Urbanization of the Christina River Basin and subsequent hydrologic alteration
Date
2016
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
The Christina River Basin is an invaluable source of water for more than half a million people. Thus, sustainable management of the basin is vital for maintaining and improving the health of this resource while addressing projected changes in climate. For this reason, the objectives of this study include examining changes in population density, land use land cover, impervious cover and precipitation, and analyzing the relationship between urbanization, streamflow and water quality. Population data from 1970 onward and land use data from 1990 onward are observed to quantify urbanization of the basin, while precipitation, streamflow and water quality data are analyzed to identify and understand effects of this urbanization on the hydrology of the Basin. ArcGIS and Microsoft Excel are utilized for this study for the analysis and mapping.
Results of the analysis show a definite increase in population density across the majority of the basin, with some of the 38 subwatersheds increasing by greater than 1,500 people per square mile on the outskirts of Newark, DE and others increasing by as much as 1,000 people per square mile on the outskirts of Downington, PA. A decrease in population density occurs in the Wilmington and New Castle, DE area with as much as an 800 people per square mile decline. Between 1990 and 2010, a significant portion of agriculture and forest land uses were lost as they converted to urban land uses. Urban uses occupy nearly 9% more of the Basin’s area by 2010, while agriculture has decreased by 7% and forest by 4.4%. This has lead to a significant increase in impervious cover across most of the basin as well.
Precipitation at two representative precipitation gages show negligible changes over the time frame. The streamflow-precipitation analysis demonstrates very little change in peak flows and highest ten discharge events, or flash floods. While trends are positive at the majority of the five station pairings selected for this aspect of the study, slopes are very small and have weak or no correlation. Likewise, minimum 7-day low flows, or baseflow, and the ten lowest discharge events, or droughts, also demonstrate very little change over the period of study. Trends of these variables are negative at most of the station pairings but slopes are very small and correlation is weak or nonexistent. As precipitation has not changed over the time frame of the study, any of the anticipated changes in these variables indicate undesired alterations in hydrology due to increased urbanization.
Despite increases in population density, urban land use and impervious cover, water quality across the basin has demonstrated some improvement since 1995. Inorganic nitrogen has increased basin wide, sometimes with seasonal statistical significance, and bacteria levels have increased at most locations as well (although with no correlation in any watershed). However, total suspended sediments and orthophosphate as phosphorus levels have decreased during most seasons in the majority of the watersheds, with statistical significance for some seasons, and dissolved oxygen has increased with strong correlation and statistical significance, indicating an improvement in the health of the basin.
This study illustrates major changes in population, land use and impervious cover of the Christina River Basin since 1990, and unexpectedly little change in streamflow and some improvement of water quality. Recommended future research includes a more in depth analysis of the relationship between land use and water quality.