Semi-arid U.S. cities are growing rapidly in regions where water is scarce. A major challenge for environmental sustainability is to help semi-arid communities thrive by ensuring that cities, agriculture, and the environment are jointly water-secure. One successfully demonstrated option to meet growing water demands with the same water supply is reducing urban water use through efficiency. Urban lawn irrigation is now the primary target for reducing urban water use, as it dominates annual water use in many semi-arid cities. Cutting urban water use through irrigation efficiency could play a critical role in water-scarce river basins, but is presently limited by the potential unintended consequences of streamflow reduction from less runoff and groundwater flowing into streams. If we transform semi-arid cities to use less water outdoors, will urban streams run dry? Will major rivers flowing through semi-arid cities become trickles, with all the associated damage to ecology, recreation, and downstream water availability? The lack of fundamental scientific knowledge on how urban irrigation affects streamflow has limited the implementation of water-efficient urban irrigation systems in some parts of the semi-arid U.S. The purpose of this work is to predict the effects of reduced urban irrigation on streamflow, enhance the capacity of urban areas to manage scarce water resources, and ultimately contribute to the goal of thriving, sustainable semi-arid communities. The project will establish a mentoring program for underrepresented minority students interested in water careers and engage these students in hands-on urban water research. This project is co-funded by the Hydrologic Sciences Program and the Environmental Sustainability Program.

Reducing urban irrigation could play a critical role in achieving water security and sustainability in river basins where water is scarce. Urban irrigation may be reduced through smart, efficient irrigation systems or lawn conversion to xeriscaping and impervious surfaces. This work will quantify the present contribution of lawn irrigation to streamflow in small, semi-arid urban watersheds. This will be achieved in the Denver, Colorado, U.S. area by developing and applying a novel method combining water isotopes as a tracer of tap water with water provider reports, and monitoring in experimental watersheds. Then, using irrigation plots and integrated subsurface-surface-land surface hydrologic modeling, this research will determine the effects of reduced urban irrigation on semi-arid groundwater recharge and surface runoff. Lastly, this work will evaluate the hydrologic significance of present and reduced urban irrigation on streamflow in the semi-arid urban corridor of a large river basin. This research will integrate tightly with an education plan to diversify the urban water workforce in Colorado.