In response to prolonged drought situations, water collectives have been promoted and exist in most water scarce farming regions to help them sustainably manage groundwater resources. However, the sustainability of the collectives itself is prone to groundwater depletion and to the risk of its irreversible future loss. The role of groundwater loss risk in determining the success or failure of groundwater collectives is inadequately understood. In this paper, a stylized dynamic optimization model of institutional participation is developed and results are analyzed for several cases of collective and non-collective farming behaviour. Results indicate that risk of groundwater loss may increase the tendency to free ride and exacerbate its depletion. This tendency may increase with a larger collective size when there are no deterrence mechanisms in place. However, when staying out threatens institutional stability, it is optimal for the profitmaximizing farmer to join the collective and help with groundwater conservation. © 2014 The Author.