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Water table fluctuation
Groundwater table which represents the upper surface of water level in the
zone of saturation is susceptible to change in the atmospheric pressure, but
signification variation occurs due to withdrawal or pumpage and also due to
groundwater recharge from rainfall and other sources of recharge like irrigation
etc. Thus in short the fluctuation of water table is a measure of storage of the
aquifer and the change in storage over an area "A" can be measured by
knowing the value of the specific yield i.e.
Change in storage = Area x Fluctuation of the water table x
Sp. Yield value.
Seasonal variation of the water table can be computed by knowing the
pre-monsoon and post monsoon water table records. Rise in the water table
indicates addition in storage; lowering of water table on the other hand indicates
withdrawal from the storage. Long term water-table data reveal the status of
groundwater situation in an area. In case there is a continuous withdrawal of
groundwater which exceeds the annual recharge in the area, long term records
depicted in a hydrograph will reveal gradual recession of water table. There
may be several other natural or artificial causes like evapo-transpiration,
earthquake, tidal effects, movement of the ground due to railways etc., which
may be small, but significant. Water-table recorders on selected observation
wells reveal daily (even hourly), weekly, monthly fluctuation of the water-table.
Fluctuation of the Piezometric Surface
Fluctuation of the water-table aquifer in the recharge area of the confined
aquifer due to rainfall or pumpage causes fluctuation of the pizometric surface
in the confined bed. Since a small amount of storage is released from the
confined aquifer, heavy pumpage will cause a significant lowering of poezometric
surface. Heavy withdrawal from the confining aquifer is thus warranted because
of large decline of the piezometric head which may cause land subsidence.
Minor fluctuation of the piezometric surface also occurs because of atmospheric
pressure, tides, passing of trains etc.
