Brahamaputra Basin

Brahamaputra Basin
4. Ground water resources
Water is essential to sustain agricultural growth and productivity. Ground water is the water below
the surface of the earth that occurs in soil pores and in cracks and fissures of rocks. Typically, ground
water is liquid flowing water but the definition also includes soil moisture, perma-frost (frozen soil)
and water trapped in very low permeability bed rocks. A unit of rock containing a usable amount of
water is termed as an aquifer. Ground water is refurbished from the surface and eventually flows
back to the surface in the form of springs or oases. The occurrence of ground water generally
depends upon the rainfall, drainage, topography and the geological conditions of the area.
4.1 Ground water observation wells
The Central Ground Water Board (CGWB) maintains a network of total 430 ground water
observation wells in the basin for study of ground water behaviour. The spatial distribution of
ground water observation wells are shown in the Map 13. Sub basin wise distributions of
observation wells are 304 located in Brahmaputra Lower Sub Basin and 126 well locations are
existed in Brahmaputra Upper Sub Basin (Table 10).
Sub surface lithological data indicate two broad groups of aquifiers for groundwater exploration (i)
shallow water table and (ii) deeper water table or confined zones, separated by a system of
aquicludes.The occurnce of ground water in the hard rock inselberg areas is controlled by foliations,
fractures / joints, and weathered zones. The physiographic features, types of drainage, depth and
duration of precipitation also play an important role.Many of the shallow open wells go dry by the
month of March. In the tertiary sediments, groundwater occurs in the sandstone and along the
fractures and solution cavities in the limestones. Groundwater occurs both under water table and
confined conditions. In the piedmont and alluvial fan deposits ground water occurs generally under
water table conditions.The water table has, in general, a southerly slope corresponding roughly to
the surface topography.
There is variation in depth of groundwater level which is mainly due to change in elevation. In the
flood plains and the wells located in Brahmaputra valley are comparatively shallow in comparison to
the wells located in upper reaches at higher elevations. The depth of water level varies from range of
2-6 m bgl and 5 to 10 m bgl in most of areas. But in the higher elevation areas deeper water level
observed, which goes upto 20 m of below ground water level depending upon the terrain feature of
the area.Based upon the pre-monsoon data for year of 2011, In the Brahmaputra lower sub basin
the ground water is found in a range of 4 to 20 m bgl mostly while in Brahmaputra upper sub basin
the water is found in a range anywhere between 2 to 20 m bgl.
Ground water is used for mainly two purposes at large scale in the basin which includes Drinking and
Irrigation.
Table 10. Sub-basin wise number of ground water observation wells
Sl. No. Sub Basin No. of Observation Wells
1 Brahmaputra Lower Sub Basin 304
2 Brahmaputra Upper Sub Basin 126
Source: CGWB
www.india-wris.nrsc.gov.in 46