FrontMatter -- Preface -- Contents -- Summary -- 1 The Hydrologic Sciences -- 2 The Water Cycle: An Agent of Change -- 3 Water and Life -- 4 Clean Water for People and Ecosystems -- 5 Hydrologic Sciences: A Path Forward -- Appendix A: Contributors to the Report: Challenges and Opportunities in the Hydrologic Sciences -- Appendix B: Biographical Information: Committee on Challenges and Opportunities in the Hydrologic Sciences.