Introduction to special section on Hydrologic Synthesis

The Hydrological Synthesis special section presents synthesis topics that have the potential to revolutionize hydrological sciences in a manner needed to meet critical water challenges that we now face. The special section also highlights topics that are important and exciting enough to compel researchers to engage in collaborative synthesis activities. This introductory paper provides a brief overview of nine papers that are included in this special section, which discuss the synthesis of tools, data, concepts, theories, or approaches across disciplines and scales. The wide range of topics that are explored include groundwater quality, river restoration, water management, nitrogen cycling, and Earth surface dynamics. Collectively, the special section papers illustrate that the challenge to deal effectively with complex water problems is not purely a scientific, technological, or socioeconomic one; it is instead a complex, 21st century problem that requires coordinated synthesis