New research opportunities to advance hydrologic sciences promise a better understanding of the role of water in the Earth system that could help improve human welfare and the health of the environment. Reaching this understanding will require both exploratory research to better understand how the natural environment functions, and problem-driven research, to meet needs such as flood protection, supply of drinking water, irrigation, and water pollution. Collaboration among hydrologists, engineers, and scientists in other disciplines will be central to meeting the interdisciplinary research challenges outline in this report.