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