Trust and transboundary groundwater cooperation

International transboundary aquifers provide important water supplies to over 150 countries. Long-term sustainability of these aquifers requires transboundary cooperation and yet only a select few (1%) transboundary aquifers are formally regulated by a treaty. To better understand the drivers and incentives that allow treaties to emerge, we develop a two-player game to model the social dilemma of transboundary aquifer cooperation. The game incorporates socio-economic and hydrogeological features of the system and highlights the importance of trust to evaluate the benefits and risks of any treaty. We validate the game through a case study of the Genevese aquifer, which is governed by the longest-running and most collaborative transboundary aquifer treaty on record. We then focus on the symmetric game between identical players to explore the role of groundwater connectivity, alternative water supply, water demand, and trust on the emergence of transboundary treaties. The solution space highlights how incentives for cooperation are greatest when the value of water is commensurate with the cost of groundwater abstraction. Cooperation requires high trust in situations characterized by water abundance or scarcity. The model further indicates how two different types of agreements are likely to emerge. Treaties that limit abstraction have greater potential when countries have access to an alternative water source, whereas treaties that restrict pumping near the border have greater potential in water-scarce regions with emerging concerns over groundwater depletion. In addition to helping explain the emergence of existing treaties, this framework offers potential to identify aquifers that may be amenable to cooperation.