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) groundwate r level monitoring was started in early 1998 in Andhra Pradesh under the Na tional Hydrology Project. Well hydrographs in the hardrock terrain during t he monsoon period show erratic rising trends interspersed with flatter or d eclining trends, indicating that monsoon recharge occurs in short bursts. T he hydrographs during the non-monsoon period show linear recession limbs. H owever, some hydrographs exhibit a slow-down in rate of recession as the no n-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, b oundary effects as well as anisotropic and heterogeneous behaviour. Coeffic ients of recession (R-2) in recession limbs of all the hydrographs are near ly equal to 1, indicating that there are no irregular abstraction rates or irregular surges of recharge in non-monsoon period, which would help in pre dicting nonmonsoon water levels. Diurnal fluctuations within both the risin g limbs and recession limbs, do not show any appreciable difference except at the precise lime of recharge surge. Exponential recession coefficient of the hydrograph at the end of recession period, when only the aquifer param eters influence the data i.e. the decline in water level, indicate approxim ate specific yield values of 0.009 to 0.038. However, apparent specific yie lds obtained using water table fluctuation from the well hydrographs in hyd rologic budget equation and commonly adopted value (10%) of rainfall infilt ration factor (r.i.), are abnormally low (0.003 to 0.022). This is partly e xplained by the deep water table (> 12 m), and also suggests higher rainfal l infiltration factor.