Abstract:
Until recently, aquifers located in hard rock formations (granite, gneiss, schist) were considered as highly heterogeneous media, and adequate methodologies for groundwater management or borehole siting were non-existent. Recent studies showed that a typical hard rock aquifer is made of two main hydrogeological units characterized by quite homogeneous specific hydrodynamic properties; namely the saprolite and the fissured layers. Therefore, hard rock aquifers can be considered as a multi-layered system.
Based on this research work, the Indo-French Centre for Groundwater Research (BRGM-NGRI located in Hyderabad, Andhra Pradesh, India) has developed an operational Decision Support Tool (DST-GW) designed for groundwater management in hard rock area under variable agro-climatic conditions. This DST-GW focuses on the impact of changing cropping pattern and artificial recharge on the groundwater levels at the scale of small watersheds (up to 100 km2). The DST-GW is based on the groundwater balance and the Water Table Fluctuation Method', well-adapted methods in hard rock and semi-arid contexts. It is a semi-automatic program developed under a MS-Excel Interface. The model integrates the natural characteristics of hard rock aquifers such as the variation in specific yield with depth, the respective thicknesses of the fissured and saprolite layers, as well as variations in both natural and artificial aquifer recharges with respect to climatic conditions. In addition to the hydraulic model, the DST-GW includes a module dedicated to future scenarios including socio-economic indicators: scenario "business as usual", implementation of supply and demand measures to mitigate over-exploitation, impact on farmer incomes, etc.
During its scientific development, the DST-GW has been implemented in a representative south Indian watershed (53 km2) characterised by a granitic basement, semi-arid climatic conditions, rural context, and groundwater irrigation. For this watershed, the DST-GW models the basin-scale piezometric levels with an average error of less than ±0.6 m from 2001 to 2005 (calibration period); this shows the robustness of the model. Due to groundwater overexploitation (more than 700 bore wells in use), the model results showed that if no measures are taken, the water table depletion will run dry about 50% of the pumping bore wells by the year 2010. Simulations of mitigation measures with the DST-GW show that a realistic change in cropping patterns could rapidly reverse the tendency and lead to a sustainable management of the resource.
