Many modern agricultural systems are structured around one dominant form of water storage and distribution, usually large reservoirs. In contrast, in peninsular India, small reservoirs (tanks), predominantly supplied by surface runoff as opposed to river canals have for centuries been the trademark of an entire agrarian civilization, with no equivalent elsewhere in the semiarid tropics. This article focuses on the physical and socioeconomic conditions that underlie the success of an indigenous technology which has for centuries exploited the potential for runoff harvesting by i) optimizing water management for agriculture; and ii) minimizing soil loss. Today, siltation of reservoirs, privatization of water resources, and generalized mining of groundwater, pose a threat to the sustainability of these water-harvesting systems. The ongoing transformation of this common resource pool is critically assessed.