The Berambadi (Kabini Critical Zone Observatory) is an 84 km2 sub-catchment of the Cauvery river basin in the west of Karnataka, India. Intensive crop irrigation in the catchment is highly dependent on the supply of groundwater from a dense network of boreholes penetrating the peninsular gneiss aquifer. Large inter-annual variations in groundwater levels are observed in the catchment, where a single, heavy monsoon can compensate for several years of groundwater level decline. Extensive specific capacity data and groundwater level monitoring form the basis of a numerical transect model, which is used to explore the functioning of the fractured aquifer. The model results are compared with hydrogeological investigations, including inorganic hydrochemical analyses, δ18O and δ2H stable isotopes, CFC-11 and 12 and SF6 age-tracers, and salt dilution tests.
Numerical modelling indicates that lateral groundwater flow comprises only a small part of the water balance and water availability is controlled by local recharge and abstraction. Limited lateral flow is thought to be the result of shallow hydraulic gradients caused by widespread high abstraction, rather than poor connectivity in the fractured gneiss. This is supported by geological observations, which indicate up to 80% of the gneiss is sheared and fractured, and groundwater level fluctuations, which show a high degree of uniformity across >200 boreholes. Salt dilution tests indicate flowing fractures up to depths of 90 m below surface, but also highlight zones of low hydraulic conductivity throughout the profile. A lack of vertical or lateral trend in hydrochemistry suggests no significant x−y anisotropy at the catchment scale and distributed recharge across the catchment (rather than from specific lakes or tanks).
In contrast to many areas with crystalline rock aquifers, where lateral connection is poor, sheared crystalline aquifers enable higher abstractions, which can only be sustained through regular local recharge. In this catchment, and probably many similar, abstraction is preventing natural lateral flows (in unpumped conditions) out of the catchment through surface and groundwater, which could have implications downstream. The improved understanding of groundwater lateral and vertical connectivity and processes gained from this integrated study will aid groundwater management in this, and similar, catchments.