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A Tool for Sustainable Groundwater Management in Semi-arid Hard- Rock areas

Congress: 2008
Author(s): S. Ahmed, S. Aulong, Z. Hrkal, P. Lachassagne, M. Samad, S. Massuel, A. Mukherji, J. Perrin, K. Raju and K. Tirupataiah

Keyword(s): Hard Rock, granite, Decision Support Tool, global modelling
AbstractHard rock (granitic and metamorphic rocks, e.g. granites, gneiss, schists, etc.) aquifers are often considered as ‘discontinuous aquifers’ as their water bearing structures are localized along fractures and/or fissures, the matrix permeability being itself very low. Thus, they have long been considered as a too heterogeneous medium to easy enable the exploitation of the groundwater resource and, a fortiori, to enable its proper management, as it is already done since decades for porous aquifers for instance. Such rocks and associated aquifers constitute the basement of the continents and, are consequently very common throughout the world, particularly in semi-arid regions, e.g., Africa, South America, India. As an example, in Southern India, they are particularly exploited (through millions of borewells) and contributed to the Indian “Green Revolution”. Recent geological and hydrogeological research results demonstrated the relationships existing between the weathering processes and the geometry and hydrodynamic properties (hydraulic conductivity, storativity) of such aquifers. A typical hard rock aquifer (approximately 100 m thick) comprises two main hydrogeological units characterized with quite homogeneous specific hydrodynamic properties: the saprolite and the underlying fissured layer. Therefore, hard rock aquifers are composite aquifers that can be considered as a multi- layer system. From this research work, a methodology for the groundwater resource management of such aquifers adapted to the semi-arid context has been developed, and emerges a Decision Support Tool (DST). DST is designed for groundwater management in hard rock areas under variable agro-climatic conditions. It focuses on the impact of changing cropping pattern and artificial recharge on basin-scale groundwater storage, piezometric levels, drying of wells, etc. DST is a program developed under a MS-Excel Interface. The model is based on the groundwater balance and the ‘Water Table Fluctuation Method’ and integrates the natural characteristics of hard rock aquifers such as the depth-variation in storativity, the layer thicknesses, as well as the variation in both natural and artificial aquifer recharges with respect to climatic conditions, the possibility to change the climatic conditions, to give basic socio-economic indicators, etc. The method has been implemented for scientific purpose in a representative south Indian rural catchment in Andhra Pradesh (53 km², Maheshwaram, Ranga Reddy Dist.). DST models the basin-scale piezometric levels with an average error of ±0.6 m from 2001 to 2005 (calibration period) that shows the model’s robustness. Due to the groundwater overexploitation (>700 borewells in use), the simulations show that if no solution is found quickly, the groundwater decline will entail the alarming loss of about 50% of the pumping borewells by around 2010, with all the accompanying serious socio-economic consequences. However, DST lets foresee that some sustainable solutions exist. The scientific tool (DST) and methodologies are now being tested at an operational scale for Policy Makers and Planners (e.g. Mandal, Gram Panchayat levels) with the data collected in routine by the local Authorities and a few other low-cost additional data. As an exercise under the SUSTWATER Project (EuropeAid funding), this challenge is currently implemented in another catchment (Gajwell), in close collaboration with the Andhra Pradesh Rural Development Department and the Andhra Pradesh Groundwater Department.
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