Due to the high temperature conditions which prevail over the Ganga basin for the greater part of the year, a significant quantum of water is lost through evaporation. In fact, except in lower Bengal and the Himalaya, the total annual evaporation is far greater than the total annual rainfall received. This fact indicates that the greater part of the basin would be hydrologicaly dry if the total annual rainfall were distributed evenly over the twelve months of the year. Since the vast majority is concentrated in a three month span in most of the basin, the water available from rainfall usually exceeds what is lost through evaporation during this period, allowing some surplus water to flow down the Ganga river and its tributaries.


Within the Ganga basin, every square kilometre of land surface area receives an average of one million cubic metres of water annually through rainfall, which is about 861 billion cum. Less than half of this total is actually available, after accounting for water lost through evapo-transpiration (30 percent) and seepage into the ground (20 percent). The gross numbers for evaporation and seepage into the groundwater have been estimated at around 293.8 billion cubic metres and 212.8 billion cubic metres, respectively. Thus, the total annual surface flow of water in the Ganga basin is estimated to be around 468 billion cubic metres of water.

 (Source: NGBRA report on Environmental and Social Management Framework)