Urban development leads to changes in stormflow and baseflow, although the magnitude of these changes varies by city.Ìý This presentation will examine trends in streamflow with urbanization at the national scale (across the United States) and in Denver, Colorado.Ìý
Across the U.S., we will examine trends in the flow duration curve in 45 watersheds during periods of peak urban development, which ranged from 1939 to 2007.Ìý We used U.S. Geological Survey streamgage records combined with pre-development and urbanization characteristics to identify twenty years for analysis in each urbanizing watershed.Ìý Each urbanizing gage was paired with a nearby reference gage to represent climatic trends over the same time period.Ìý Urbanization, as measured by housing density, did not homogeneously alter the flow duration curve.Ìý Urbanization led to widely variable trends in low flow, where half of the urbanizing gages had increasing flow at the 10th non-exceedance percentile.Ìý Eight urbanizing watersheds had trend slopes greater than 20% flow change per year, with semi-arid and arid watersheds overrepresented.Ìý The largest changes in the flow duration curve were due to human wastewater infrastructure and flood control facilities.
In Denver, Colorado, we used water stable isotopes to quantify the contribution of tap water to stream base flow.Ìý Stream baseflow in thirteen urban watersheds and two grassland watersheds were sampled in summer 2019.Ìý Precipitation and tap water from five local water provider service areas were also sampled.Ìý Urban streams flowed for 90%-100% of the summer of 2019, whereas the grassland streams flowed for 60% of the summer.Ìý A two end-member mixing model revealed that tap water made up the majority of urban streamflow in late summer in urban streams.Ìý We used water infrastructure loss estimates to separate contributions of tap water from loss from that of lawn irrigation return flow and found that lawn irrigation return flow made up larger fractions of streamflow in urban watersheds with higher baseflow.Ìý
Isolating urban hydrologic signals from those of climate will serve to better identify and manage synergistic effects of urban development and climate change on flooding and water availability.