A HISTORICAL, EMPIRICAL ANALYSIS OF THE RELATIONSHIP BETWEEN IMPERVIOUSNESS AND STREAM BASEFLOW IN THE EASTERN UNITED STATES. DOES URBANIZATION IMPACT BASEFLOW DURING DRY WEATHER?

Anthropogenic influence and urbanization has led to increased impervious surface area in historically rural watersheds. In permeable soils, precipitation infiltrates and recharges the groundwater levels which are the source for streams during dry weather; this groundwater sourced streamflow is called baseflow. With an increase in urbanization in a watershed one would theoretically expect a decrease in infiltration and therefore a decrease in ground water recharge and baseflow. Reduced baseflow can directly limit water availability for approximately half the U.S. population that depends on surficial waters for their drinking water. While in theory the relationship is sound, the connection between urbanization and baseflow is confounded by additional processes associated with urbanization such as irrigation and, leaking water pipes and storm/sanitary sewers. The goal of this project is to empirically investigate how urbanization has impacted stream baseflow on a geographic and temporal scale that has not been previously investigated. The project uses USGS gage data from rivers in 10 states (NY, CT, PA, DE, MD, VA, NC, SC, GA) that are unregulated and have a minimum recorded time period of 25 years. Three metrics of annual baseflow used are 1)baseflow per unit drainage area (BF); 2) ratio of BF to precipitation 3); and BF as a fraction of total flow (BF/TF). The non-parametric Mann-Kendall statistical test is used to detect trends in the entire hydrological record of the gages as well as each 10-year block of data to detect both short term and long term trends. Initial results from 31 unregulated streams with significant trends were 13% increasing and 6% decreasing for the BF metric, 16% increasing and 23% decreasing for BF/P, and 13% increasing and 13% decreasing for BF/TF. Preliminary results show the need for long-term records if trends are to be detected. The next step is to expand on the initial results by running statistical analysis on additional gages of all 10 test states in the study. The results will determine if there is a relationship between urbanization and baseflow as well as provide water managers with a way to assess the threat that urbanization poses to dry-weather water availability.