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