Water security is a multi-faceted problem, going beyond mere balancing of supply and demand. Early attempts to quantify water security relied on static index based approaches that failed to acknowledge that human action is intrinsic to the water cycle. Human adaptation to environmental change and increasing spatial specialization in the modern world necessitate a more flexible and dynamic view of water security. Starting from first principles, and through application of simple water balance concepts to human-impacted water systems, we first develop a set of indicators for water insecurity. We then offer an approach to model these indicators as outcomes of coupled human-water systems to anticipate watershed trajectories under human impacts, predict water insecurity and inform appropriate action. In this way, far from being a static index, water security signifies a “safe operating subspace” within a three dimensional space that maps physical resource availability, infrastructure and economic choices.