Effects of Urbanization on Stream Baseflow in NJ

Kirk R. Barrett (1) and Seth Kwaku Xeflide

 Passaic River Institute, Montclair State University, Montclair, NJ 07043

(1)  973-655-7117; kirk.barrett@montclair.edu; www.csam.montclair.edu/pri

Urbanization (converting fields and forests into buildings, roads and parking lots) increases the amount of impervious surfaces such as pavement and rooftops, which decreases the amount of rain water that soaks into the ground (groundwater recharge).  This groundwater is, in turn, the source of water in streams during dry weather, which is termed baseflow.  Therefore, as urbanization increases in historically rural water supply watersheds, decreases in recharge and baseflow could have an important impact on surface water supplies during drought.  

The goal of this project is to empirically investigate how/if urbanization has been related to stream baseflow over the past 50 to 100 years in New Jersey, concentrating on rural (but urbanizing), water supply watersheds. The project will help assess the threat to surface water availability posed by rural urbanization. 

We will use flow data from approximately 70 USGS stream discharge gages throughout NJ, with 20 to 107 years of record.  Drainage area for each gage (which range from 2 to 785 square miles) will be precisely delineated from maps of streams and HUC14s.  Summer and annual baseflow will be separated from hydrographs by a standard method.

To compensate for variable gage drainage area and variable precipitation, we will normalize baseflow by dividing by gage drainage and rainfall during the period of analysis for each gage, expressing baseflow as inches per inch of precipitation.  Monthly precipitation data is available from about 60 precipitation gages in NJ with 15 having over 100 years of record.   Historical imperviousness will be estimated by correlation with historical population density, based on a correlation between from 1995 statewide imperviousness determination and census data.

We will study sets of comparable (e.g., in terms of slopes and soils) drainage areas with different imperviousness historical trajectories over identical time periods to determine how/if imperviousness was related to baseflow.  We will also perform time-based comparisons within particular drainage areas.  Statistical tests will be applied to determine if/how stream baseflow was correlated to imperviousness.  

Separate analysis will be performed for different physiographic regions and using only drought years.  We will also perform a summer-only analysis because that is when drought can combine with high water demand for severe effects.  Confounders of the theoretical urbanization-baseflow relationship (notably regulation by reservoir, lawn irrigation and treated wastewater discharge) will be considered and incorporated into the analysis as possible.

This project was supported by the National Research Initiative of the USDA Cooperative State Research, Education, and Extension Service, grant number 2005-03915.