The design and implementation of water policies to address scarcity problems are largely shaped by the behavior of interest groups and their ability to influence policymakers. Different and opposed interests of stakeholders regarding policies trigger water conflicts and, frequently, lead to the failure of the implemented regulation. Departing from political economy theory, we empirically estimate the determinants that affect the level of lobbying effort and effectiveness by water interest groups for influencing water policy. The findings are based on data collected by a survey administered among different irrigators' groups, in a water-stressed river basin (the Jucar River Basin in Spain) that vie to increase their water allocations. Our results demonstrate how lobbying effort depends on the involvement of the interest groups, the energies exerted to sway water authorities, and on the variation among the group members. Lobbying effectiveness is a function of the effort exerted. Furthermore, both functions depend on the intrinsic characteristics of the group's members. While the empirical results corroborate several main statements of the theory of lobbying and interest groups, some deviations based on the empirical application remain.