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[1] The hydrodynamic properties of the weathered-fractured layer of a hard-rock pilot watershed in a granitic terrain are characterized using hydraulic tests at different scales. The interpretation of numerous slug tests leads us to characterize the statistical distribution of local permeabilities in the wells. The application of flowmeter profiles during injection tests determines the vertical distribution of conductive fracture zones and their permeabilities. It appears that the extension of the most conductive part of the weatheredfractured layer is limited down to 35 m depth. The partition of drainage porosity between blocks (90%) and fractures (10%) is determined thanks to the interpretation of pumping tests using a double-porosity model. The application of anisotropic and single-fracture analytical solutions on pumping test data allows us to determine, respectively, the degree of anisotropy of permeability (K r /K z = 10) and the radius (4-16 m) of the horizontal conductive fractures crossed by the wells. Two different scales of fracture networks are identified: the primary fracture network (PFN), which affects the matrix on a decimeter scale by contributing to an increase in the permeability and storage capacity of the blocks, and the secondary fracture network (SFN), which affects the blocks at the borehole scale. SFN is composed of two sets of fractures. The main set of horizontal fractures is responsible for the subhorizontal permeability of the weathered-fractured layer. A second set of less permeable subvertical fractures insures the connectivity of the aquifer at the borehole scale. The good connectivity of fracture networks is shown by fractionaldimension flow solutions. The absence of scale effect in the study area suggests that the hydraulic conductivity at the borehole scale is laterally homogeneous. Finally, the analysis and synthesis of the hydrodynamic properties allow us to propose a comprehensive hydrodynamic model of the fractured-weathered layer. Many geological and hydrogeological indicators suggest that a continuous and laterally homogeneous weathering process is responsible for the origin of the fractures and permeability encountered in the aquifer. These results confirm the major role played by weathering in the origin of fractures and on resulting hydrodynamic parameters in the shallow part of hard-rock aquifers.

Terra Nova, 23, 145–161, 2011 Abstract The hydrogeology of superficial (∼0–100 m b.g.l.) Hard Rock Aquifers (HRA; i.e. plutonic and metamorphic rocks) has so far been dominated by a few concepts considered to be relevant by a large majority of the HRA community. One of the most fundamental of these concepts is that their (secondary, fissure/fracture) permeability is either of tectonic origin or related to unloading processes. We will show that these genetic concepts are erroneous. We will demonstrate how the hydraulic conductivity of HRAs is a consequence of the (palaeo) weathering processes, with a stratiform fissured layer located immediately below the unconsolidated saprolite and, to a lesser extent, a verticalized fissured layer at the periphery of (or within) pre‐existing discontinuities (veins, joints, ancient faults, lithological contacts, etc.). This result opens up large perspectives in terms of applied hydrogeology and applied geology. A specifically dedicated methodological toolkit well adapted to the operational survey, management and protection of HRAs is briefly presented.

A water budget approach is developed to jointly estimate specific yield and natural recharge in an unconfined aquifer with significant seasonal water table fluctuations. Water table fluctuations are due to distinct seasonality in groundwater recharge. The separation of the hydrologic year into two (or more) extended seasons of recharge (wet season) and norecharge (dry season) with accompanying changes in water table allows for a split use of the water table fluctuation (WTF) method, first to estimate specific yield from the water table drop during the dry season (no recharge) and, second, to estimate recharge from the water table rise during the wet season, after considering all other water budget components explicitly. The latter includes explicit computation of groundwater storage with the WTF method. The application of the WTF method requires a large number of water level measurements throughout the unconfined aquifer before and after each season. The advantage of the method is that specific yield and recharge are estimated at the scale of interest to basin hydrologic studies and that the method requires no extensive in situ instrumentation network. Here, the method is demonstrated through a case study in a fractured hard-rock aquifer subject to intensive groundwater pumping for irrigation purposes.

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Until recently, aquifers located in hard rock formations (granite, gneiss, schist) were considered as a highly heterogeneous media, and no adequate methodology for groundwater management was available. Recent research studies have shown that when hard rocks are exposed to regional and deep‐weathering processes and when the geology is relatively homogenous, a typical hard rock aquifer is made of two main superimposed hydrogeological layers each characterized by quite homogeneous specific hydrodynamic properties: namely the saprolite and the fissured layers. Therefore, for these cases, hard rock aquifers can be considered as a multi‐layered system. Based on these works, an operational decision support tool (DST‐GW) designed for the management of groundwater resources in hard rock area under variable agro‐climatic conditions has been developed. The tool focuses on the impact of changing cropping pattern, artificial recharge and rainfall conditions on groundwater levels at the scale of small watersheds (10 to about 100 km2 in case well‐developed weathering profile). DST‐GW is based on the groundwater balance and the ‘water table fluctuation method’, which are well‐adapted methods in hard rock and semi‐arid contexts. Based on field data from an overexploited South Indian watershed (58 km2), the model allows calibrating, at watershed scale, the variation in specific yield of the aquifer with depth, as well as the rainfall‐aquifer recharge relationship. Seasonal basin‐scale piezometric levels are computed with an average deviation of ± 0·56 m compared to measurements from 2001 to 2005. The model shows that, if no measure is taken, the water table depletion will induce the drying‐up of most of the exploited borewells by the year 2012. Scenarios of mitigation measures elaborated with the tool show that change in cropping patterns could rapidly reverse the tendency and lead to a sustainable management of the resource. This work presents the developed tool and particularly the hydraulic model involved in and its application to a case study. However, the purpose tool is applicable at watershed scale but not design for the groundwater management of a very small area or for a single borewell. Copyright © 2010 John Wiley & Sons, Ltd.