Coping with variable and unpredictable freshwater resources represents a profound challenge to climate adaptation. Rainfall, snowmelt, soil moisture, and runoff can vary from zero to large quantities, over a range of time scales and in ways not well predicted by climate models. Extreme floods and droughts are the most obvious manifestations, but hydrologic variability can also have chronic impacts. Water security involves managing these risks so that they do not place an intolerable burden on society and the economy (1). We discuss interlinked roles of institutions, infrastructure, and information in managing those risks.