Exploitation of aquifers has traditionally been regarded as a typical example of management of a common property resource where collective ownership induces inefficient use due to strategic interactions between users (Provencher and Burt, 1994; Roemer, 1989). Indeed, an increasing number of regions have unsustainable groundwater balance due to overdraft (Shah et al, 2000). This chapter takes groundwater extraction rights owned and administered collectively by groups of farmers' and considers the potential impact of changes in groups' operational rules on the resource use.