering the region's climate logic.

The Cwg classification breaks down as follows:

• C (Mesothermal): This indicates a 'warm temperate' climate where the average temperature of the coldest month is between 18°C and -3°C. It reflects the distinct seasonality of North India.
• w (Dry Winter): This signifies that the winter season is dry. Most of the basin's precipitation is concentrated in the summer monsoon months, while winters see very little rain, often under the influence of stable high-pressure systems Geography of India, Majid Husain, Climate of India, p.34.
• g (Gangetic): This is a special suffix denoting that the hottest month occurs before the arrival of the summer solstice and the monsoon rains (usually in May or early June).

While the entire basin shares the Cwg label, it isn't uniform. A powerful rainfall gradient exists: precipitation is heaviest in the East (around 1600 mm in West Bengal) and steadily decreases as you move West toward Haryana (about 800 mm) INDIA PHYSICAL ENVIRONMENT, NCERT Class XI, Climate, p.38. This happens because the Bay of Bengal branch of the monsoon loses moisture as it travels inland. Additionally, the northern fringe (the Tarai belt) receives more rain due to the orographic lift provided by the Himalayas, whereas the southern edges transitioning toward the Peninsular plateau are relatively drier.

Sources: Geography of India, Majid Husain, Climate of India, p.33-34; INDIA PHYSICAL ENVIRONMENT, NCERT Class XI, Climate, p.38

5. Impact on Agriculture: Agro-Climatic Zones of the Ganga Basin (intermediate)

The agriculture of the Ganga Basin is a direct reflection of its rainfall gradient, which decreases from the humid east (West Bengal) to the semi-arid west (Haryana/Rajasthan border). Because the Bay of Bengal branch of the monsoon loses moisture as it travels inland, the basin is divided into distinct Agro-Climatic Zones. In the Lower Ganga Plain (West Bengal), where rainfall often exceeds 150 cm, water-intensive crops like paddy (rice) and jute dominate. As we move to the Middle Ganga Plain (Eastern UP and Bihar), rainfall moderates to 100–150 cm, making the Rice-Wheat rotation the backbone of the economy Environment and Ecology, Major Crops and Cropping Patterns in India, p.20. In the Upper Ganga Plain (Western UP and Haryana), annual rainfall drops further to 60–100 cm. Here, agriculture relies heavily on canal and groundwater irrigation to support wheat and sugarcane. In the drier southern fringes of the basin, such as the Bundelkhand region (Southern UP), where rainfall is between 50–100 cm, the landscape shifts toward hardy crops like millets (Bajra), pulses, and oilseeds Geography of India, Spatial Organisation of Agriculture, p.28. This spatial organization ensures that crop selection matches the available natural moisture and thermal conditions of each sub-region. To maintain soil health in these intensive zones, farmers employ strategies like Crop Rotation and Relay Cropping. For instance, legumes (pulses) are often grown between two cereal crops to fix nitrogen back into the soil Indian Economy, Agriculture - Part II, p.337. This is particularly vital in the Middle and Upper plains where the soil undergoes heavy nutrient depletion due to the continuous rice-wheat cycle.

Region	Avg. Rainfall	Dominant Cropping Pattern
Lower Ganga Plain (East)	>150 cm	Rice-Rice, Jute-Rice
Middle Ganga Plain (Central)	100 - 150 cm	Rice-Wheat, Rice-Pulses
Upper Ganga Plain (West)	60 - 100 cm	Wheat-Sugarcane, Cotton-Wheat
Southern Fringe (Bundelkhand)	50 - 100 cm	Millets, Pulses, Oilseeds

Sources: Environment and Ecology, Major Crops and Cropping Patterns in India, p.20; Geography of India, Spatial Organisation of Agriculture, p.28; Indian Economy, Agriculture - Part II, p.337

6. Rainfall Gradients in the Indo-Gangetic Plain (exam-level)

To understand why a farmer in Bihar experiences a very different monsoon compared to a farmer in Haryana, we must look at the "moisture journey" of the Southwest Monsoon. The rainfall in the Indo-Gangetic Plain (IGP) is primarily governed by the Bay of Bengal branch. This branch enters the plain from the east and travels inland, creating two distinct rainfall gradients that define the agriculture and ecology of North India.

The first and most prominent is the East-to-West Gradient. As the moisture-laden winds move away from the Bay of Bengal, they continuously shed their water content. By the time these winds reach the western reaches of the plain, they have lost much of their moisture. Consequently, annual rainfall drops significantly from approximately 1600 mm in West Bengal to about 800 mm in western Uttar Pradesh and Haryana INDIA PHYSICAL ENVIRONMENT, Chapter 4, p. 38. This is why the eastern plains are characterized by lush rice paddies, while the western plains transition into wheat and mustard belts CONTEMPORARY INDIA-I, Chapter 4, p. 30.

The second is the North-to-South Gradient. The northern edge of the plain, lying along the sub-Himalayan foothills (the Tarai belt), receives higher precipitation due to orographic lifting. When the monsoon winds encounter the Himalayas, they are forced to rise, cool, and condense, resulting in heavy rainfall between 100–200 cm INDIA PHYSICAL ENVIRONMENT, Chapter 4, p. 38. As you move southwards, away from the mountains and toward the fringes of the Peninsular Plateau, the rainfall generally decreases.

Direction of Movement	Rainfall Trend	Primary Reason
East to West	Decreasing	Progressive loss of moisture as winds move further inland.
North to South	Decreasing	Decreasing influence of Himalayan orographic (mountain-induced) lifting.

Sources: INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.38; CONTEMPORARY INDIA-I, Chapter 4: Climate, p.30; Geography of India (Majid Husain), Chapter 4: Climate of India, p.31

7. Solving the Original PYQ (exam-level)

Now that you have mastered the mechanics of the South-West Monsoon and the physiography of India, this question serves as the perfect application of those building blocks. To solve this, you must synthesize two specific concepts: the moisture depletion of the Bay of Bengal branch and the orographic effect of the Himalayas. As the Bay of Bengal branch enters the Ganga plain from the delta, it moves inland toward the northwest. Because it is moving away from its primary moisture source, the air gradually becomes drier, leading to a natural rainfall reduction from east to west. You can visualize this by comparing the lush greens of West Bengal to the semi-arid landscapes of Haryana.

To determine the north-south gradient, think back to the role of mountain barriers. The northern boundary of the Ganga basin is hugged by the Himalayan foothills (Tarai region). When the monsoon winds strike these mountains, they are forced to rise, causing heavy orographic precipitation. As you move further south away from the mountains toward the Peninsular Plateau, this lifting mechanism weakens, resulting in less rainfall. By combining these two geographical truths, we arrive at the correct answer: (B) east to west and north to south. This logic is well-documented in INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT) and Geography of India, Majid Husain.

UPSC often designs distractors to test if you are guessing based on generalities. Options (A) and (C) are classic traps that suggest a west to east decrease; this would only be true if the primary moisture source were the Arabian Sea branch reaching the basin first, which is geographically impossible due to the direction of the monsoon trough. Option (D) tries to trick you on the vertical axis by suggesting rainfall decreases from south to north. Students often make this mistake by assuming the ocean's proximity to the south (the Bay of Bengal) means the southern plains are wetter, forgetting that the Himalayan barrier in the north is the actual trigger for the heaviest rainfall in this specific basin.