3.3 Observed Groundwater data
We downloaded local (observed) groundwater level data from Central
Ground Water Board (CGWB) for period January 2004 to January 2017 of
Bankura district. Usually CGWB measures groundwater level with respect
to pre monsoon, monsoon, and post monsoon kharif and post monsoon rabi
seasons. According to the CGWB, for the states of Kerala, Odisha and
West Bengal the month of April is taken as pre monsoon month for
groundwater level measurement. August, November and January months are
representing monsoon, post monsoon kharif and post monsoon rabi
respectively. The downloaded groundwater data represents the depth to
groundwater table which is mentioned as meters below ground level
(mbgl). Groundwater level head by subtracting depth to groundwater table
from 30m (because the observed wells in study area are shallow wells
with the depth of 30m) is calculated. The obtained value is multiplied
with the specific yield value which is mentioned in the table 1. (Y.B.
Katpatal et al. 2017). These values are converted into GWS variations
with reference to the average groundwater level of the time period
January 2004 to December 2009.
∆GWSobs = [(30-GWL) –
(30-GWL2004-2009)] × Sy (3)
Where,
∆GWSobs = Observed groundwater storing variations
GWL = Groundwater level at time
GWL2004-2009 = average groundwater level during
January 2004 to December 2009
Sy = Specific yield of the study area