The terrestrial water cycle consists of multiple interacting reservoirs, including the atmosphere, the soil-vegetation system, rivers, streams, and other forms of surface water, and groundwater. All of these components have been affected by climate change in the past, and will continue to be so affected into the future. Before we can predict future changes in the water resources that are so critical for healthy human communities and ecosystems, we must face three challenges. First, we must develop a theoretical framework that emphasizes the continuous cycling of water through all the land and atmospheric reservoirs of the terrestrial water cycle and allows us to study the dynamical interactions among these reservoirs over a wide range of space and time scales. In other words, we need to study climate and hydrology together, not as isolated components, emphasizing the linkages among all the reservoirs. Therefore, we must develop integrated tools to study joint climatic and hydrologic responses. Second, using these new tools, we must achieve greater understanding of the mechanisms that drive changes in these reservoirs over different space and time scales so that we can quantify potential changes in, e.g., the amount of water in our lakes and rivers, the extent of our coastal wetlands, and the amount of water in and rate of replenishment of our groundwater aquifers as a result of climate change. In other words, we must gain the ability to translate climate change into hydrologic change. Third, we must build into this scientific framework the ever-increasing human influences, occurring at global and regional scales, that may strongly affect climate and the terrestrial water cycle. The current approach to regional climate downscaling and climate change impacts on the terrestrial water cycle, i.e., the practice of decoupling the fundamental reservoirs, does not permit us to address these issues. Therefore, in the proposed study, we intend to adopt a new approach, embodied in a new modeling tool. The interdisciplinary project team has recently built the a new modeling system for exploring coupled climatic-hydrologic processes by fully integrating all surface and subsurface terrestrial water cycle reservoirs, and their governing dynamics, into a state-of-the-art regional climate model, the Regional Atmospheric Modeling System (RAMS). With this tool, RAMS-Hydrology, the project team now has a unique ability to produce downscaled scenarios of regional climate change impacts on the terrestrial water cycle and investigate the two-way interactions between the atmospheric, surface, and subsurface water reservoirs that modulate these impacts. The work proposed here is to combine this new downscaling tool with our best understanding of large-scale climate variability over the past decades, and our best estimates of the range of potential global climate changes in the future, to examine coupled climate and water cycle change over North America during the 20th and 21st centuries. The proposed research strives to view the terrestrial water cycle as a dynamically coupled system among all its reservoirs in the atmosphere, the land surface, and the subsurface. The project crosses disciplinary boundaries between atmospheric science and hydrology: our research team includes experts in global climate modeling, regional atmospheric modeling, soil moisture, and hydrological modeling, providing an integrated approach to the water cycle from the atmosphere into the soil and through the vadose zone into the water table and the saturated store below. The project takes advantage of a unique new tool developed by the project team that fully and dynamically couples the atmospheric, surface, and subsurface branches of the terrestrial water cycle in a single, internally self-consistent modeling framework. This tool will be used as a better system for doing downscaling of global climate change scenarios to regional-scale combined climatic-hydrologic impacts. The proposed work clearly addresses many aspects of the NSF Water Cycle program and cannot readily be pursued by individual core programs within NSF. As such, our findings are expected to shed new light on the interactions and feedbacks among the various components (including humans) of the terrestrial water cycle and the regional climate system. This work is also expected to enhance our ability to predict, and thus prepare for, changes in future water resources as a result of changes in global and regional climate, as well as our understanding of the fundamental causes and mechanisms of these changes. All our findings will be reported in peer-reviewed journals, presented at meetings, and disseminated on the World Wide Web. The proposed project will also integrate research and education at Rutgers University. In addition to training a postdoctoral fellow and a graduate student (we will make an effort to offer the graduate assistantship to a student who is a minority), we will integrate the modeling tool we have developed into an existing graduate course in climate modeling to allow the students to have hands-on access to state-of-the-art climate and hydrologic research techniques. We will also incorporate the results of this project and its tools into a senior-level capstone course we are developing for Meteorology undergraduate majors.