from the standpoints of scientific curiosity and societal need. At the most fundamental level, critical gaps remain in the knowledge about hydrologic fluxes. Evaporation, transpiration, and groundwater fluxes interconnect the water, energy, and biogeochemical cycles yet adequate measurements and estimates of these fluxes are elusive, even for relatively pristine natural systems. The perturbations to the hydrologic cycle from “replumbing” through human activities add a dimension of complexity, and urgency, to this research area. A challenge for the hydrologic community is to understand replumbing; for example, the downstream consequences of urban growth or changes in the severity, duration, and occurrence of floods and droughts as a result of climate change, and to apply this understanding to making predictions for the future. Furthering understanding of the processes that link components of the water cycle is no less important than understanding the human impacts on the water cycle.

The processes that define water fluxes occur at many time and space scales, for example, the first drops of water that initiate streams to the complex systems of rivers that define drainage basins. Research questions are continually raised regarding the quantitative relationships among variables and across scales. Because interactions at overlapping scales change hydrologic patterns in subtle ways, disentangling the causality of subtle shifts and regime changes in streamflow and understanding their environmental impact is a challenge. The climate system can vary at long time scales as well as shift rapidly into new modes of behavior that are radically different from the historical experience. Understanding the hydrologic response to abrupt climate change over short time scales and to slowly varying natural climate change is far from complete. Exploration of how the water cycle has affected the evolution of other planets may provide important insight into Earth’s water cycle and its dynamics as an agent of change and determinant of life. The study of hydrologic processes on other planets defines the new field of “exohydrology,” and research in this area is only just beginning.

Water and Life

Water is essential for all living organisms, and, on land, the magnitude of the water supply and the timing of water delivery structures biological systems at all spatial and temporal scales. Recently ecologists, geomorphologists, climate scientists, and hydrologic scientists have found a common frontier lies at the nexus of life and water because water plays a critical role in driving the environmental patterns that exist and evolve on Earth. The past, with radically different biota, topography, and atmospheric and ocean chemistry, presents an opportunity for hydrologists to explore how key processes in the hydrologic cycle differed, and how these processes contributed to Earth’s evolution. Hydrologic flow regimes, river channel



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