ABSTRACT:

Urban streams in the Denver, Colorado, USA region flow more often than undeveloped grassland streams. We sought to identify the sources of this increased flow using water stable isotope data and an analysis of streamflow responses to rain events. We collected and assessed 402 urban stream, 522 tap, and 38 precipitation samples across 2019, 2021, and 2022. Two endmember mixing analysis was utilized to obtain the percentage of precipitation-derived groundwater and tap water contributing to urban baseflow. Our endmember mixing results revealed that a major portion of stream water came from tap water, through excess lawn irrigation returning to the stream and leaking water pipes. The average portions of streamflow that come from tap water and lawn irrigation return flow were 76% and 47% respectively across 2019, 2021, and 2022. Uncertainty related to estimation of tap contribution and lawn irrigation return flow ranged from 3 – 29%. We also observed an increasing correlation between lawn irrigation return flow in urban streams and imperviousness of the watersheds in the Denver area. In semi-arid and arid cities in the USA, including in Denver, urban irrigation consumes a large portion of city water. Through an analysis of spatiotemporal variations in streamflow, we observed that tap water is a larger contributor to urban streamflow than increased stormflow during most months. The joint contributions of tap water and directly-connected impervious area driving increased stormwater lead to profound alterations in the urban streamflow regime compared to grassland streamflow. This study provides insights into how urban irrigation and stormwater together increase streamflow, aiding water managers in implementing effective water management strategies in water-scarce cities.