Computer simulation models have emerged in recent decades as increasingly prominent technologies within the toolkit of modern democratic governance. Despite and/or because of this centrality, however, formerly 'technical' domains of modeling have been opened up to new forms of public debate, scrutiny and critique, with uncertain policy consequences. This paper traces such dynamics through one field of contemporary relevance: the joint evolution of simulation models and water management in California. Rather than decrying the politicization or debasement of expertise, I argue that broadening the deliberative basis of model design and use is likely to improve both the technical and political functioning of models. The paper concludes by sketching a model of 'virtual accountability' meant to inform the actions of future model builders, users, and stakeholders in contested realms of public policy.