Altered hydrology — including changing patterns of precipitation, river flow, and evapotranspiration — has profound effects on ecosystem dynamics. Such effects are especially pronounced throughout the forests, rangelands, rivers, and cities of the arid and semi-arid American Southwest, where climate change and direct human appropriation radically alter water fluxes. A warming, drying climate combined with highly regulated surface flows and variable groundwater pumping result in novel hydrologic regimes that can reshape the nutrient cycles, species compositions, and biotic interactions in terrestrial and aquatic ecosystems. Furthermore, changes in the amount, timing, and spatial extent of precipitation, stream flow, and water availability can create new natural resource management challenges and influence socio-economic drivers on ecosystems in both rural and urban areas. To what extent can we predict the ecological changes arising from altered hydrological patterns in the Southwest? This session will bring together scientists examining the ecological effects of a changing water cycle in the Southwest to develop a more integrative understanding of altered hydrology across ecosystems, and the consequences for community dynamics and ecosystem processes.