Water is one of the most important natural resources of the United States and is essential to our health and economic well-being. Increasing competition for water to meet the needs of a growing population, municipalities, agriculture, industry, ecosystems, and recreation weighs on the sustainability of this resource. The sustained use of water resources is contingent on understanding the hydrologic implications of various alternative development strategies and properly evaluating their short- and long-term implications at scales that make sense. Compounding this complexity are unforeseen factors such as climate variability and change, which can further exacerbate an already challenging situation.