State Water Resources Research Institute Program (WRRI)

Details for Project ID 2009NJ210G, 2009

Does urbanization decrease baseflow? A historical, empirical analysis in the coastal states of Eastern United States

Institute: New Jersey
USGS Grant Number:
Start Date: 2009-09-01 End Date: 2011-08-31
Total Federal Funds: $82,489 Total Non-Federal Funds: $87,935

Principal Investigators: Joshua Galster, Kirk Barrett

Abstract: Urbanization increases the amount of impervious surfaces such as pavement and rooftops, which decreases the amount of rain water that infiltrates into the ground, and, theoretically, decreases baseflow in streams. Therefore, urbanization poses a potential threat to surface water availability during dry weather, relevant both to drinking water supplies and stream ecology. Although theoretically compelling, the relationship between urbanization and baseflow is confounded by other processes which are also associated with urbanization, for example, lawn irrigation and treated wastewater discharge. In fact, it is not clear to what extent this theoretical relationship actually occurs in real watersheds. The relationship has not been thoroughly investigated empirically using available stream flow data sets. The goal of this project is to empirically investigate how/if urbanization is related to stream baseflow on a larger geographic and temporal scale than has been previously investigated. The project will cover the watersheds with USGS streamflow gages in 10 states (southern NY, CT, NJ, PA, DE, MD, VA, NC, SC and GA) that meet the specific criteria, namely a gage’s watershed must have experienced a substantial increase in percent imperviousness over a minimum of 25 years with daily streamflow records. As a pseudo-control, we will also analyze trends in 50 gaged watersheds that showed near constant imperviousness. We will estimate historical imperviousness by first developing a correlation between current housing density and current percent impervious coverage. Then, we will compute historical housing density in the watersheds and then apply the correlation to estimate historical imperviousness. For selected gaged watersheds, we will perform a baseflow separation using a digital filtering method to produce an annual time series of baseflow and two other metrics, baseflow divided by precipitation and baseflow divided by total flow. We will then perform a regression between concurrent, historical annual imperviousness and each of the three baseflow metrics to assess if they are correlated. If the theory holds, we should see a negative correlation -- increasing imperviousness is correlated with decreasing baseflow. To help assess whether climate change is affecting baseflow, we will test for trends in the control watersheds. This analytical method is well developed. We have applied it to 25 gaged watersheds in New Jersey. This project will provide the most complete assessment about whether/how urbanization actually affects baseflow. Water supply managers and land development regulators can make use of this information to help better understand the effects of land development and manage it accordingly, especially in rural and water supply watersheds. The project results should be useful in assessing the threat posed by urbanization to dry-weather water availability and stream ecology.