Présentations Colloques

In South India, water scarcity is a major threat due to the economic dependence on groundwater-based agriculture, to crystalline-rock aquifers lacking important groundwater resources and to the variability of the monsoon. To tackle this problem, states and federal governments develop large-scale managed aquifer recharge plans.**We explored the potential and dynamic of a percolation tank in close relation to crystalline aquifer structure and hydrodynamic (i.e. decrease of the storage and transmissivity with depth, anisotropy, aquifer compartmentalization and impact of preferential flow paths). Numerous and regular field investigations (e.g. GPS, piezometric measurements, borehole logging) coupled with remote sensing over the 2000-2014 period provides a rare dataset in such context. Such assessment shows the high variability of the potential storage, the risk of inequity between farmers in water recovery and the difficulty of assessment of the tank potential. **In the studied case, despite a limited potential, important works started in 2016 aiming to increase the tank depth by removing the upper saprolite layer. It may reduce evaporation through the reduction of the surface area for an equivalent volume of stored water, however, due to the low porosity of the fissured layer, underground storage will also partly decrease thus counterbalancing part of the beneficial effects. Following the modifications of tank geometry, it is very likely that water levels will rise rapid and strongly in the nearby boreholes leading to increased yield for some of them. However, rising water levels will be due to the low storage coefficient whereas the overall stored volume will remain low, so that the pluri-annual efficiency the system may be lower than expected. **This detailed assessment shows 1) the need for conceptual hydrogeological models adapted to hard rock saprolite aquifers, 2) the need of developing robust decision support on tank design and potential efficiency improvements and 3) the need of developing assessment procedures for such structures upstream of infrastructures development.**