Brahamaputra Basin

Brahamaputra Basin
4.2 Ground water level fluctuation
Water table undergoes a seasonal fluctuation following a natural cycle. Water table rises during
rainy season due to recharge received from rainfall and falls back during summer season due to lack
of recharge and continuing extraction of ground water. In geo-hydrology, the highest water level is
referred to as the post-monsoon water level and the lowest level in summer as pre-monsoon water
level. The difference between highest water level value and lowest water level value is termed as
fluctuation in ground water level. This value can be positive as well as negative depending upon
various factors.
Ground water fluctuation in the Brahamaputra basin is assessed for recharge and draft conditions
based on the available groundwater level in four different seasons. Ground water recharge is
estimated by the difference between pre and post monsoon seasons. Similarly, groundwater draft is
estimated by the difference between post-monsoon and post-monsoon (rabi) seasons data.
Interpolated maps for ground water recharge and irrigation draft (rabi) are prepared based on the
available ground water fluctuation data in the basin. Ground water fluctuation due to recharge and
draft in Brahmaputra basin is depicted through chloropleth map given in Map 14 & Map 15.
The ground water fluctuation map (Map 14) shows the fluctuation of water level due to recharge in
the basin for the year 2011. It revealed that there is positive fluctuation in groundwater level in
major areas of basin with a rise of less than 2 m bgl in the post monsoon season. Few pockets of
Jalpaiguri region shows rise in ground water level of 2 - 4 m bgl.
But on the other hand there is also negative fluctuation with a fall in water level less than 2 m bgl
occurred in a very small part of basin which constitutes a minor area of Jalpaiguri, KochBihar and
Kokrajhar districts of West Bengal and Assam respectively.
Ground water fluctuation due to draft in the basin shows a larger part of basin experienced negative
fluctuation in groundwater level with a fall in water level less than 2 m bgl. Negative fluctuation with
a fall in range of 2 - 4 m bgl and 4 m bgl is noticed in Jorhat, Karbi Anglong, Golaghat, Sibasagar,
Nagaon, Kamrup districts of Assam and Jaintia Hills and Ribhoi districts of Meghalaya due to overexploitation
of ground water for different purposes and other natural cycle disruption like less
rainfall etc.
www.india-wris.nrsc.gov.in 48