Trend Analysis of Groundwater Fluctuations in a Typical Groundwater Year in Weathered and Fractured Rock Aquifers in Parts of Andhra Pradesh

High frequency (i.e., continuous record set at 6 hours interval) groundwater level monitoring was started in early 1998 in Andhra Pradesh under the National Hydrology Project. Well hydrographs in the hardrock terrain during the monsoon period show erratic rising trends interspersed with flatter or declining trends, indicating that monsoon recharge occurs in short bursts. The hydrographs during the non-monsoon period show linear recession limbs. However, some hydrographs exhibit a slow-down in rate of recession as the non-monsoon period advances, thereby causing the trend to become curvilinear and fitting into exponential or geometric curve. Curvilinear trends are due to the effect of a complex aquifer system that exhibits double porosity, boundary effects as well as anisotropic and heterogeneous behaviour. Coefficients of recession (R²) in recession limbs of all the hydrographs are nearly equal to 1, indicating that there are no irregular abstraction rates or irregular surges of recharge in non-monsoon period, which would help in predicting nonmonsoon water levels. Diurnal fluctuations within both the rising limbs and recession limbs, do not show any appreciable difference except at the precise time of recharge surge. Exponential recession coefficient of the hydrograph at the end of recession period, when only the aquifer parameters influence the data i.e. the decline in water level, indicate approximate specific yield values of 0.009 to 0.038. However, apparent specific yields obtained using water table fluctuation from the well hydrographs in hydrologic budget equation and commonly adopted value (10%) of rainfall infiltration factor (r.i.), are abnormally low (0.003 to 0.022). This is partly explained by the deep water table (>12 m), and also suggests higher rainfall infiltration factor.