Correct Option
The correct option isProgressive loss of moisture in monsoon winds
Explanation
The rainfall distribution in the Ganga valley is primarily governed by the trajectory of the Bay of Bengal branch of the Southwest Monsoon. These winds carry moisture from the ocean and travel inland, parallel to the Himalayan foothills.
Detailed Analysis
- Progressive loss of moisture (Correct): The Bay of Bengal branch enters India from the east (West Bengal/Bangladesh) and is deflected westwards by the Himalayas. As these winds traverse the Indo-Gangetic plains from east to west, they continuously shed moisture in the form of rainfall. By the time the winds reach the western parts of the valley (such as Delhi and Punjab), their moisture content is significantly depleted due to the long distance traveled over land (continentality effect).
- Increase in altitude (Incorrect): While the altitude of the Ganga plains does gradually increase from east to west, this factor is not the primary cause of reduced rainfall. Typically, an increase in altitude facilitates orographic rainfall; the decrease observed here is strictly due to the increasing distance from the moisture source.
- Proximity to the Equator (Incorrect): The proximity of the eastern part to the Equator influences temperature and general tropical characteristics but does not explain the longitudinal (east-west) decline in rainfall across the plains.
- Shielding effect of Aravalli (Incorrect): The Aravalli range primarily affects the Arabian Sea branch of the monsoon by running parallel to the wind direction, resulting in low rainfall in Rajasthan. It does not cause the gradual decrease in rainfall observed along the length of the Ganga valley, which is driven by the exhaustion of the Bay of Bengal branch.
Key Takeaway: Rainfall in the Ganga valley decreases from east to west because the Bay of Bengal monsoon branch progressively loses moisture as it travels further inland, away from the sea.