Streamflow Signature Analysis of Long-Term Effects of Urbanization on Hydrologic Alteration in the Southeastern United States

Abstract

Urbanization in the southeastern United States has risen exponentially over the past few decades. This increase causes runoff to have a greater surface flow, a smaller interflow, and sometimes a reduced baseflow by changing the natural infiltration characteristics of a watershed. As development continues, there is an increasing need for a better understanding of these effects on water resources and aquatic habitat. A study was performed to determine the overall effect urbanization has on hydrograph alteration over time. Streamflow and precipitation gauge data were analyzed over 36 years and compared with landcover changes to produce datasets for four watersheds. Datasets were split into pre-impact and post-impact periods to determine trends in hydrologic alteration. The 1-day, 3-day, 7-day, 30-day, and 90-day minimum and maximum were investigated for hydrologic alterations across each watershed. Maximums flows decreased over time in the urbanized watersheds whereas minimums flows varied in alteration. Streamflow signature analysis was performed on each watershed to quantify long term trends in each watershed. The runoff ratio, slope of the flow duration curve, streamflow elasticity, and baseflow index were analyzed. The urbanized watersheds experienced increases to streamflow elasticity over time leading to a greater sensitivity to changes in precipitation. Runoff ratios increased with increases to urbanization in Alabama urban watershed but were reduced in all other cases. The change in the slope of the flow duration curve varied depending on the watershed and baseflow index decreased in most cases but increased in the urbanized Alabama watershed. Watershed modeling with HEC-HMS was used to determine the effectiveness of large-scale hydrologic modeling methods in representing upstream tributary flows. Streamflow signature and hydrologic alteration analysis results derived from simulated flows proved less accurate to represent low flows and runoff ratio the farther away from the streamflow gage. However, increasing and decreasing trends between the gage location and upstream tributary locations were mostly maintained, even though the magnitude of alteration varied, showing that gage locations can at least provide a glimpse into the hydrologic alterations across the entire watershed. These trends can help identify the type of changes to the aquatic habitat that should be expected, but not necessarily an accurate depiction of the magnitude of the change.