By determining who can extract water, how much and for what purpose, water allocation regimes are essential policy instruments to tackle water scarcity. Traditionally operated by the state, they have proved difficult to implement in agricultural groundwater dependent economies due to the difficulty of regulating thousands of extraction points. Many countries are thus decentralising allocation decisions to agricultural users and other stakeholders. These user groups have the challenging task of producing robust allocations that reconcile potentially conflicting objectives on equity, economic efficiency and ecosystem health. This session will explore how stakeholders craft collective allocation rules that resolve competing societal objectives while harnessing the temporal and spatial variability of groundwater resources. Examples on novel institutional arrangements, original engagement strategies, innovative information systems and decision-support tools are sought. Interdisciplinary work showing how scientists interact with stakeholders to harness the complexity of hydrogeological systems for developing effective groundwater policies and institutions is welcome.