Several major river systems flow through India, Pakistan and Bangladesh. The Gangetic Plain, a belt of flat and fertile lowlands as much as 200 miles wide, bordering the Ganges River, includes some of the most productive agricultural land in India. In many of the river plains, groundwater is rather plentiful and close enough to the surface to make irrigation possible. An important key to sustenance in India, Pakistan and Bangladesh is the reliability of the wet monsoon. In June, the prolonged dry season is broken as the wind shifts from the north and northeast to the south and southwest, ushering in welcome monsoon rains that typically last into September.
Agriculture is heavily dependent on the normal arrival of the monsoon.
When it's late or weak, famine results, but if it's early and unusually vigorous, flooding can ruin crops and wipe out villages. The monsoon rains are also very important in replenishing ground water. Without it, irrigation is a must. Although the major rivers will still flow even when the summer rains are erratic, since they're fed by glaciers and melting snow, the land will be quickly parched if the monsoon is late arriving. Because the Himalayas act to block incursions of cold air from central Asia, agriculture is not so much limited by low temperatures as it is by lack of moisture.