HYDROLOGIC RESPONSE OF AQUIFER TO ARTIFICIAL RECHARGE IN PART OF UPPER PONNAIYAR BASIN, KARNATAKA

Abstract

The optimal utilisation and management of water resources is essential to supplement the demand of growing population, needs of development and to ensure its long term availability. Existing development takes care of complete utilisation of available surface water resources. Hence any excess demand or further development needs groundwater utilisation. However in pockets of high demand areas even the groundwater utilisation has reached a critical stage. Alternative strategies like water conservation, artificial recharging of the sources of groundwater and such other measures require consideration. This study was taken up with the aim of ascertaining the hydrologic response of aquifer to artificial recharge in the crystalline terrain in parts of Ponnaiyar Basin in Karnataka state of India by simulation modeling. Due to indiscriminate pumping necessitated by increasing water demand for irrigation, the water table in the area has shown a persistent decline. The objectives of the study were to identify possible zones for artificial recharge under the existing stage of development and to understand the behavior of the groundwater system for different scenarios of artificial recharging and thereafter to identify the best possible scheme. Initially, a water balance study for the lumped model has been carried out in the study area and it is found that there is nearly a complete utilization of available storage with a net result of change in the annual storage of 1.4 mcm, which is considered nominal. Subsequently, distributed modeling has been attempted with a model grid of 12 X 11 cells using a modular three dimensional finite difference groundwater flow model (MODFLOW) using the results of the lumped model as starting parameters. The groundwater in the basin occurs in the weathered zone of the granitic gneiss underlain by fractured rock. Accordingly, a two layer transient simulation model has been considered for groundwater modeling. The water budget of the simulation study shows that there is an annual draft of 132.81 mcm for a recharge of 124.21 mcm thereby showing a change in storage of -8.6 mcm. This indicates that there is a need for creation of additional storage which in turn will result in rise of water table in the long run which may help in arresting the declining water level trends in the study area. Accordingly, hydrologic response of the aquifer to artificial recharge for the changing stresses has been studied using simulation techniques. Two scenarios with 20% and 30% increased recharge distributed over monsoon and non-monsoon season and with equal increase in draft have been generated. A predictive simulation period of three years has been considered for studying the response of aquifer to the change in hydrologic stresses. It is found that the Scenario II (with annual draft and recharge being 168.93 mcm and 160.43 mcm respectively), is suitable for further detailed consideration. The study has some limitations due to paucity of data on geology and structure, surface water availability, agricultural statistics and sparse field control points for aquifer parameters. Feasibility of this study should also be ascertained in varying geological environments, keeping in view the availability of surface water. The application of Geographical Information System (GIS) for the selection of artificial recharge sites can also aid in the identification of a better strategy.