Assertion: The catchment areas of Ganga, Brahmaputra and Barak Rivers account for only about one-third of the total area in the country, but have about 60 per cent of the total surface water resources.
Reasoning: Precipitation is relatively high in the catchment areas of the Ganga, the Brahmaputra and the Barak rivers.

Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

Some of the rivers in the country like the Ganga, the Brahmaputra, and the Indus have huge catchment areas. Given that precipitation is relatively high in the catchment areas of the Ganga, the Brahmaputra and the Barak rivers, these rivers, although account for only about one-third of the total area in the country, have 60 per cent of the total surface water resources.