Regional Differences in the Effects of Urbanization on Seasonally-separated Flow Duration Curves

Urbanized watersheds have impervious surfaces that impede infiltration, which can reduce groundwater recharge. They also often exhibit disturbed riparian zones with channelized or other alterations that can affect floodplain and watershed evapotranspiration (ET) rates. Studies of small, urbanized watersheds have often reported reductions in stream baseflow in urban streams. Urbanization and channelization of riparian corridors in larger watersheds (50 -200 km2) may result in decreases in evapotranspiration and thus increases in baseflow in comparison with adjacent non-urban streams. In this study, we selected paired (urban, non-urban) watersheds with similar basin areas, geology, and climate in four hydro-climatic regions of the U.S.: The Northeast, Pacific Northwest, Upper Midwest and the Southeast. USGS daily average discharge data were separated into cool season and warm season data sets and the resulting flow duration curves for each set of paired watersheds were compared. We found warm season baseflow was higher in urban than in non-urban streams in regions that received significant warm season precipitation (upper Midwest, Southeast), suggesting reduced evapotranspiration in these regions. This effect was not observed in regions with low warm season precipitation (Pacific NW) or during cool seasons. These data indicate significant impacts of urban land use changes on ET and both water and energy flows in urban streams.