A Comparison of 7Q10 Low Flow between Rural and Urban Watersheds in Eastern United States

Saeed Zabet


Abstract: Increased runoff peaks and volumes from urbanizing watersheds have been well documented where watershed hydrology becomes modified after 10 to 25% of land area is developed. Lowering of baseflow has also been reported to be modified from urbanization; however hydrology thresholds related to percentage of land area developed are not well quantified. In this study, 100 watersheds in eastern USA were investigated to examine the potential effects of urbanization on low flows. The low flow metric chosen for this analysis is the 7Q10. Historical flow records were obtained from the USGS stream gauges, in which a minimum of 10 years of data were used for computing the 7Q10. Corresponding with flow data records, USGS Seamless land cover images for years 1992 and 2001 were used to quantify the percent land area urbanized. Using ArcGIS, land cover data for these two years were used to estimate percentage of urbanization by summing the land cover areas for industry, commercial, and high-density residential and dividing by the total watershed area above the USGS gauging station. Differences in 7Q10 values between the two periods were statistically analyzed using the Wilcoxon signed rank test. Results showed a significant decrease in low flow due to increased urbanization percentage from 0 to 11%. Decreases in low flows were sporadic as urbanization percentage increased from 11 to 23%, but for urbanization percentage more than 23% the increment of low flow were not significantly different.