Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 39

The Ganga essentially is a rainfed seasonal river sustained by the summer rain of the Indian monsoon. The annual total rainfall decreases from the east to west, from 1600 to 500 mm. The southwestern basin is relatively dry. As expected, rainfall increases on the Himalayan slopes, reaching 1500–2300 mm (Singh 2007). The upper basin also receives some summer snowmelt from the Himalaya. More than 70% of the annual rainfall, rising to 80% in certain locations, arrives between July and early October in the wet monsoon system. The rain often falls intensely and in episodic tropical storms, some reaching cyclonic status, over the lower basin leading to floods.

The flow of the Ganga reflects both seasonality of rainfall and stepwise increment in discharge where the major tributaries, such as the Yamuna, Gomati, Ghaghara, Gandak, Son, and Kosi join the trunk steam. Half of the annual rainfall enters the river as surface runoff, 30% is lost by evaporation, and 20% seeps to the subsurface. During the dry season, part of the subsurface water flows through the high alluvial banks of the Ganga to its channel as baseflow. The mean discharge of the Ganga at Farakka, before it divides into its deltaic distributaries, is 70 547 m³ s⁻¹. About 60% of this arrives from the Himalaya and the northern plains (Das Gupta 1984).