The Arani–Kortalaiyar (A–K) groundwater basin is located in the state of Tamil Nadu, close to the city of Chennai, in south India. It is a large alluvial basin having an area of c. 1500 km2. The aquifer consists of relatively thin deposits of sands and gravels. Recharge is from infiltration of rainfall during the monsoons, river flow losses and irrigation returns. Abstraction in the last 35 years, largely for irrigation, has led to a progressive dewatering of the aquifer to about one-third of its original saturated thickness, resulting in a substantial reduction of the yield of the municipal wellfields supplying the city of Chennai. In addition, seawater has intruded the coastal part of the aquifer, rendering the groundwater brackish or, closer to the sea, saline. Modelling of the aquifer to evaluate its sustainable yield confirmed that there has been a steady depletion of aquifer storage. Significantly, it also showed a relationship between recharge and abstraction. Under conditions of no abstraction, the aquifer quickly fills up, and further recharge is rejected. Abstraction creates space in the aquifer that allows more recharge to enter. The paper examines the implications of recharge–abstraction relationships in the evaluation of aquifer sustainable yield and how sustainable yield is affected by the mode of exploitation.