Seasonal behaviour of spatial variability of groundwater level in a granitic aquifer in monsoon climate

Groundwater flow in a small watershed in a hard rock region of Andhra Pradesh, India mainly exists in a coupled system of weathered and fractured rock aquifers. However, due to heavy extraction, groundwater level has declined and the weathered part has become unsaturated. In general, the water-table aquifer exists in the area under semi-confined or unconfined conditions. Monthly water-levels from 32 wells fairly evenly distributed over the area were collected to analyse the variability and finally to calibrate a groundwater flow model. Universal kriging technique with a linear drift was applied to analyse the available groundwater levels during different periods for one cycle during the year 2000. The bounded variograms, i.e. without the effect of a drift, were calculated using the directional variogram in the direction perpendicular to the major mean flow. All the variograms were then cross-validated to get final acceptable models for each time period. Subsequently, two variograms, one for the monsoon period with the presence of recharge component and the other for the non-monsoon period without the presence of recharge component were evolved. A common variogram was also determined to represent the average over all time periods. Crossvalidation tests were done to check that the variograms representing monsoon and non-monsoon periods were able to reproduce field values for the corresponding periods more satisfactorily than a single common variogram. Thus application of geostatistics could be simplified for time-varying parameters such as water level.